In Considerable Variety': Introducing the Diversity of Australia's Insects

‘In Considerable Variety’: Introducing the Diversity of Australia’s Insects

Tim R. New

‘In Considerable Variety’: Introducing the Diversity of Australia’s Insects

Tim R. New Department of Zoology La Trobe University Melbourne, Victoria Australia [email protected]

ISBN 978-94-007-1779-4 e-ISBN 978-94-007-1780-0 DOI 10.1007/978-94-007-1780-0 Springer Dordrecht Heidelberg London New York Library of Congress Control Number: 2011934968 © Springer Science+Business Media B.V. 2011 No part of this work may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission from the Publisher, with the exception of any material supplied specifically for the purpose of being entered and executed on a computer system, for exclusive use by the purchaser of the work. Printed on acid-free paper Springer is part of Springer Science+Business Media (www.springer.com)

Preface

H.M.S. Endeavour, captained by James Cook, visited the east coast of Australia from April to August 1770. Amongst the far-reaching accomplishments from that visit, Joseph Banks and Daniel Solander initiated study of the animals and plants of this island continent. Banks, and his assistants, collected the first suite of insects to be taken back to Europe for study – from several localities from Botany Bay northward. Some insects clearly impressed Banks – he reported the towering mounds of termites he saw at Cooktown as reminding him of ‘the Druidic monuments I have seen in England’. Almost 240 new species from this collection were described formally in 1775 by J.C. Fabricius, a leading disciple of Linnaeus and one of the most influential entomologists of the era. His later ‘Philosophia Entomologica’, published in 1778, is regarded by many people as the first real entomology textbook and Fabricius was perhaps the first entomologist to appreciate the massive variety of insects. In the ‘Philosophia’ he noted that the number of species ‘is almost infinite’ and that ‘if they are not brought in order, entomology will always be in chaos’. Before the Endeavour voyage, Fabricius had met Banks and Solander in London in 1767–1768 and, no doubt, urged them to bring back insects from the voyage. Several of those first-collected Australian insects are common species that are easily recognisable today – the common brown butterfly (Heteronympha merope), the yellow-winged grasshopper (Gastrimargus musicus), and the green mantis (Orthodera ministralis) are examples. Another was a widely-distributed bull ant, Myrmecia gulosa, collected first at Botany Bay, and it is tempting to speculate that Banks might even have been the first European to suffer the pain of their attack! We know that members of his party were stung by the green tree ant (Oecophylla smaragdina) further north – for, in the Endeavour Journal, Banks recorded ‘their stings were by some esteemed not much less painfull than those of a bee’. These insects and many others, named under Linnaeus’ then radical binomial system, were allocated to genera described from other parts of the world and with which European workers were familiar: most have since then proved to be far more distinctive and transferred to more recently-described genera, many of them restricted to Australia and, sometimes, also its neighbours. For example, the very first Australian insect (and, indeed, animal) to be named was a scarab beetle that Fabricius called v

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‘Scarabaeus barbarossa’, but is now referred to the genus Haploscapanes, which contains only a handful of named species, all from the Australian region. Numerous similar revisions have helped to emphasise the unusual nature of Australia’s insects – and, indeed, they are no less characteristic and endemic than the better-publicised mammals and birds. As further exploration occurred, the richness and complexity of insects became gradually clearer, with few species from Australia even remotely familiar to their describers in the northern hemisphere. It seems that no insects were collected on Cook’s second voyage, but the visit to Adventure Bay, Tasmania in January 1777 (on the third voyage, with H.M.S. Resolution) yielded more, with ten species described by Fabricius in 1787. Cook recorded that the insects seen there were ‘in considerable variety’. Insects are the most diverse of all animal groups, and characterising and understanding the Australian insect fauna is ‘a work in progress’. From the First Fleet onward, changes began to occur, with arrivals of alien animals and plants either accidentally or being introduced for settler commodities and agriculture. The insect fauna was no longer pristine, with progressive arrivals from overseas of insects (including fleas and lice as parasites of domestic stock and companion animals), some having substantial impacts on human welfare as consumers of crops and stored products. Those early arrivals were not documented, of course, but many later introductions have been – honeybees, for example, were brought to Australia in 1822 as amongst the first of suites of insects deliberately imported, for a variety of purposes but without consideration of any future impacts in the Australian environment. In parallel, however, visiting and resident naturalists had greater opportunity to collect and study Australia’s native fauna, and are still doing so. More than 200 years later, we still have only vague ideas about the diversity of many groups of our insects, with various ‘scientific guesstimates’ based on collection contents and expert opinions. Many species have not been studied or, even, collected and it is common for any visiting specialist working on a particular family of beetles, flies, wasps or other large group to discover large proportions of hitherto undescribed species to augment the total. Many surprises remain. Our largest known stick insects, giants of the order with one given the appropriate species name ‘gargantua’, have been described only in the last few years – the female of this particular giant, with body length exceeding 30 cm (and spanning more than twice this with legs extended) is known only from a small area of tropical rainforest in northern Queensland, and is one of the world’s largest insects. But at the other extreme, many minute insects are amongst the ‘black holes’ of our formal know­ ledge. Enormously diverse, of serious interest to only a few specialists (most of them based far from Australia), they remain undercollected and difficult to appraise. Some tiny wasps, that pass their early life within a single egg of a small barklouse or leafhopper, are only about one fifth of a millimetre long: considerably smaller than a large single-celled Amoeba but with all the structural complexity of much larger insects miniaturised into this speck of life. This variety of size was familiar to early entomologists, but is still surprising to many other people, and the practical difficulties of studying the richness of insect life renders estimates of their diversity somewhat intangible.

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We simply do not know how many native insect species occur in Australia. Recent suggestions of more than 200,000 different species assure them an easy top place for diversity amongst the entire fauna; and the figure is debated – it cannot be refuted, and may even be an underestimate. Methods of collecting and studying insects have advanced from Banks’ day, but the principles of needs for capture, preservation, curation, and expert examination and diagnosis are constant. Fabricius initiated the taxonomic foundation on which we must still build, with the realisation that even now perhaps only a quarter or fewer of our native insects have been given formal names. Early descriptions of species are brief, commonly only one or two lines of Latin and addressing a very limited range of characters. They contrast with the lengthy diagnoses now the norm for differentiating similar or allied forms. Fabricius’ contention (in Mantissa Insectorum 1787) that ‘Too many words are the real trouble of entomology’, was founded in an era when recognising entities regarded as species was altogether a simpler exercise than it is now. For example detailed measurements and good illustrations, involving morphological details often necessarily based in delicate dissections and microscopical examination, are now almost mandatory in describing insect species and differentiating related forms. Differences based on structural features are increasingly being augmented by molecular data and statistical analyses to clarify relationships. This book is about this ‘considerable variety’: what it comprises, how and where insects live, their peculiarities and roles in Australian environments, and their interactions with humanity. It is an introduction to the natural history of insects in Australia, and some of the remarkable features of the fauna that render insects the richest and most successful animals with which we, sometimes uneasily, share the planet. I hope to introduce the study of insects, entomology, through their evolution and adaptations to the variety of Australia’s terrestrial and freshwater environments they so capably dominate. This is not a formal textbook, but covers much of the ground that an elementary entomology text may include, in a framework intended to help people lacking formal biological training or knowledge of insects to begin to understand the major general features and causes of insect variety, and emphasising the importance of Australia’s insects, how they ‘work’, and the needs for conserving their diversity and sustaining their participation in ecological processes and systems. The sequence commences with several general introductory chapters on insect structure, evolution, biology and ecology, helping to illustrate the richness, variety and peculiarities of the Australian fauna. Later chapters summarise the main entomological features of some of Australia’s key environments, and the final chapters address aspects of interactions between people and insects and the importance of increasing efforts for documentation and conservation. I have tried to avoid much of the technical ‘jargon’ that readers can find so offputting and an impediment to understanding, and the sequence of general themes are each treated from basic principles; an Appendix summarises main features of the different insect orders. Each chapter contains suggestions for further reading but, except in a few cases in which I have referred directly to specific papers, close referencing is not given, as likely to disrupt the book. I hope that biologists who recognise allusions to their work without direct citation will forgive this approach. Many of the references cited

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are classics, and for many I have indicated their relevance: all are available readily. With similar intent, I have not attempted to provide a full illustrated synopsis of Australia’s insects; more comprehensively illustrated books are cited in Chap. 1, for example. My purpose has been, rather, to provide a limited range of illustrations of some representatives of major insect groups that help to ‘tell a story’, to consolidate points in the text, and that indicate particular features or habits that aid understanding of insect variety. Deliberately, many are of common or widespread species that can be discovered easily, some in home gardens, and so that can become familiar with relatively little effort. Much of the book’s content draws on basic information and principles, and so transfers easily to the insects of any other part of the world. The book is, I hope, based in good science and is intended to be accessible to non-entomologists as a means of introducing insects to a wide non-specialist rea­ dership and, in particular, of demonstrating the bases of the immense – indeed ‘considerable’ – ecological, functional and taxonomic variety that renders the Australian insect fauna so intriguing, and also so important to sustain. Two major strands of modern conservation are education (linked with informed understanding and advocacy) and scientific knowledge. Insects have long suffered from both image problems and that non-entomologists, including the great majority of ecologists and managers charged with conserving Australia’s unique biota and ecosystems, do not appreciate their taxonomic and biological subtleties and complexity that influence the scale of attention needed to sustain them. The book indicates some of the ambiguities and complexities of documenting insect diversity in Australia and discovering how insects have exploited this vast geographical arena – and so contributes to defining the steps needed to assure the wellbeing of this unique biological heritage. Acknowledgments  The contents of this book are derived from many sources, only a few of which are cited specifically. New information on Australia’s insects is published in a range of relevant journals, such as Australian Journal of Entomology, Australian Entomologist, Australian Journal of Zoology, Austral Ecology, and Invertebrate Systematics, all of which focus on research in the region, and many syntheses of relevant topics can be found in the Annual Review of Entomology and elsewhere. Selection of examples to include or omit for limitations of space has been a complex and idiosyncratic exercise, and informed readers may justifiably consider some suboptimal and would opt for a different array from which to discuss general themes and principles. Photographs supplied by colleagues are acknowledged individually in the legends, and it is a pleasure to reiterate my thanks to these friends who responded so gene­rously to my requests for this use. I appreciate comments from reviewers of the original proposal, and the continuing support of Zuzana Bernhart at Springer, together with the friendly cooperation and advice during production from Elisabete Machado. Production has been facilitated immensely by the careful help of Ms. Juno Martina George. La Trobe University Victoria 3086 Australia

T.R. New

Contents

  1 The Basic Insect Pattern: Theme and Variations.................................. Introduction: Insects and Their Close Relatives........................................ The Insect Body Plan................................................................................. Inside Insects.............................................................................................. Further Reading.........................................................................................

1 1 3 7 12

  2 Fossils and Major Insect Adaptations.................................................... Introduction: The Process of Insect Evolution........................................... Wings and Flight........................................................................................ Wings and Ecology.................................................................................... Insect Diversification................................................................................. Further Reading.........................................................................................

13 13 14 18 19 22

  3 Insect Life Histories................................................................................. Introduction: Modes of Development........................................................ Diversification Within Metamorphosis...................................................... Seasonal Development............................................................................... Further Reading.........................................................................................

23 23 25 32 36

  4 Origins, Distributions and Diversity...................................................... Introduction: Australia as an Environment for Insects.............................. Insect Species?........................................................................................... The Yellowish Skipper and Donnysa Skipper, Hesperilla flavescens and H. donnysa................................................... The Swordgrass Brown, Tisiphone abeona........................................... Further Reading.........................................................................................

37 37 43

  5 Environments and Habitat for Insects in Australia.............................. Introduction: Places to Live....................................................................... Resources for Insects................................................................................. Further Reading.........................................................................................

55 55 61 66

44 44 53

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Contents

  6 Foods and Feeding Biology...................................................................... Introduction: The Variety of Food and Feeding Habits............................. Exploitation or Partnerships?..................................................................... Searching for Food..................................................................................... Insect Herbivores....................................................................................... Insect Carnivores........................................................................................ Insect Parasitoids................................................................................... Insect Parasites...................................................................................... Insect Decomposers................................................................................... Further Reading.........................................................................................

69 69 70 71 75 79 85 87 88 91

  7 Insect Behaviour and Lifestyles.............................................................. 93 Introduction: Behaviour and Adaptation................................................... 93 Sensory Mechanisms................................................................................. 95 Social Existence......................................................................................... 101 Further Reading......................................................................................... 111   8 Insect Communities................................................................................. Introduction: Living Together.................................................................... Richness and Variety.................................................................................. Evolutionary Radiations............................................................................. Assessing Richness.................................................................................... Further Reading.........................................................................................

113 113 116 119 125 127

  9 Insect Populations.................................................................................... Introduction: Population Size and Structure.............................................. Population Fluctuations............................................................................. Further Reading.........................................................................................

129 129 131 137

10 Insects in Inland Water Environments.................................................. Introduction: Inland Aquatic Habitats for Insects...................................... Insect Variety............................................................................................. Further Reading.........................................................................................

139 139 141 147

11 Australia’s Alpine Insects........................................................................ Introduction: Environmental Extremes...................................................... Alpine Insects............................................................................................. Further Reading.........................................................................................

149 149 150 155

12 Lowland Insects and Their Environments: Non-forest Habitats.................................................................................. Introduction: Terrestrial Open Habitats..................................................... Grasslands.................................................................................................. Arid Environments..................................................................................... Mallee Environments................................................................................. Further Reading.........................................................................................

157 157 157 162 163 165

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13 Forest Insects............................................................................................ Introduction: Forest Habitats..................................................................... Forest Insects............................................................................................. Further Reading.........................................................................................

167 167 168 174

14 Insects and People in Australia............................................................... Introduction: Interest and Involvement...................................................... Pest Insects................................................................................................. Further Reading.........................................................................................

177 177 179 185

15 Australia’s Alien Insects.......................................................................... Introduction: Variety and Impacts.............................................................. Importation and Establishment.................................................................. Consequences............................................................................................. Australian Insects Abroad.......................................................................... Further Reading.........................................................................................

187 187 188 190 194 197

16 Conservation............................................................................................. Introduction: Rationale and Needs for Insect Conservation...................... Species Conservation................................................................................. Habitat Conservation................................................................................. Wider Views............................................................................................... Further Reading.........................................................................................

199 199 201 206 208 210

17 Diversity and Its Implications for Understanding Australia’s Insects.................................................................................... Introduction: Relevance of Basic Documentation..................................... Surveying Diversity................................................................................... Increasing Understanding.......................................................................... References.................................................................................................. Further Reading.........................................................................................

211 211 212 214 221 222

Appendix........................................................................................................... Australia’s Insects: The Players................................................................. Apterygota............................................................................................. Pterygota............................................................................................... Palaeoptera................................................................................................. Neoptera..................................................................................................... Polyneptera............................................................................................ Paraneoptera.......................................................................................... Oligoneoptera........................................................................................

223 223 225 226 226 229 229 235 239

Index.................................................................................................................. 249



Chapter 1

The Basic Insect Pattern: Theme and Variations

Introduction: Insects and Their Close Relatives Even defining ‘an insect’ can be difficult! But any understanding of their massive variety must start from a clear picture of the basic structural template that forms the foundation for any such definition and later diversification. That progressive differentiation has taken place through adaptive modification of almost any structure present, and defining that body plan is vital in distinguishing true insects from other animals. Examining some of the evolutionarily older kinds of insects helps us to characterise that pattern, as well as to suggest some of the reasons why insects as a group appear so successful and have persisted largely unchanged in their fundamental design for so long. Insects are arthropods, members of that vast phylum of invertebrate animals that share a hard external skeleton and have jointed limbs, ancestrally a pair for each body segment. Within the arthropods, they are accompanied by spiders, mites, crabs and other crustaceans, myriapods such as centipedes and millipedes, and a host of others, each of which has a reasonably consistent body plan that enables us to recognise them. So, also, with insects. Characteristically, insects have six legs and their body is divided into three major regions, the anterior head, central thorax and posterior abdomen. Sometimes these regions are clearly separated – as in a ‘waist’ (although, paradoxically, in wasps and their relatives, that waist is actually after the first bit of the abdomen!) between thorax and abdomen; hence the name ‘insect’ (cut into), in marked contrast to the body of many other arthropods. This much is straightforward, but the integrity of defining insects in this way is disrupted by the existence of several other groups of small arthropods that share this pattern and so join them in the Hexapoda, the six-legged arthropods. These, the springtails (Collembola), proturans (Protura), and diplurans (Diplura) have historically all been placed in the class Insecta, but each is now considered an entire independent class equivalent to the whole of the true insects. The reasoning for this is complicated, and rests on the form of the mouthparts. The three small groups are collectively called ‘Entognatha’ (or entognathous hexapods) to emphasise that their mouthparts

T.R. New, ‘In Considerable Variety’: Introducing the Diversity of Australia’s Insects, DOI 10.1007/978-94-007-1780-0_1, © Springer Science+Business Media B.V. 2011

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1  The Basic Insect Pattern: Theme and Variations

Fig. 1.1  Broad groupings of insects and other hexapod classes. The three classes of Entognatha were all earlier considered to be insects, but all ‘true insects’ are ectognathous, with the winged forms (Pterygota) comprising several major lineages (see text)

are enclosed in extended folds from the front of the head, in contrast to the exposed mouthparts of the true insects, which are thereby ‘Ectognatha’. However, even Entognatha is probably not a single lineage, and Diplura are probably nearer to the basal insect line of evolution than the other two, again assessed on the articulation of the mouthparts. Even within the unambiguous Insecta, the most ancestral forms include one small order, bristletails (Archaeognatha), with jaw (mandible) articulation different from all others. These broad groupings are summarised in Fig. 1.1, and at least enable us to define ‘insects’ in discrete taxonomic terms that are universally accepted. However, simple observation of many insects reveals many departures from the above basic pattern used so far to define them. Many butterflies have only four legs; some adult insects have even lost all their legs (as in female Strepsiptera, living inside their hosts) and the traditional body divisions may be difficult to discern; when we include highly modified immature stages (larvae), of which more later, the appearance is often very different. A typical ‘maggot’, the larva of many true flies (members of one of the largest orders of insects, Diptera, see Chap. 3) is basically a tapered cylinder, without any head, the modified mouthparts enclosed in the anterior end, no obvious differentiation between any body regions, and no legs. Such radical departures emphasise the extent of modifications that insects may undergo to exploit different environments and ways of life, and also make it very difficult for field biologists to associate early stages of many insects with the corresponding adults. Nevertheless, the basic pattern forming the foundation for these is at least reasonably consistent, and the basis also for classification of insects into their major

The Insect Body Plan

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groups, orders. Insect systematics and recognition is based largely on external structural features, with relationships inferred from patterns of change and transition that occur. Nowadays, this information can be augmented, and in some cases questioned, from results of molecular analyses, but each of the approximately 30 orders of insects alive today can be recognised, and diagnosed formally, on a particular combination of structural features common to all its members and differentiating the order from all others (Appendix, p. 223). Simplistically, recognition of a dragonfly, beetle, moth, grasshopper, or many others is generally straightforward, even though assessing the relationships between the orders may not always be so. Even experienced entomologists can be misled by the bizarre appearance of some forms. Just as the first specimens of the platypus sent back to England caused naturalists to speculate that they had been manufactured, or birds of paradise were presumed to lack feet, some Australian insects have at first seemed not to fit any conventional ideas. The initial formal description of the orthopteran known as the ‘Cooloola monster’ (Cooloola propator, from Queensland) was introduced by the following comment: ‘After some amusement at the technical excellence of the apparently manufactured monster, it was determined that it was a genuine complete cricket-like insect’. Occasional other oddities have proved difficult to allocate even to order, but discovery of entirely new orders is unlikely to occur very often, although specialists continue to debate whether some of the long-recognised orders should retain their current boun­daries or divisions. The most recently erected insect order, the southern African Mantophasmatodea (heelwalkers, rock crawlers, with features of both praying mantids and stick insects), was named following discovery of living insects in 2002. However, since then it has been relegated to a suborder and combined with another small non-Australian group within the existing order Notoptera. But, in short, the basic body plan of insects is both definable and has become differentiated to produce the largest suites of animals ever to grace Earth.

The Insect Body Plan The structural plan of true insects is exemplified well by a rather ‘basic’ insect such as an adult of the field cricket (Teleogryllus commodus) or the Australian plague locust (Chortoicetes terminifera), both members of the order Orthoptera and notable for their intrusions into pasture and cropping systems. This ‘basic body’ (Fig. 1.2) comprises three very different-looking regions. The head is a solid capsule, with the antennae (chemosensory structures), three pairs of mouthparts, and large compound eyes the major features. Small ‘simple eyes’ (ocelli) are also present. The thorax is also a solid box, with three pairs of legs and in most crickets and other insects, two pairs of wings (p. 14). The abdomen is elongate, more delicate in appearance and with posterior elaboration used in mating by both sexes and egg-laying by females. All of the appendages noted are derived from the serial appendages of the basic arthropod, so also help to indicate the number of segments in the theoretically complete insect body. The pattern is clearest for the thorax, where the three segments

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Fig. 1.2  Generalised adult body pattern of an insect, indicating the three main regions; head (with large compound eyes, antennae and mouthparts), thorax (dotted, with legs on all three segments and two pairs of wings), and abdomen: insets show some variations on the form of antennae (a–d), legs (e–i) and mouthparts (j–l) to indicate some of the extensive modifications that enable insects to adopt different ways of life. Main diagram shown with slender ‘filiform’ antennae: others are (a) clubbed, or ‘clavate’, as in butterflies; (b) flanged or ‘flabellate’, as in some beetles; (c) feathery, ‘plumose’ as in some moths; (d) elbowed, ‘geniculate’, as in ants. Legs: (e) fore leg of mantid, adapted for grasping prey (cf Fig. 6.3); (f) fore leg of mole cricket, broadened for digging; (g) parasitic louse, for firm gripping of host hair or feather; (h) enlarged hind leg of grasshopper, for jumping; (i) hind leg of water beetle, flattened and with fringe of long hairs, as paddle for swimming. Heads; main diagram with chewing mouthparts, as in a grasshopper: (j) fly, with mouthparts composed of labium, expanded as ‘sponge’ for semiliquid diets; (k) butterfly or moth, a long coiled proboscis adapted for sucking nectar from flowers; (l) plant bug, with long slender stylets for piercing vegetation and ingesting plant sap

each retain the pair of appendages, as legs and contributing also to construction of wings on the second and third segments. The six segments of the head are fused together, and appendages modified to form antennae (segment 2), and the mouthparts (segments 4–6), and the abdomen appendages are lost other than for those

The Insect Body Plan

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constituting genitalic structures on segments 8 and 9 of the 11 total. Remnants of the limbs are present on the more anterior segments in some very primitive wingless insects, giving us clues to the derivation of the more posterior structures in crickets and others. The insect body as a whole thus comprises 20 segments arranged into three ‘blocks’ (tagmata) adapted primarily for rather different roles: the head for sensory perception of new environments as the insect encounters them, and for feeding, the thorax for locomotion, and the abdomen for reproduction. Because solid muscle attachments are needed for mouthparts, legs and wings to operate effectively, the head and thorax are indeed usually very solid structures. Conversely, the abdomen is flexible, with a lateral membrane rather than solid ‘wall’. It can be distended for storing food, food reserves, or eggs, and also allows versatility in mating – some insects adopt postures that could be envied by compilor(s) of the Kama Sutra, but which usually involve juxtaposition of the abdominal tip of the two sexes – and depositing eggs in many hidden habitats such as under bark, in the ground, or in other insects or even other animals. In some insects, the abdomen also has two posterior ‘cerci’, filamentous sensory organs, broadly analogous to ‘posterior antennae’ and which tend to be reduced or lost in the most advanced insects. Any or every appendage and other structure can be changed massively from this basic form, and it is useful to look at some of these divergences here, as a prelude to seeing how they operate in different kinds of insects with differing ways of life, and keeping in mind that the variations have both functional roles, and value in diagnosing and recognising different kinds of insects. Several representative variations of antennae and mouthpart form are shown in Fig. 1.2, each condition diagnostic for some insect group(s), and collectively helping to emphasise the strong linkages between structure and function, the last reflecting ‘way of life’. Again, the cricket provides basis for comparison. The cricket’s antennae are long (although not as long as in many other similar Orthoptera, in which they can reach several times the body length) and slender. They are technically ‘filiform’ or threadlike, and are made up of numerous small jointed lengths, commonly (but not embryologically) termed ‘segments’ but more properly ‘antennomeres.’ Many other insects have much shorter antennae, and the basic appearance can be much different, with branches, flanges (Fig. 1.2b), numerous lateral projections, or apical thickenings so that they can seem feathery (plumose: Fig. 1.2c) or clubbed (clavate: Fig. 1.2a). The effect of these ornamentations is to increase the surface area available for chemical receptors. In some moths, the form of the antennae differs markedly between the sexes within the same species: those of males are strongly feathery, and of corresponding females, slender. Such differences indicate rather dissimilar needs. In this case, for example, females of many moths, such as codling moth (Cydia pomonella) and Oriental fruit moth (Grapholita molesta), both pests of orchard crops, do not fly but attract mates by emitting a highly specific pheromone scent. The males detect the scent through their antennae and respond by flying upwind along the increasing concentration gradient to encounter a potential mate. This behaviour, by which males of some species may be attracted from up to several kilometres away, has been used in aspects of pest management for these species, by attracting males to artificial pheromones on crops and so keeping them

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from mating, thereby reducing the next generation of the pest. Of less economic importance, collectors may use a female moth to attract males as specimens. Not all moths ‘work’ in this way, but this example illustrates well how the appearance and structure of an insect appendage allows us to interpret or infer some important aspect of its biology. Likewise for mouthparts. Diversification of feeding habits is a major component of insect evolution and their spread of ecological roles and interactions, and is reflected in modification of any or all of the mouthparts. In the cricket, again regarded as a ‘basic’ representation, these comprise a pair of strong jaws (mandibles, on segment 4) that dismember food – in this case, predominantly vegetation; behind these are the paired maxillae (on head segment 5), very different in appearance from the tough mandibles and including a sensory structure (maxillary palp) and ‘accessory jaws’; and the third pair on segment 6 are structurally similar maxillae with reduced palps but fused in the midline to constitute a single structure, the labium. This arrangement is found in many different insects with chewing habits, whether herbivores or carnivores, but this structure is clearly not well-adapted to ingest liquid diets, such as plant sap or blood. The functional need is then for some structure with a role equivalent to that of a drinking straw or hypodermic syringe that can probe or penetrate the plant or animal surface and imbibe the liquid. This is accomplished independently, and by rather different modifications in several widely disparate groups of insects. In some it involves transformation of the cricket-like mandibles and maxillae into slender piercing ‘stylets’, each forming part of the circumference of a tube (proboscis) through which liquid is passed. The whole of this delicate structure in sucking bugs (Hemiptera, Fig. 1.2l) and some Diptera (such as mosquitoes) is supported by a broadened protective labium. The functionally similar structure of Lepidoptera is formed from maxillae alone, and can be coiled under the head when not in use (Fig. 1.2k), so not impeding manouverability: because of their need to take nectar from flowers, some Lepidoptera have a proboscis several cm or more long – the record is perhaps of a hawk moth from Madagascar, in which this structure extends about 30 cm, enabling it to gain nectar from orchids with very deep flowers and act as a pollinator for these! Other mouthpart variations occur. In bushflies and other advanced Diptera, the major structure is from the labium alone, flattened and expanded at the apex which is adpressed to surfaces of dung, carrion, plants or other foodstuffs to sponge up semiliquid materials (Fig. 1.2j). As with many other structures, mouthpart form can be taxonomically diagnostic as well as functionally informative. Moving to the thorax appendages, any or all the parts of the typical insect leg (Fig. 1.2e–i) can also be changed – whereas we naturally think of walking or running as their primary function, other roles are common. In our cricket/locust examples, the hind legs are conspicuously elongated and strengthened for jumping; many aquatic insects have legs broadened and/or fringed with long hairs to increase their surface area for ‘rowing’ on or under water; mole crickets have unusually strong and broad front legs for digging into soil; and the spined and grasping front legs of mantids are used to capture other insects (and on occasion other animals, even small birds) as prey – in an adaptation paralleled by some predatory bugs, lacewings and

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flies in which the forelegs have assumed similar form. Insect courtship may involve elaborate displays including ‘leg-waving’, and legs are also involved in the sound production (stridulation) and reception in many Orthoptera. Wings are discussed more fully later, as a key feature of insects, but the basic principle of multiple modifications from a single basic form is common to all the structures we have noted, with numerous cases of parallels – as in the grasping forelegs of some predatory insects, above. Adoption of a similar habit or way of life by groups of insects that are only distantly related can commonly lead to a problem of capability being solved by the same basic adaptation evolving independently. But, however unusual or bizarre any insect may appear, it has a fundamental structure derived from a pattern similar to that of the cricket with which we started this section. The abdomen is more uniform, largely reflecting that the serial appendages so prone to modification are restricted to the posterior end. However the most ancestral groups of true insects, the silverfish (Zygentoma) and bristletails (Archaeognatha) show us how these reproductive structures may be derived from the same basic limb form. In these very primitive lineages the underside of some (even, most) of the abdominal segments have paired narrow ‘styles’ projecting rearwards from the posterior margin. These represent part of the base (coxa) of the leg, and have disappeared from the more anterior abdominal segments in advanced insects. The coxae of normal thoracic legs of some of these insects also bear a style, clearly indicating he homology described above.

Inside Insects The internal structure of insects, also, follows a rather basic and consistent pattern to accommodate the needs for digestion, respiration, reproduction, movement and the variety of other metabolic and developmental processes and responses to the local environment. This pattern, summarised in Fig. 1.3, shows the characteristic relative positions of the major anatomical systems. Thus (1) the alimentary system (dotted in Fig. 1.3a) is the most conspicuous as a continuous tubular ‘gut’ from the anterior mouth to the posterior anus, along the whole length of the body; (2) the circulatory system is predominantly dorsal to this, with a mid-dorsal vessel (sometimes called the aorta anteriorly and the ‘heart’, posteriorly, but a single tube) along the midline; (3) the central nervous system, in contrast, is ventral with anterior concentration of nervous tissue as a brain, encircling the gut in the head; (4) the respiratory system, as a series of tubular tracheae, extends throughout the body, with air admitted from the exterior through a series of paired lateral openings, spiracles; and (5) the reproductive system is predominantly posterior, with reproductive openings for mating and oviposition situated below the anus. All these systems are within a body cavity, the haemocoel, and can be displayed easily in dissection of a freshly killed cockroach or grasshopper. The body cavity contains haemolymph, in which all the above structures are bathed. Haemolymph is

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1  The Basic Insect Pattern: Theme and Variations

Fig. 1.3  (a) General internal anatomy of an insect, to show organisation of various organ systems. (b) Digestive tract, indicating different regions

the blood of insects but, unlike vertebrate blood, has only a minor role in respiration and is involved more in nutrient and waste metabolite transport and in some immune interactions. Haemolymph can provide defence against disease, parasites, or physical injury, such as by forming clots that can seal wounds through the cuticle, or cellular reactions that encapsulate disease-causing organisms (including eggs of parasitoid wasps, p. 85) and isolate them. Waste products are filtered from the haemolymph by special structures, the Malpighian tubules, and discharged into the gut and, thence, to the exterior. The haemocoel also contains material loosely termed the ‘fat body’, a more-or-less evident layer of fat around the gut or lining the body cavity and which has complex metabolic roles in storage and reorganisation of nutrients and regulating their supply to the insect. Both the haemocoel and the gut of insects contain microorganisms of various kinds. Some are clearly needed by the insects, and are mutualistic. Wood-feeding termites depend on single-celled protistans to break down cellulose into digestible components, for example, but the true roles of many of these characteristic symbionts are still unclear. Each of the major anatomical systems can undergo modifications for particular ways of life, but they are used only to a very limited extent in classifying insects – not least because the hard external structures are much more accessible for study, and remain available and unchanged in long-dead specimens in collections.

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However, some understanding of the variety of internal structure helps in interpreting the variety of life styles that insects may exhibit. Thus, the alimentary canal is divisible into several distinct regions, differing in appearance and primary function The relative development of these may reflect the diet of the insects, predominantly whether it is solid or liquid, or plant or animal material, so that the gut may be generalised, or specialised to deal with particular food materials. Insects such as grasshoppers and caterpillars that ingest solid vegetable food tend to have simple, short, muscular guts, strong enough to resist abrasion from plant or animal fragments and wide enough for those particles to pass easily. In contrast, many liquid-feeders have long, more convoluted narrow intestines that allow greater surface contact with the liquid, and protection from abrasion is not an issue. Many sap-sucking bugs and other liquid-feeders have to take in large volumes of food, because the nutrients they need are very dilute in excess water; some bugs have a special ‘filter chamber’ to eliminate excess water and concentrate the food for digestion. Whatever the diet, the gut may also need to store food at times, because many insects can feed only intermittently. The basic pattern of the gut divisions is shown in Fig. 1.3b. Food enters through the mouth, near which the salivary glands open. The saliva is sometimes used to commence digestion of food outside the body, and may be injected by liquid-feeding insects for this purpose and to aid ingestion. The gut itself is conventionally divided into three main regions, the anterior foregut, central mid-gut and posterior hind gut. The fore gut comprises the oesophagus, through which food passes to the crop (in which food may be stored) and insects taking solid food have a proventriculus (or gizzard, often muscular and with internal hardened spines or ridges by which food particles are broken down). The mid-gut is the major region for digestion, with the surface area for the enzymes produced in this region to interact with food sometimes increased markedly by pouches or gastric caeca from the central ventriculus. In many insects it is lined by a peritrophic membrane separating the food from the gut wall and increasing circulation of enzymes and which is shed at intervals together with any post-digestion food residue. Thus, the diet of dragonfly larvae can best be studied by allowing wild-caught larvae to eject their faecal pellets surrounded by peritrophic membrane, and which encapsulate the remains of their arthropod prey from their recent meals. Dissection of these pellets provides many characteristic fragments (such as hard parts of arthropod mouthparts or legs, that cannot be digested) that can be identified, often quite precisely. The Malpighian tubules, carrying metabolic wastes as noted above, open to the intestine at the junction between mid-gut and hind gut, and their number and form can also characterise particular insect groups. Major functions of the hind gut are absorption of useful materials from faeces and urine before these are egested, and the three successive regions, commonly differentiated as ileum, colon and rectum, differ in relative extent across taxa. In some insects, such as larvae of antlions and other lacewings (Neuroptera), the hind gut is blocked, so no faecal material can be passed until the insect matures. A liquid-filled body cavity also provides an internal ‘hydrostatic skeleton’ that enables soft-bodied insects such as maggots and similar larvae to crawl, through

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waves of muscular contraction being transmitted along the body. Insects with a hard exoskeleton move mainly through direct action of muscles attached to this. Insects have only striated muscles, and those associated with the wings and powering flight or other strenuous movement are particularly well developed. Circulation of haemolymph occurs through passing it along the dorsal vessel, with segmental openings with valves to help ensure a one-way flow toward the anterior. The ventral nerve cord consists of a series of segmental concentrations of nervous tissue (‘ganglia’) linked by paired longitudinal connective nerves. Their basic pattern is of a pair of ganglia for each body segment, but considerable modifications have occurred through these becoming fused or concentrated to varying extents. Most consistently, all the head ganglia are joined to form the brain (dorsal) and suboesophageal ganglion (ventral) around the fore gut, and the numerous other patterns range from all thoracic and abdominal ganglia being distinct to, at the other extreme, all forming a single mass within the thorax. However, nerves radiate from the ventral cord to all muscles and sensory structures to govern the insect’s responses and behaviour. In addition to the conspicuous sense organs, such as eyes and antennae noted earlier, the variety of less obvious structures reflect the needs to respond to both internal and external changes, and to a great variety of environmental cues. Numerous specialised hairs, bristles and related structures on the body surface are linked with individual nerve fibres and are highly adapted receptors for mechanical, positional, chemical, audial or temperature or humidity cues – so that sounds produced by other insects, (whether mates or antagonists), and chemicals such as pheromones, and changes in the external environment can all be detected effectively and appropriate responses be made. The repertoire of sensory structures and responses for any insect may include many individualistic components that facilitate precise responses. Insects obtain oxygen and eliminate waste carbon dioxide through a system of ramifying internal tubes, tracheae and smaller tracheoles, with external openings (spiracles) through which air is taken from or eliminated to the outside environment. One pair of spiracles opening laterally from each thoracic and abdominal segment is the primitive pattern, but Recent insects never have more than two thoracic and eight abdominal spiracles, and many have far fewer. Dragonfly larvae (‘mudeyes’) and some others have no spiracles at all – in larval dragonflies, gaseous exchange takes place across the wall of the rectum, into which water is pumped, and mayfly larvae obtain oxygen by diffusion across the lateral abdominal gills. Likewise larvae of some internal parasitoids (p. 86) also lack spiracles and have finely ramifying tracheoles over much of the body surface to enable gas exchange across the body wall. Many insects with spiracles can close them, reducing water loss in more arid environments. Reproductive structures also follow a rather basic pattern, as a foundation for innumerable variations in size, shape, complexity and development in both sexes, in relation to functions and reproductive behaviour. These functions are complex. A female insect of a bisexual species needs to mate, store and transmit sperm, produce varying numbers of eggs (from few to many, all together or over an extended period and perhaps store these over many weeks or months) as well as lay them, perhaps in precisely selected locales. Some taxa are viviparous, so that larvae are the

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first stage to be released to the outside world, and many insects are parthenogenetic. Males need to produce and store sperm and transmit it to the female. They may also have adaptations to enhance their chances of success in competition for mates: males of some butterflies (such as the big greasy, Cressida cressida, a swallowtail from Queensland) actually plaster the female reproductive opening with a secretion that hardens to form a ‘chastity belt’ (technically, a ‘sphragis’) to prevent subsequent matings with other males. And some male dragonflies use spines on their reproductive appendages to ‘rake out’ sperm from any previous matings before depositing their own. Much of the intricate behaviour of insects relates to increasing chances of reproductive success and larger numbers of progeny, and their often complex behavioural strategies link strongly with structural adaptations in the reproductive system. As in other animals, hormones in insects play pervasive roles in moulting, development and many aspects of reproduction and metabolic regulation. They are produced from various internal organs and transported by the haemolymph. Three major groups of hormones are usually distinguished as central to reproductive and growth functions, and these are termed the ecdyosteroids, the juvenile hormones and the neurohormones (or, more commonly, neuropeptides). The first are concerned with moulting as a critical process in insect growth and maturation; the second are involved with control of metamorphosis and reproductive development; and neuropeptides influence almost all other aspects of metabolism as well as reproduction and the regulation of juvenile hormone production. They are integral drivers of ‘how insects work’. Entire texts have been written on almost every aspect of insect structure and physiology, and numerous articles in scientific journals and reviews continually present new information and interpretation of their functions and evolution. Part of the story of insect variety is linking structure and function, and appraising how all aspects of insect morphology and metabolism enable the insect to cope with (and capitalise on) its environment, to fit it to develop, disperse, find and use the resources it needs throughout its life, and to regulate its behaviour and lifestyle to persist and cope with changes in that environment in both space and time. The idiosyncrasies of any insect species or group reflect these needs. With a few exceptions of apparent or relative environmental uniformity (such as the flour or grain storage environments of some stored products beetles) insects live in environments that are patchy and variable, and within these they may encounter a range of conditions of humidity and temperature, of food supply, and of other species that may facilitate or oppose their own wellbeing. But, even within a warehouse or sack of flour, conditions change – in aeration, nutritional quality and in the number of individual insects present as populations increase with little initial opposition, so that density of insects may lead to crowding and competition for food and space, and change the interactions between individuals (and species) as more frequent and less easily avoided encounters occur between them. Such situations can induce changes in hormone balance that, in turn, induce behavioural or other changes. Parallels are numerous in more open environments, but there may be a greater variety of ‘escapes’ possible. However, increased density is associated with, for example, changes in

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some normally solitary grasshoppers to induce them to enter a gregarious phase, as ‘locusts’, in which changed behaviour reflects modifications in both hormonal ­balance and sensory responses. Sensory capability to select food, oviposition sites, mates and other necessities, and how and when to disperse are all critical components in an insect’s life. Not all such decisions may be positive, particularly in ­interactions with plants or other animals, from which a wide array of outcomes may be possible, as we see later. Interactions between individuals and different species are mediated largely by sensory mechanisms.

Further Reading The first three references below are to entomology texts of varying complexity, and provide more formal information on insect structure and biology. The next two are amongst several wellillustrated guides to insect recognition and biology in Australia. The last is a major, indispensable, source of information on published knowledge on Australian insects CSIRO (1991) The insects of Australia. Melbourne University Press, Melbourne (The most comprehensive, two-volume, text on Australian insects) Gullan PJ, Cranston PS (2010) The insects. An outline of entomology, 4th edn. Wiley/Blackwell, Oxford (Latest edition of a very successful general entomology text) (Note that various chapters of either of the above are valuable ‘further reading’ to most chapters of the current book) New TR (1996) Name that insect. A guide to the insects of southeastern Australia. Oxford University Press, Melbourne (An introduction to the regional fauna) Brunet B (2010) Australian insects: a natural history. French’s Forest, New Holland Zborowski P, Storey R (2010) A field guide to insects in Australia. French’s Forest, New Holland Daniels G (2004) Bibliography of Australian entomology, 1687–2000 (2 volumes). Privately published, Mt Ommaney

Chapter 2

Fossils and Major Insect Adaptations

Introduction: The Process of Insect Evolution This structural diversity of insects, and the biological variety it reflects, did not develop all at once. Insects as we would recognise them from modern forms have been around for at least 300 million years, as amongst the first major diversifications of arthropods on land. Over this vast period, we can detect several changes and transitions in structure that appear now to have been ‘pivotal events’ in leading to their success and fostering their recent abundance. However, the fossil record from which we infer those changes remains cryptic in places: assembling unambiguous evidence from ancient insect fossils is not always easy, and it is not surprising that uncertainties persist – or that the opinions of various specialists may differ widely over how particular fossils may be interpreted! In this chapter, some background to the information on insect evolution derived from the fossil record is outlined, together with its relevance to study of the insects around us today. The conventional belief is that insects evolved during the Devonian period, about 360 million years ago in the middle of the Palaeozoic era. Until recently, the few relevant fossils available from that early time, or even earlier, are Archaeognatha (from about 380 to 390 million years ago) and the non-insect Collembola. They have no trace of wings. The oldest of these early fossils, from Scotland, are almost 400 million years old and the collembolan Rhyniella praecursor was long believed to be the world’s oldest hexapod. The ‘true insects’ appeared first in the Devonian of North America. Very recently, however, a fossil from the Scotland deposits (Rhyniognatha hirsti) has been reappraised and is now considered a ‘real’ insect, leading to the implication that insects actually originated in the Silurian period and that wings may have evolved considerably earlier than is commonly supposed. The challenge to find supporting fossil evidence for this remains. Modern Archaeognatha are still wingless, and exemplify the insects termed ‘Apterygota’ (non wing-bearing) as the ancestral condition from which other insects arose. They and silverfish are the modern representatives of this truly ancient lineage.

T.R. New, ‘In Considerable Variety’: Introducing the Diversity of Australia’s Insects, DOI 10.1007/978-94-007-1780-0_2, © Springer Science+Business Media B.V. 2011

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Wings and Flight The subsequent development of wings, conventionally presumed to have taken place during the Carboniferous period (extending between about 360 and 285 million years ago), is the most significant single event in insect evolution. Insects are the only winged invertebrates, and the ability to fly probably gave them enormous advantage in escaping from local threats (such as being eaten by ground-dwelling predators) and exploiting new habitats and resources as these arose. Wings remain the most obvious distinguishing feature of recent insects and all but a tiny fraction of modern insect species belong to the ‘winged insects’ (Pterygota: wing-bearing). The Carboniferous, the period during which most modern coal measures were laid down – largely from tree fern vegetation, is widely depicted as moist, with predominantly swampy environments in which these plants flourished. It has been suggested to mark the period when insects moved from the ground to exploit this taller vegetation, and from which the complex suites of interactions between insects and plants have developed. Tall vegetation was present some 30 million years before the Carboniferous, so that the earlier development of wings suggested above might have had similar evolutionary stimulation. However, simply climbing up vegetation poses hazards – falling off, or being dislodged by rough weather, for example, may expose insects to ground-dwelling predators, particularly should they ‘crash land’. One possible way to avoid this, and survive, is through wings. Hypothetically – for there were no human observers to record the process – development of lateral processes on the body might facilitate posture control in our falling insect: if it lands the right way up in a hostile environment, it might be able to run away and hide quickly. If not, it may perish. Should those processes become larger during evolution they might form gliding planes (whereby a dislodged insect might reach another tree fern and avoid the ground altogether, using principles similar to the ‘webbing’ of gliding possums, for example), and from there it is a relatively small (but significant) step to progressive enlargement and eventual hinging of these structures at the base to constitute wings with both directional and postural controls – as true locomotor structures affording aerial capability. The Carboniferous also marks the era of gigantism in the early winged insects. Some fossil dragonflies from that time are the largest insects that have ever existed. They span around 60  cm from wingtip to wingtip, and some mayflies were also much larger than any modern species. It is tempting to suggest that – because there were no other aerial organisms at that time – the exuberance of these forms was facilitated by the innovation of flight occurring in an environment in which there was little risk or competition. Gigantism may also have been facilitated by the high atmospheric oxygen levels during the later Carboniferous and much of the following Permian era, during which oxygen levels reached almost 30% – far higher than they have ever been since then. However attractively simple the above hypothesis on wing origins may be, as based on interpretation of wings and their possible precursors from fossils, two rather different origins for insect wings have been discussed extensively – and each

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15

has its proponents. First, that wings developed as above from the two more posterior thoracic segments as dorsolateral ‘paranotal lobes’, initially as small flanges and then becoming more elaborate as they are enlarged. Second, that structures equivalent to the lateral respiratory gills still found in larvae of some primitive winged insects (the mayflies) may have led to wings – this assumes that the insect may be ‘lifted’ by winds when left exposed and moved passively, with subsequent elaboration of the flaps leading to control. The ‘gill theory’ is now accepted more widely, with the ancestral structure being one of the two branches of the so-called ‘biramous’ arthropod legs still widespread amongst modern crustaceans. With either scenario, however, we may be endorsing an entirely false presumption, that the role of any lateral process was indeed evolved as a wing precursor: this is not certain or perhaps even likely. Simply because wings are used so widely for flight, the need for flight was not necessarily the impetus for initiation of adaptations leading to wing development. Other possible roles of lateral processes, besides being gills, could include camouflage (through colour, ornamentation, or decreasing shadows), defence (by making the insect effectively larger as a refuge from predators, appear more fearsome, or simply tougher), providing extended bases for muscle attachment for locomotion (perhaps aiding sudden movement), or reproduction (elaboration for attracting mates, and perhaps found in one sex only). Each of these has parallels in the forms of modern insects and, as examples, indicate the considerable variety of adaptive advantages any such structure might convey. Once present, however, the structures could form a basis for wings. Some early fossil insects (in the extinct order Palaeodictyoptera) have lateral lobes on all three thoracic segments, but only two pairs of wings have ever developed. These are always on the second segment (mesothorax: fore wings) and third segment (metathorax: hind wings), and the two segments together are often termed the pterothorax (wing-bearing thorax) which, because of need for large flight muscles, can become large. Wings have fundamental importance in tracing evolution of insects and inferring some relationships between orders, because it is almost certain that they have evolved only once – so that all winged insects (the vast majority of those alive today) are derived from a common ancestor, and the enormous variety of wing forms all relate to the same basic pattern. This pattern is consistency of longitudinal struts (veins) radiating along the wing from the base, with the pattern (wing venation) of immense value in classifying insects. The veins form a consistent sequence from the anterior margin of the wing towards the posterior; each vein has a name, with equivalent notational shorthand based on initial letters, as shown in Fig.  2.1. Each, with the pattern consistent in the two pairs of wings, may change in track, branching pattern, and extent of development so that different insects can differ markedly in the numbers and intensity of venation, and differences between fore wing and hind wing. The longitudinal veins are linked transversely by variable numbers of cross-veins, very numerous in insects such as dragonflies, grasshoppers and crickets, many fewer in some advanced insects, but all helping to delimit closed areas (cells) on the wing, also of taxonomic significance. As one example, most members of two major families of parasitoid wasps (p. 86), can be differentiated on the absence (Braconidae)

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Fig. 2.1  Wing pattern and venation. The general pattern and nomenclature of the main longitudinal wing ‘veins’ of an insect, as a series of struts from the base to apex of the wing, and standardised from anterior to posterior; the veins may branch as shown, and are named from the initial letters of each, with branches of the same vein numbered sequentially from the anterior. Any of these veins may be modified or lost and linked by crossveins to produce characteristic patterns of wing venation important in diagnosis and classification of insects

Fig. 2.2  Example of use of wing venation in classification: the two large related families of parasitoid wasps, Ichneumonidae and Braconidae. Most members of these families are separated by the presence (a, Ichneumonidae) or absence (b, Braconidae) of a single fore wing crossvein between veins R and M, furnishing a useful ‘spot character’ for recognition; note the highly reduced venation from the general pattern shown in Fig. 2.1

or presence (Ichneumonidae) of one particular fore wing crossvein (Fig. 2.2). A typical ichneumonid is shown in Fig. 2.3. Even small differences in wing venation may be characteristic diagnostic features for groups or species. The qualifying ‘almost’ in ‘almost certain’ above refers to ambiguity over the independent evolution of the mayflies and dragonflies, as the two most ancient insect orders with functional wings. Many authorities accept that there are in fact two insect lineages with wings, but there is still some uncertainty over whether

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Fig. 2.3  A living ichneumonid wasp to indicate venation outlined in Fig. 2.2a

these are ‘mayflies and (dragonflies and all the rest)’ or ‘dragonflies and (mayflies and all the rest)’. Mayflies (Ephemeroptera) are the only insects in which there are two winged instars (growth stages, p. 23), with a winged ‘subadult’ (subimago) preceding the adult (imago) stage. Whichever association is accepted, it is clear that mayflies and dragonflies are the earliest winged insects that have persisted to the present, albeit as rather smaller forms than their enormous ancestors. Although the term is commonly discarded, reflecting their possible independent origins noted above, they are still commonly referred to as Palaeoptera (‘ancient-winged insects’, to distinguish them from the later ‘Neoptera’). The main unifying feature for this juxtaposition is that their wings cannot be flexed – folded back to rest along the insect’s body when not in use – so that all modern mayflies have the wings held at right angles, upward from the body, and the two suborders of Odonata have the wings held horizontally extended (dragonflies, Epiproctophora, often noted under the earlier less-embracing but more familiar name Anisoptera) or vertical (damselflies, Zygoptera). The flexing is an important change, found in all the more recent groups of winged insects – although lost in butterflies, that is a secondary specialised development. Wing flexing means that the insects can reduce their effective body size, much as many naval aircraft are modified for hangar storage on board by ‘folding their wings’. In locusts and many others, this habit may be accompanied by ‘pleating’ of the hind wing, which is much larger than the forewing, so that it folds fanwise. The adaptation may aid rapid movement on the substrate, as escape from predators and to reach refuges (such as under vegetation, or in cracks in the ground or under bark); the smaller effective size enables insects to crawl into small spaces more easily; wings are not as likely to get damaged on vegetation, and so on – in short, one can postulate advantages for this structural change in increasing chances of the insects surviving.

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Wings and Ecology Numerous subsequent adaptations of wings have taken place – many insects have highly modified structures derived from wings, have lost one or other pair, or have even become secondarily wingless. Or wings may be used for purposes other than flight – for example, a few stoneflies raise their wings as sails enabling them to be blown across water surfaces. Variations within a species are also frequent: many aphids have winged and wingless individuals in a population, either together or in different generations, and some moths have fully winged males and flightless females with rudimentary, stunted wings. As examples of changes to the basic pattern of two pairs of typical wings used for flight, beetles have the fore wings hardened as elytra, to constitute protective ‘shells’ and give Coleoptera their characteristic appearance so that only the hind wings are employed in flight; flies (Diptera, the name meaning ‘two-winged’) rely on fore wings alone for flight, with the hind wings transformed into small gyroscope-like structures (halteres) that aid aerodynamic manouverability; and ectoparasitic insects such as lice and fleas have lost their wings completely. They can rely on their mammal or bird hosts for transport, and do not ‘need to fly’. Reasons for secondary flightlessness in so many insects have been debated extensively. It has evolved many times independently in different groups and, with the exception of ectoparasites, loss of wings is likely to restrict possibilities of long distance dispersal, and it is indeed common in insects living in very restricted but for long suitable habitats (such as caves, on isolated islands, and amongst denizens of social insect colonies) from which dispersal might be very risky. Wings may be lost in only one sex – most commonly the female. In some moths wingless females can attract flying males by pheromones (p. 5), and one evolutionary ­suggestion is that , because they do not have to allocate energy to wing development during growth, that energy is then available for enhanced reproduction such as producing more or larger eggs. Some other insects undergo generations of winged and wingless individuals. Some aphids disperse by flight early in the warmer periods of the year, at the start of a breeding season, and the populations then ‘stabilise’ in a habitat by offspring being wingless, perhaps over several generations, before winged individuals are again produced. Wings can be textured, haired or naked, coloured to facilitate camouflage, display, defence or other purpose. The elaborate colour patterns that render Lepidoptera so attractive to collectors are formed from the covering of flattened scales on the wings, and the pattern may be highly characteristic for particular taxa, but also influenced by temperature and other factors. The well-known phenomenon of ‘industrial melanism’ in the peppered moth (Biston betularia) in Britain has been discussed extensively – with rise of heavy industry, and sooty emissions blackening tree trunks on which the nocturnal moths were purported to rest by day, proportions of black (melanic) moths increased, and the normal paler forms declined in frequency, as they were more conspicuous and vulnerable to bird predators. Somewhat intri­ guingly, in view of so much of Australia’s forest being burned at intervals and the trunks of eucalypts commonly blackened by charring, we have very few black

Insect Diversification

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moths. One southern geometrid moth, Melanodes anthracitaria, however, does have a wholly black form as one of its two colour patterns: as a forest species it may gain some benefit from crypsis, but this has not been investigated experimentally. A few other blackish moths, including some Noctuidae, are not primarily forest species. Wings are the most numerous insect fossils from the older geological periods, from the Carboniferous on. They are tough, decay-resistant, and commonly discarded by consumers, and their essentially two-dimensional structure renders them far easier to study than the more complex fossils of insect bodies – even though interpreting the patterns they show can be much harder. That wings have persisted so long and diversified so widely attests to their contribution to insect success; they are regarded widely as the single major advance that has facilitated insect domination of so many ecosystems. They have been pivotal in demonstrating the increasing diversification of insects from the Carboniferous onward, as global conditions changed. With the rise of gymnosperms during the Permian period, for example, some herbivorous groups proliferated – and the Palaeoptera began to decline from their Carboniferous glory.

Insect Diversification Fossils laid down over the next 150 million years or so demonstrate the changing balance in global insect fauna, and responses to environmental changes – essentially, as new resources arose, insects developed to exploit them – but also that many basic insect types (orders) have actually changed very little in form from those times. Many present-day cockroaches (Blattodea), for example, closely resemble their ancient ancestors in general appearance. Much later, with evolution of flowering plants and warm-blooded vertebrates in the Cretaceous period, the most recent insect orders became distinctive. The nectar-sucking Lepidoptera did not appear in the fossil record until flowering plants were present, and parasites such as fleas could not develop in their modern forms until mammals and birds became available as warm-blooded hosts. Other new resources such as warm dung also fostered specialist insect consumers – and strong suggestion of parallels with some adult beetle and flies using nectar, whilst breeding in dung or vertebrate carrion aiding the ecological divergences between adult and larval stages of the same species. Few such opportunities on land or in freshwater have been ignored during insect evolution. However, a major anomaly arises from this picture – simply, if insects are so successful in these terrestrial and freshwater environments, why are they not also predominant in marine environments? Marine insects do exist, and range from bugs that skate on the water surface in parts of the Pacific (some passing their whole life up to thousands of kilometres from land), to a variety of species on the shorelines – some flies and beetles exploit debris such as seaweeds and dead fish or birds washed up on the strand, for example. But they have indeed not fully exploited the ocean environments. Physiologically, some are certainly capable of doing so – insects regularly exploit highly saline waters elsewhere (p. 139) – and the most

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likely suggestions reflect that insects arose on land, from immediate terrestrial ancestors, at a time when the land environment was not overly crowded and where they could establish their evolutionary claims without undue competition. However, at the same times, other arthropods were proliferating in the seas – so that land might have been the ‘easier option’, whilst possible forays into marine systems were thwarted by the organisms already present. Oceans were perhaps just too difficult for insects to exploit, but whether this was because of competition or the complex physiological demands of respiration and osmoregulation – or both of these – must remain speculative. Some entomologists suggest that simply the physical problems of coastal wave actions, turbulence and tides may effectively prevent land insects invading the seas. Most marine insects occupy coastal habitats such as intertidal zones, salt marshes, mangrove swamps or others that may be regarded as intermediate between sea and land. One caddisfly, Philanisus plebeius, representing an order in which larvae are most typically found in fresh water, lives in rockpools in south east Australia and New Zealand. Some insect orders are known only from fossils, and have clearly become extinct as they were replaced by ever more efficient forms adapted to more recent conditions. The older insect fossils, from which most of this long-term story is inferred, are shale impressions in two dimensions, and the amount of detailed information derived from them varies considerably, depending on the individual condition and orientation of structures. Much later, the record was augmented by a second category of fossils, those in amber, most commonly from the Baltic region and with insect inclusions highly desirable for jewelry. Amber is the fossilised resin of conifers, in which insects were trapped soon after it was exuded and glue-like – modern functional equivalents are common in Australia, where the ‘gum’ oozing from wounded or stressed acacias or gum trees often captures visiting insects attracted by scent, by the promise of a meal by feeding on insects already trapped, or simply blundering in. More resin (sometimes referred to as ‘sap’, but in reality somewhat different in origin) eventually covers the insect, to enclose it, in course hardening and persisting indefinitely. Amber fossils are thus whole insects, preserved in manner equivalent to mounting in gum on a microscope slide and available for very detailed examination. Some are up to about 100 million years old, from the Cretaceous period. Amber from different periods and widely separated locations gives us snapshots of the insect assemblages present. Older Cretaceous deposits include those from Canada and Lebanon. The Baltic amber is much more recent, ‘only’ 40–50 million years old. Particularly for smaller insects (essentially those not available in the sedimentary fossil record, from which most specimens detected are relatively large) very fine details can be discerned, to the extent of being able to measure individual hairs and leg parts after careful polishing and grinding of the amber bead to provide clear viewing from a variety of angles. In general, Cretaceous amber insects are all referable to extant orders, and most to families and genera that can be related easily to modern insects and, in many cases are identical to these. Baltic amber insects are even closer to modern forms. Many, indeed, are apparently of the same species as insects found today, although usually far from northern Europe. Amber fossils also

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reveal the long term stability and ancient nature of some intricate ecological relationships – for example, they include a number of tiny wasps likely to share the habit of their modern relatives of developing inside other insects, and we can sometimes even postulate what kind of insects their hosts may have been. Aggregations of fossils can allow at least tentative ecological interpretations. Australian insect fossils have contributed to this framework – several hundred species, spanning about 20 orders, have been described, and several classic fossil localities are well known to palaeontologists. The earliest undoubted insect fossil from Australia is a palaeodictyopteran from an Upper Carboniferous site in Tasmania. The Permian deposits in New South Wales (notably from Belmont and Warner’s Bay) yielded a considerable variety of taxa, very distinct from those of the northern hemisphere. Perhaps the two most fruitful insect fossil sites in the country are (1) the Ipswich Triassic series in Queensland, with more than a thousand specimens, some (such as the oldest known aphids) especially notable, and (2) the freshwater lake deposit from the lower Cretaceous at Koonwarra, Victoria, discovered during road construction activities. The latter gained fame also as yielding the first bird feathers from Australia, in conjunction with the earliest known fleas. It includes more than 80 species, in 12 orders, and is notable for the array of aquatic larval forms represented. More recent Tertiary fossils are known from Queensland and New South Wales. The most exciting insect fossil discovery in Australia in recent years (first publicised in 2006) has been that of amber on beach strand washes in northern Queensland, with subsequent findings also elsewhere in the country, and the first knowledge of amber being present in the country. Australian amber is thought to be about 15 million years old, on initial estimation, and derived from kauri trees (Agathis, still found in rainforests of Queensland). Little has so far been published on its insect inclusions, but they are considerable and diverse, with flies, termites, ant, wasps, beetles and others signaled, together with feathers, hairs, and a wealth of other fossils. The first formal description of an Australian amber insect, of a dolichopodid fly in 2009, is surely the precursor of many other exciting discoveries from this amber to help understanding the origins of Australia’s insects. So far we have seen three of the major features of insect evolution, in conjunction with changing representation in the fossil record: development of wings, the basal articulation of wings with the thorax changing to allow wing flexing, and the progressive diversification of feeding habits and mouthpart structure. The fourth critical feature is the development of different developmental patterns within the life cycle, to achieve a ‘complete metamorphosis’. This is outlined next, but for completeness here, the development by some insects of a true social existence (Chap. 7), whereby some aspects of environmental variation may be buffered or countered, has also frequently been noted as a further factor contributing to their success. ‘Success’ in insects can be considered superlative in both evolutionary and ecological terms – reflecting long persistence of early origins and well-established lineages, many of which have generated vast numbers of species but within which the basic pattern has essentially changed little. The second category also reflects that some insect groups have gained predominant ‘key’ roles in ecosystems. The social insects

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(Chap. 7) could be considered amongst the paramount evolutionary successes of insects. Only modestly rich in species compared to many far more diverse groups, termites (globally with around 3,000 described species), have ecological importance far beyond this tiny proportion of insect richness, as the major consumers of cellulose in dead plant biomass. The ubiquity and ecological importance of ants also confers major influences in many terrestrial ecosystems, both in Australia and elsewhere. But these two social insect groups have very different developmental pathways, and understanding the major life history patterns of insects is a critical aspect of interpreting insect evolution and diversity.

Further Reading Bickel DJ (2009) The first species described from Cape York amber, Australia: Chaetopogonopteron bethnorrisae n.sp. (Diptera: Dolichopodidae). In Berning B, Podenas S (eds.) Amber – archive of deep time. pp 35–39. Denisia 26 Grimaldi D, Engel MS (2005) Evolution of the insects. Cambridge University Press, Cambridge (comprehensive, fully-illustrated, recent survey of the insect fossil record and the features and evolution of Recent insects) Jell PA, Roberts J (eds) (1986) Plants and invertebrates from the Lower Cretaceous Koonwarra fossil bed, South Gippsland, Victoria. Association of Australasian Palaeontologists, Sydney (comprehensive, well-illustrated descriptions and appraisal of a major insect fossil deposit in southern Australia) Poinar GO (1993) Insects in amber. Annu Rev Entomol 46:145–159 (overview of information then available) Wootton RJ (2001) How insect wings evolved. In: Woiwod IP, Reynolds DR, Thomas CD (eds.) Insect movement: mechanisms and consequences. CABI Publishing, Wallingford, pp 43–64

Chapter 3

Insect Life Histories

Introduction: Modes of Development In common with other arthropods, being bounded by the confines of a hard external shell means that insects cannot grow continuously but must undergo a series of moults in order to increase in size. The insect body inside separates from the exoskeleton, which is then split and shed, leaving a larger new and initially soft exoskeleton for the insect to expand into. Successive moults separate different stages (stadia, instars) that approach the adult stage with reproductive capability. Along this path, the body increases in size and may change in appearance in various ways. Most commonly, an insect life cycle starts from an egg, hatching to a larva (initially a ‘first instar’ that eventually moults to a ‘second instar’ and so on in a sequence that can be numbered to denote the individual stage). Eventually the adult stage is reached and in most insects no further moults occur and the cycle is completed as adults mate and reproduce. However, the changes between egg and adult differ markedly in extent in different insects and define several widely adopted patterns of insect life cycle. The simplest pattern is that found in the primitive Apterygota which, by definition, do not develop wings. A hatchling silverfish resembles its parent in most details – it is simply much smaller, and cannot reproduce. At each successive moult, it becomes larger but changes very little in appearance – but an adult silverfish continues to moult at intervals throughout its life. Presence of wings in more advanced insects constrains this – in part reflecting that fully developed wings are large delicate structures that can be damaged easily by moulting and may render the insect especially vulnerable whilst doing so – so that (with the single exception of the primitive mayflies, amongst the ancestral forms of winged insects, p. 17) insects with fully developed functional wings do not moult. The more primitive orders of Neoptera develop wings gradually, on the outside of the body. The field cricket egg hatches to a wingless active miniature cricket, feeding in the same way as the adult. After the second or third moult, the appearance changes somewhat, with development of small posterior lobes, a pair each on the

T.R. New, ‘In Considerable Variety’: Introducing the Diversity of Australia’s Insects, DOI 10.1007/978-94-007-1780-0_3, © Springer Science+Business Media B.V. 2011

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upper hind angle of the mesothorax and metathorax. These lobes (‘wing buds’) enlarge with each successive moult and at the final moult are transformed to proper wings by becoming hinged at the base. Crickets thereby show a progressive stepwise transition from wingless to winged form, with the different instars broadly similar in their life style and occurring in the same habitats. This can be contrasted with the far more dramatic changes developed in the more advanced insects, in which the appearance of the larva and adult differ greatly – consider a caterpillar and a moth, or a maggot and a fly, for example. In these, the larva and corresponding adult occur in different environments and have very different feeding habits and biology. A typical caterpillar is a chewing plant-feeder, the adult a nectar-feeding butterfly or moth exploiting this food by means of a long sucking proboscis. Further, the caterpillar or maggot has no external trace of wings. The two phases of larva and adult appear as discrete biological entities, and the transition between these represents one of the major adaptive advances in insect evolution. This transformation results from establishment in the life cycle of the pupa, derived from the last instar larva as a stage that (1) does not feed, and so allows the insect to survive periods when food is not available, (2) in consequence provides a refuge to survive seasons that may not be suitable for continued development because of being too cold, too hot or from other environmental factors, and (3) allows for the larval body to be broken down whilst enclosed in a protective case, and reorganised into the adult form (including formation of external wings) before the latter resumes development in the outer world. Thus, a caterpillar ‘pupates’, and a moth or butterfly subsequently emerges from the pupa (Fig. 3.1). But one evolutionary principle is important to clarify here – whereas a larval silverfish or cricket has always resembled the corresponding adult insect in general form, a caterpillar has never resembled a moth, nor a maggot resembled a fly. The pupa stage facilitates generating changes in both ‘directions’ – so that, simplistically, both the caterpillar and the moth have been derived from the same basic pattern but diverged as they exploit different lifestyles, so that their various distinguishing characters have largely been acquired independently. From this, we have three broad patterns of insect development, in summary: 1 . Silverfish and their allies: little or no change in form as they grow. 2. Crickets and many others: gradual change of form, with development of wings as projections on the outside of the body. 3. Most more advanced insects: considerable change of form from larva to adult, with an intermediate pupa stage and internal development of wings. Post-embryonic change of body form is termed ‘metamorphosis’, and the above categories therefore show different kinds of metamorphosis. With no clear change of form, Apterygota are termed ‘ametabolous’; external development of wings, literally ‘exopterygote’, is referred to as an incomplete metamorphosis, more formally the insects are ‘hemimetabolous’; and the pupa results in internal wing development (‘endopterygote’) with a complete metamorphosis, and the insects designated as ‘holometabolous’. Holometabolous insects are by far the predominant group, with close to 90% of living insects showing this pattern, essentially of having two different ways of life

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Fig. 3.1  Emergence from the pupa and expansion of wings is a vulnerable phase in the life cycle of many insects. This female common brown butterfly (Heteronympha merope, Nymphalidae) is completing this phase whilst hanging from vegetation and with the mottled brown underside possibly aiding its camouflage

within the same species. But, as we would expect, some complications occur, with needs to operate in different milieux leading to other divergences between adult and larval forms. The primitive Palaeoptera arose long before any complete metamorphosis – yet the transition between aquatic larvae and terrestrial adults, with accompanying changes in feeding habits and other ecological features also results in very different life forms without the intervention of a pupa. The final instar larva (having developed external wingbuds as in early terrestrial winged insects) leaves the water and moults directly to the land-based adult stage. A similar pattern of highly derived larval forms for aquatic life is found also in the stoneflies (Plecoptera). All of these larval forms face the same problems – they need gills for aquatic respiration, for example – but their transitions do not represent holometabolous development. The larvae of mayflies and dragonflies are sometimes referred to as ‘naiads’ (water nymphs), and all larvae of exopterygote insects are sometimes termed ‘nymphs’.

Diversification Within Metamorphosis In later chapters we will see many individual departures from the generalities implied by these patterns – emphasising the difficulties of attempting any sweeping statements to encompass insect variety! Not all insects have an egg stage,

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Fig. 3.2  The egg-case (ootheca) of a large mantis, Archimantis latistylus (Mantodea), is formed from proteinaceous foam exuded by the female, that hardens to enclose her eggs as they are laid, and is attached to vegetation. The eggs are protected from desiccation and some attack by predators and parasitoids, but can still be attacked heavily by small wasps (Podagrion), whose females have a long slender ovipositor that can reach the eggs through the foam

for example, as some are viviparous. Many aphids, some flies and others (so both exopterygote and endopterygote insects may be represented) produce active larvae as the first stage and the egg retained inside the parent’s body to (1) shorten the life cycle – so that aphids can undergo generations in very short periods and ‘capitalise’ on suitable environments by building up numbers quickly and (2) increasing precision, so female parasitic flies can deposit individual larvae, as an active feeding stage, directly onto a suitable host to exploit it without delay and avoiding possibly hazardous exposure. And insects may lay from few to many offspring, and either leave these exposed, or protect them in some way from harm (Fig. 3.2). The numbers of larval instars also differ substantially in different insect groups. From an indefinite number with silverfish, many Palaeoptera undergo 20–25 larval instars but this number can decrease, characteristically to only 3–5 in many holo­ metabolous insects. The precise number may be influenced by food supply and other environmental conditions, together with the length of the developmental period. Larvae of holometabolous insects differ greatly in appearance across different orders and, because of their extensive adaptive characters, they may be just as diagnostic as the corresponding adult stages. However, most Australian insects have been described and named from the adult alone (many from only one sex, so that even associating males and females correctly may be difficult in interpreting samples or survey results) and only a small proportion have been associated clearly with larval stages, by rearing from them. If we are confronted with a beetle larva or caterpillar from a field collection, we may only be able to infer its identity from

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what we know of identified adults found in the vicinity, with that knowledge usually highly incomplete. In contrast, the extensive differentiation between larvae and adults of exopterygote aquatic insects sometimes renders the immature stages better studied than adults: they are often abundant and easy to collect, many are long-lived, and are important components of freshwater fauna. Larvae of Ephemeroptera, Odonata, Plecoptera and holometabolous Trichoptera (caddisflies) have all been studied well because of their practical importance in helping to assess water health. Water quality can sometimes be appraised by looking at the number of species and relative abundance of these insect groups. In North America, the ‘EPT Index’ (for ‘Ephemeroptera, Plecoptera, Trichoptera’) is a commonly used measure and, in general, larvae of these orders may be amongst the better known groups of immature insects. Three very broad categories of endopterygote larvae have been a useful framework in helping to describe their variety, and each of these reflects capability and needs of a particular way of life – primarily the extent of mobility and features of the environment in which they must feed. Historically, these categories have been likened to different embryonic stages, in relation to a sequence of development and later suppression of the paired limbs on each body segment, and the names themselves are derived from embryological terms use to denote stages of development. The parallel is simplistic because the ‘simplest’ larval form is in fact a highly specialised derivative, rather than a primarily early stage, within the most advanced of all insect orders, Hymenoptera. This, the ‘protopod’ larva, appears rather simple in structure, as if at an early stage of development. The body may be incompletely segmented with the abdomen sac-like and sometimes scarcely distinct from the thorax, no thoracic limbs are present, the head lacks eyes and antennae, and only the mouthparts are well-developed. At first impression, this seems an odd and seemingly incapable animal, but it is one whose features are in fact highly adapted to a special way of life. It is typically the offspring of some parasitic (strictly, parasitoid, p. 85) wasps that lay their eggs inside a host (such as a caterpillar) in which the whole larval development takes place. The larva is thus bathed in abundant food (caterpillar body fluids and tissues), does not need to move around much or to hold on in its small operating universe, has no need of vision or other major sensory systems – it simply persists and feeds in an environment that is sheltered, nutritious and largely predictable. The countering possible disadvantage is that it cannot escape – if anything happens to the host (for example, by it being eaten by another arthropod or a bird, or killed by a virus or other pathogen), the internal parasitic wasp is assuredly also lost, and the precise placement by the parent female may then be futile. One evolutionary way to, in turn, counter this eventuality is for the wasp to invest rather little energy in each individual offspring but compensate by producing large numbers, distributed amongst many different individual hosts, some of which may survive. Many such wasps are highly dispersive and search efficiently for hosts – and energy spent in searching activity is then not available for reproduction; but with the sheltered environment, small protopod larvae can be produced at this early stage with their necessary capabilities assured. A related feature of some parasitic wasps is ‘polyembryony’ whereby a single egg or embryo deposited in a

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Fig. 3.3  Caterpillar of the vine moth (Phalaenoides glycine) (Noctuidae, Agaristinae), experimentally attacked by two polyembryonic wasp parasitoids at different times. Each single oviposition has led to proliferation of larvae, as exposed here by pricking the caterpillar skin, so that high numbers of offspring are produced with minimal expenditure by the parent wasp

host divides to produce large numbers of offspring (Fig. 3.3) – so that a host that survives until the parasitoids have developed can supply a whole cohort of wasps, perhaps of several hundred individuals, to the next generation (Fig.  3.4): a very effective counter to some other hosts being lost. Surviving caterpillars eventully proceed to adulthood (Fig. 3.5). The second of the major categories of larvae (Fig. 3.6a) is associated with living on vegetation, with needs to feed by chewing, to move around and to be able to hang on during wet and windy weather – with some exceptions due to mobility, a caterpillar dislodged from a tall tree is doomed just as surely as a wasp inside an eaten caterpillar! These larvae are termed ‘polypod’ (many-legged). They have a well-developed head, with chewing mouthparts and a group of simple eyes replacing the normal adult compound eyes, and the usual insect pattern of three pairs of thoracic legs – but, in addition, they have a variable number of fleshy lobelike ‘prolegs’ on the abdomen, and a pair of these constituting posterior ‘claspers’. The prolegs are used to grip the substrate, such as to clamp the edges of leaves, and their broad surface can have circlets of small hooks or spines, as a natural-world precursor to some human ‘fastener’ inventions, such as ‘velcro’®. Lepidoptera and plant – feeding sawflies (Hymenoptera: the black or dark green ‘spitfire’ grubs found on eucalypts, sometimes in large numbers or dense clumps, are familiar to many people: Fig. 3.7) exemplify this category. Finally, but with much more extensive diversification from its basic form, the ‘oligopod’ larva (Fig. 3.6b–d) fits the typical insect pattern of three body regions, primarily chewing mouthparts, and limbs restricted to the thorax. As in polypods,

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Fig. 3.4  Caterpillar of the vine moth, above, from which fully grown parasitoid larvae have emerged, each to spin a small silken cocoon in which to pupate

Fig. 3.5  The adult Phalaenoides glycine is a colourful, day-flying moth with aposematic colouring, perhaps deterring vertebrate predators from attacking it. It is an important pest of vines in southern Australia

eyes are simple (other than in scorpionflies, Mecoptera, in which compound eyes are retained) and comprise a small group of individual stemmata, commonly six or seven in number. The variety of oligopods can be demonstrated by contrasting two forms of beetle larvae, representing very different ways of life. Active predatory

Fig. 3.6  Representative larvae of holometabolous insects to indicate some broad functional categories. (a) polypod larva of Lepidoptera with normal thoracic legs, several pairs of abdominal prologs, and posterior claspers; (b) oligopod larva, a typical beetle to indicate basic pattern of thoracic legs and no prolegs; (c) modification of this as active predatory ‘campodeiform’ larva as in ground beetles – head orientated forward so mouthparts anterior, legs long, and body streamlined; (d) a contrasting sedentary ‘scarabaeiform’ beetle larva adapted for an inactive life style spent underground and feeding on plant roots; (e) a ‘maggot’ of higher Diptera, lacking limbs or a distinct head, and with mouthparts as small hooks retracted into thorax, lateral spiracles absent and large posterior spiracle present

Fig. 3.7  Representative larvae of polypod, the ‘spitfire larvae’ of a pergid sawfly on Eucalyptus

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Fig. 3.8  Scarabaeiform larva of a stag beetle, Lucanidae

beetle larvae such as those of many rove beetles and ground beetles chase their prey, detect them by sight, and capture them with their mandibles. They tend to have slender bodies, raised above the ground on long legs, used for running, and the head axis ‘tilted’ so that jaws are orientated forward and encounter prey organisms in front of the insect, rather than underneath it. Pasture scarab beetles differ greatly from this appearance and have a very characteristic body form (giving them a common name of ‘curl grubs’). The head is small, with mouthparts in the usual ventral position, thoracic legs are short, and the abdomen is greatly enlarged and ‘curled’. They live underground, feeding on plant roots, so do not need to move fast, but must accommodate and process considerable volumes of food material (Fig. 3.8). These two contrasting kinds of larvae retain the full oligopod complement of features. But, in response to exploiting different feeding habits and habitats, many other oligopod larvae have undergone considerable modification, with the major trends including loss of limbs and heads to varying extents. A bushfly maggot (Fig. 3.6e) living in dung has no distinct head, and its mouthparts are reduced to small hooks retracted into the front of the body; likewise it has no legs. Larvae of some timber-boring beetles retain the head, needing strong mandibles to chew open their tunnels and fragment the wood on which they feed: again, they lack legs but may have various processes on the body, these aiding purchase on the sides of their excavations. Many such variations occur. As we might expect, insect pupae also provide similar structural variety related to adaptive need. They can be long-lasting, providing a refuge for the insect over much of the year, and can thus be vulnerable to attack or loss. Several trends toward apparent protection occur, sometimes in conjunction – crypsis (camouflage if exposed, or formed in less exposed sites such as under bark or in holes in the ground), toughening (with the outer case hard, ornamented or ‘polished’ to resist bites), or a protective covering (cocoon). The large pupae of the gum emperor moths

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Fig. 3.9  Representative pupae: some basic forms. (a) decticous exarate pupa (e.g. Megaloptera) with limbs and jaws free; (b) adecticous obtect pupa, with all appendages sealed to the body, and usually with a toughened outer covering (e.g. Lepidoptera); (c) adecticous exarate pupa, with legs and antennae free (e.g. some Hymenoptera)

(Opidiphthera) are enclosed in a hard egg-shaped cocoon, constructed of bark fragments and silk produced by the caterpillar before it pupates, and this is a very tough structure formed usually on or under eucalypt tree bark. Some other moths incorporate stinging body hairs into a more delicate silken cocoon. However, becoming captive in this way poses the problem of emerging later; being enclosed in a barrier sufficient to repel intruders also necessitates being able to break through it from inside. The more primitive endopterygotes such as Neuroptera have pupae with functional mandibles and some mobile legs – they are able to bite their way out of a cocoon, or move to more exposed areas before moulting to the adult. This mobility is lost in more advanced orders, in many of which the jaws are non-functional and all limbs are sealed to the body surface – the basic pupal forms (Fig. 3.9) are thereby termed (1) decticous (jaw-bearing) and adecticous (without jaws) with the second divided into ‘obtect’ (all limbs cemented to the body and confined within the pupal case) and ‘exarate’ (with some use of legs to aid limited movement). The adecticous pupae commonly have spines and processes on the body, to help the emerging (pharate) adult push out of the pupa case, sometimes through predetermined lines of weakness, or through ‘one way funnels’ in the pupal case or cocoon.

Seasonal Development The various growth stages integrate into sequences of development that fit any insect species for its environment, and may have strongly seasonal patterns that reflect the stability and predictability of the resources needed, and the physical environment in

Seasonal Development

33

which these occur. Some environments vary much more than others, of course. Domestic environments – insides of houses, warehouses or other food storage areas may be well-buffered against climatic variations, so that conditions vary little throughout the year. Insects living there may be able to breed year-round. At least part of the reason why some beetles found in stored grains or termites in buildings gain pest status is simply that the enclosed conditions allow them to build up vast populations continuously, unhampered by climate or food shortage. Most insects living out-of-doors cannot do this. Seasonal timing of insect development has two main constraints, both intuitively obvious but which are fundamental considerations in the evolution of life history patterns. First, feeding stages (larvae, adults – the latter with some exceptions that do not feed) must occur at times of the year when suitable food is available. Second, particular seasons may be too cold or too hot to permit development, so that ‘resting’ (non-feeding, inactive) stages may be needed to overcome them. The times optimal for development may be short – perhaps only a few months in spring and summer when temperatures are warm enough and a good supply of food is present – and in the most clearcut cases, the rest of the year may be too cold and food in short supply or of low quality. Insects have collectively evolved two major strategies to help them cope with this variation. They can take refuge in space, or in time. The first necessitates movement, most commonly migration by flight to track resources in the landscape – as one area becomes unsuitable, another is exploited. The second involves some form of dormancy, with the insects in non-feeding stages or protected sites where development ceases (or is reduced markedly) until conditions improve. The two can be combined, because migration only ‘works’ in tracking resources as long as physical conditions within the dispersal range allow continued feeding or if the insect does not need to feed whilst migrating. Movement may also be made to dormancy sites. The Bogong moth (Agrotis infusa) is one of Australia’s most iconic migrant insects. It breeds in lowland areas of the southeast, where caterpillars eat grasses, cereal crops, and similar vegetation. However, as conditions warm in early summer the moths migrate to higher ground and spend the summer sheltering gregariously in caves or under rocky overhangs in the Australian Alps in a dormant state of ‘aestivation’ (the summer equivalent to winter hibernation). Their migrations (p. 150) involve many thousands of moths, and cause comment in most years – vast numbers of moths are attracted to the lights of evening sporting events, and a parliamentary report in 2006 discussed methods of preventing the nuisance they create by entering Parliament House in Canberra (for which the recommendations included closing doors and turning off lights late at night!). In autumn, as conditions become cooler, moths return to lowland areas and resume breeding on fresh growth of the food plants. This migration has other ecological interests but contrasts with that of many other insects in which winter refuge is needed. Inclement periods are most commonly passed in non-feeding stages, so that the Bogong moth (in which the adult becomes dormant) is relatively unusual – overwintering eggs or pupae are much more commonly adopted, with each produ­ cing feeding stages as conditions again become suitable. This broad dormancy is

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undertaken in many insects by the particular situation known as diapause. The term was coined (in the 1890s) to describe a period of embryonic pause within the egg, but is now used to denote arrested development in any life stage. Within different insects, it may be obligatory (the most common condition, affording a consistent feature in the life cycle) or facultative. The seasonal regulation of diapause, both its onset and termination, necessitates some recurrent or cyclic environmental cues as ‘triggers’. The most predictable of all these factors is photoperiod (day length), commonly with temperature also involved and mediated through changing hormonal balances within the insects. In temperate regions such as southern Australia, day length varies considerably throughout the year – between the shortest day in June it increases to the longest at the summer solstice in December, and thereafter decreases again so that day length varies in a wholly predictable cyclic pattern. Insects may cue in to these changes to ‘anticipate’ (in evolutionary terms) when living conditions become adverse and subsequently suitable. Thus, many insects active during the summer period respond to shortening days to enter diapause at or near a particular day length threshold in autumn, so that their refuge is to reach a safe stage in their life cycle by that date. In spring, those same insects respond to increasing daylength to resume development: eggs and pupae hatch (breaking developmental diapause), diapausing reproductively immature adults mature and reproduce (breaking reproductive diapause), and so on. The strategy is then to exploit the suitable environment as effectively as possible before conditions again degenerate. The role of diapause is to maximise survival in circumstances in which seasonal conditions do not permit normal development to continue. Whereas photoperiod is a very common regulator, reflecting its predictability, temperature is also important in modifying the insect’s response to critical photoperiods – high temperatures may prevent or delay diapause, and cooler temperature hasten its onset. Likewise a minimum temperature, even a cold snap, may be needed to force ending of diapause. Dietary and genetic influences may also occur. Broadly, diapause is associated with (1) reduced metabolism and development, (2) reduced activity and (3) resisting environmental conditions that are unsuitable for the insect. With obligatory diapause, the most common syndrome, a very well-defined seasonal pattern can be established, particularly when only one generation each year is undertaken. Greater developmental flexibility occurs with more numerous generations, because conditions within the favourable period may affect these in different ways. The adoption of holometabolous development, allowing insects massive opportunity to diversify (even within individual species), vies with evolution of wings as one of the most critical phases of their evolution. Its importance is indicated by all of the ‘hyperdiverse’ orders exhibiting this pattern. The major lineages of Recent insects can be summarised as follows, and the constituent orders are listed in Table A.1 (p. 224), simply to introduce them: some will be very familiar to most readers as components of ‘nature’ that impinge on our daily lives in various ways – others will probably be less familiar, but also just as important in the roles they pursue. 1. Apterygota Primitive wingless insects with ametabolous development, and continued moulting as adults.

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2. Pterygota (all winged insects) Palaeoptera (‘ancient-winged insects’). The most primitive winged insects, lacking ability to flex wings; exopterygote but with strong secondary specialisations for aquatic larval life. Neoptera (‘new-winged insects’). Can flex wings. Polyneoptera. The most primitive series of orders; chewing mouthparts, exopterygote. Paraneoptera. Based on sucking mouthparts for liquid food, and derivations from these, exopterygote. Oligoneoptera. Most advanced insects, very variable orders, united by being endopterygote. Of these series, Apterygota are a tiny remnant fraction of modern insects, and Palaeoptera have decreased markedly since their Carboniferous predominance. The three series of Neoptera are noted here: Polyneoptera are the first-evolved, and include those insects most similar to the basic pattern, such as crickets; almost all are terrestrial. Paraneoptera are dominated by the sucking bugs (Hemiptera) and apparently arose first as plants with sap became available. Later evolution of a complete metamorphosis allowed the Oligoneoptera to diversify immensely and become by far the predominant modern insects. Several of the extant orders of insects do not even occur in Australia, and these are noted here to aid clearer focus on those that are present. As noted earlier, Mantophasmatodea is the most recently diagnosed insect order, discovered alive only in 2002, and testament to the novelties that may still await discovery. They are a curious intermediate group within the Polyneoptera, native to southern and eastern Africa where they occur in a variety of humid to arid semidesert habitats, as predators. They seem to be closely related to another group absent from Australia, the Grylloblattodea (ice crawlers, rock crawlers), small cricket-like organisms confined to the temperate northern hemisphere zones – indeed, as noted earlier, there is now some consensus amongst entomologists that these should be treated as the same order, under the name Notoptera. Grylloblattids are most commonly scavengers, and have been reported from caves and along the edges of retreating snowlines feeding on insect carrion as it becomes exposed. The other major absence from Australia is the Raphidioptera, snakeflies (named for their elongated thorax), related closely to Neuroptera amongst primitive holometabolous insects. They are predo­ minantly northern hemisphere insects, feeding on small arthropods on vegetation and larvae commonly living under bark, but extend into south eastern Asia as far as Thailand. Finally, Zoraptera are included here but in a slightly different context. They are unknown from the Australian mainland and immediate region but a species has been described from Christmas Island – politically Australian, but biogeographically a part of Indonesia. These small Polyneoptera occur in leaf litter and under bark, and have been reported also from New Guinea, so it is still possible that they might be found in the forests of Cape York Peninsula. The other orders listed all occur in Australia, but to varying extents and reflecting different origins and distribution patterns. Some background to these themes aids

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understanding their ecology, as well as characterising the peculiarities of Australia’s insects a little further. A brief systematic synopsis of Australia’s insect orders is included in an Appendix to this book.

Further Reading (The texts listed under Chap. 1 are all useful adjuncts to this chapter) Tauber MJ, Tauber CA, Masaki S (1986) Seasonal adaptations of insects. Oxford University Press, New York (comprehensive synthesis of early work on diapause and seasonal patterns of life histories in insects)

Chapter 4

Origins, Distributions and Diversity

Introduction: Australia as an Environment for Insects Unlike many other parts of the world, we still have the privilege of being able to evaluate and study much of Australia’s insect life under reasonably natural conditions. Despite the extensive and severe changes to Australian environments over the rather short period since European settlement, and continuing human pressures on natural ecosystems, many of them – at least in remnant form – still maintain sufficient naturalness to support understanding of patterns of insect distribution and evolution, species richness, endemism, genetic and ecological variety within species, and other fundamental aspects of ‘diversity’. Changes to Australia’s natural ecosystems have almost certainly already led to the demise of many insects and other invertebrate species, so that there is some urgency about how we may be able to proceed on such study within the island continent. Without increased commitment to conserving Australia’s remaining natural environments, and enhanced resources to survey and study our biota, the opportunity to do so will be lost. It can never be regained. The array of insect orders in Australia differ enormously in their size, variety, distributions, and relationships with insects in other parts of the world – but a predominant feature of almost any group is that most species do not occur naturally outside the Australian region, so are endemic. This area has long been recognised as one of Earth’s major biogeographical regions but, reflecting the size and climatic variety of Australia, most native insects do not occur throughout the country. They are restricted by environmental factors and dispersal prowess, as well as by their geographical origins – and the last helps to delimit a number of ‘faunal elements’ that are highly characteristic of parts of the continent. However, interpreting the patterns of insect distribution can really only be accomplished if we know something of their relationships – so that we can infer the ways in which the species of – say – a particular family of beetles or moths have differentiated by looking at character changes as gradual transitions that may show taxonomic relationships, and also know how the group concerned reached or has evolved in Australia.

T.R. New, ‘In Considerable Variety’: Introducing the Diversity of Australia’s Insects, DOI 10.1007/978-94-007-1780-0_4, © Springer Science+Business Media B.V. 2011

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Sound biogeography includes considerations of both dispersal and of evolution, and what influences these. Modern Australia is thought of as an isolated island but although it now has no contiguous boundaries with any other place, this has not always been the case. To the north, Torres Strait has existed only for about 8,000 years, to separate Australia from New Guinea, and Australia itself is a fragment of the former great southern supercontinent Gondwana. This broke up progressively since the Jurassic period (somewhat more than 150 million years ago), but with the fragment comprising Australia, Antarctica and South America dividing later – so that Australia became isolated ‘only’ about 40 million years ago (Fig.  4.1). The Australian continental plate continued to drift northward through more than 20° of latitude, meeting south east Asia with its very different origins and biotic remnants. Origins of the Oriental realm fauna are also complex, and biogeographers continue to debate the boundaries between this and Australasia through the complex region of the Indonesian archipelago. Over this period, Australia bore with it parts of the biota shared with other components of Gondwana, and subsequently related to those developing independently on the other ‘southern continents’ having the same ancient origin. These insects thereby have close affinities with taxa in temperate South America, southern Africa or New Zealand, and are ancient lineages largely restricted to – usually – some of these areas. Many freshwater insects, several groups confined largely to cooler streams in the south east, fall into this category. As Australia approached other more northerly places by continental drift, elements of the insect fauna of south eastern Asia were able to disperse to meet it, so that much of the northern fauna now has strong relationships with these taxa, which are often very easily distinguished from more southern forms. Some, however, indeed extend to the south east: a number of dragonflies, mostly strong-flying insects, appear to have originated in the Oriental region, having progressively migrated from ancestral stocks in the Indonesian archipelago or beyond. Many insects in the tropical north are closely related to those of Indonesia and New Guinea – the islands of Torres Strait form a series of ‘stepping stones’ for dispersal of organisms in both directions, so that Cape York Peninsula is a region of faunal interdigitation. As with Gondwanan fauna, ­elements from the north and northwest have undergone extensive differentiation in the new environments – but northern invasions have continued over long periods, with some separation of older and more recent elements, reflected in the extent of distributions within Australia and the degree of taxonomic radiation and diversification they show. Simplistically, several ‘waves’ of invaders can be inferred from the extent to which they have diversified since arrival, but the process is clearly continuous. Two other components of the insect fauna are important to note – one that is understood quite well and one that is decidedly puzzling. The first, with major influences on Australia’s ecology and economy are the ‘recent insects’, those that have joined the fauna since European settlement and widely termed ‘alien’ or ‘exotic’. These have either arrived naturally or been brought (deliberately or unintentionally) by people – an activity that still occurs, with the vigilance of quarantine authorities helping to prevent arrivals of many further species at our ports and airports every year. Representatives of this category are the best-known insects to many people, and some

Introduction: Australia as an Environment for Insects

39

Fig. 4.1  The gradual isolation of Australia from the breakup of Gondwana, to indicate origins and geographical relationships of ancient Gondwanan biota now found in Australia: (a) Gondwana at 150 million years before present (MYBP); (b) 70 MYBP, Australia (Au) narrowly separated from Antarctica (An), Africa (A) and India (I) more so; (c) 40 MYBP, Australia more distinctly separated from Antarctica, New Guinea not yet distinct; (d) New Guinea separated from Australia; (e) Present, with Australia considerably further north, having moved about 20° of latitude since initial separation. Present south pole shown (black spot) for reference; in ‘b’, Nothofagus forests from South America, across eastern Antarctica to south eastern Australia; in ‘c’ Nothofagus throughout Antarctica and Australia; by ‘d’ Eucalyptus widespread through central and western Australia (After Attiwill and Wilson 2006)

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have become widespread and naturalized. They include species variously regarded as beneficial (honey bees, Apis mellifera, but see Chap. 15) and highly damaging pests of crops, commodities and livestock. Species in the second category remain anomalous – their origins are often entirely speculative. This Archaic element is small and comprises a number of taxonomically isolated ancient insects that occur in highly disjunct parts of the world. The scorpion fly family Meropeidae, for example, is known globally from single species in different genera in Western Australia and North America. This ancient family might have a Gondwanan origin, and one authority has speculated this origin, with movement into North America from South America and its loss (or non-discovery: it might really be there!) in the latter continent. Whereas we can infer possible origins of many of our characteristic insects, and trace their evolution and diversification in the country, others remain anomalous – but remarkable – elements of our fauna. However, within Australia, distribution patterns give many clues to origins. Rather few species or genera are found Australiawide, and distributions of individual species range from regional to highly localised. Many are known from only single sites or regions, and some are supposed strongly to be narrow-range or point endemics, found nowhere else on Earth and almost inevitably of high conservation interest. Assessment of broad distribution patterns starts with the delimitation of several broad biogeographical regions as a guide to distinguishing meaningful general elements. Our major focus is on the mainland and Tasmania, but ‘political Australia’ includes also some distant outliers such as Christmas Island (to the south of Java, Indonesia), Lord Howe and Norfolk Islands in the Pacific, and the subantarctic Macquarie Island, some 1,500 km south-south-east of Tasmania. The main scheme for regional division of ‘Australia proper’ flows from that initiated by Baldwin Spencer in 1896, but is still a valuable framework to consider. The 500 mm isohyet was used to distinguish the more humid coastal perimeter of much of Australia from the semiarid/arid interior and western Eyrean region (Fig.  4.2). The northerly Torresian region is the tropical/subtropical monsoon-dominated area to which many of the more recent northerly incursives from the Oriental region are limited, whilst the southern cool temperate Bassian region harbours most of the Gondwanan elements. However, the Bassian region is now divided into three disjunct ‘provinces’, each with its peculiarities. Thus many insects found in the West Bassian province (the south west of Western Australia) occur only in that region, but are related to those in the southeast of the continent, long separated by the dryer Nullarbor Plain. Converse examples include several families of lacewings (Neuroptera) found in the southeast but not in the western province, and parallels can be found in many insect groups. The East Bassian province is itself divided, into the mainland area and the island state of Tasmania – with the intervening Bass Strait formed most recently around 12,000 years ago. Each part supports numerous endemic insects, again many of them highly localised. Biogeographers have sometimes sought to divide these broad regions into finer levels, and any of a substantial number of features may be used to do so – vegetation type, altitude, and humidity are cited widely, but much subdivision has reflected the distribution patterns of the individual animal or plant groups assessed. For our purpose it is sufficient to recognise that different distribution

Introduction: Australia as an Environment for Insects

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Fig. 4.2  Map of Australia to indicate political divisions (States and Territories) indicated by initial letters as ACT [Australian Capital Territory], NSW ([New South Wales], NT [Northern Territory], Q [Queensland], SA [South Australia], T [Tasmania], V [Victoria], WA [Western Australia]) and main biogeographical regions (Bassian, Torresian, Eyrean); the 500  mm isohyet separates the inland semiarid/arid Eyrean region from the more humid coastal areas; the Bassian region comprises three separate areas, the south west, the south east, and Tasmania; the transition between Bassian and Torresian along the east coast is gradual and various points of demarcation have been proposed

patterns of insects occur, that many may be categorised broadly into origin elements, and that insects are continually mobile and evolving, so that distributions may contract or expand as conditions (such as climate) alter – and, importantly, that insects are sufficiently varied to provide distributional exceptions to almost any generality we seek to define or impose! One important faunal consideration, however, is the interdigitation of Torresian and Bassian insects along the east coast: in short, ‘southern’ insect elements can extend far to the north, some reaching New Guinea, and the converse occurs with some more Torresian taxa extending far down the east coast, particularly along the lowlands east of the Great Dividing Range. Substantial intermingling occurs in the far northern areas of New South Wales and far south east Queensland, with increased species richness of some insect groups there. For many insect groups, these broad zones give us a picture of their major centres of distribution and evolution and indicate hotspots of higher species numbers and biogeographical interest, together with clues to the environmental gradients (such as latitude or elevation) affecting these. High species richness may reflect a source area, as well as particularly suitable conditions for the resources they need. But one possible bias must be mentioned here – simply, much of Australia is still very poorly explored for many insect groups. Few higher taxa are well known, and the collecting intensity may create false impressions of true distributions. Even for the best-known insect group, butterflies – long popular and the focus of hobbyist collectors for well over a century – many gaps in detailed knowledge of species’ distributions remain. Collection intensity has been biased toward regions accessible to major cities and more distant ‘classic’ localities where collectors with limited time available can reasonably expect to find the rarer species they covet,

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Fig. 4.3  The tiny (fore wing length about 22 mm) ‘living fossil’ damselfly Hemiphlebia mirabilis (Odonata, Hemiphlebiidae) is known from several widely-separated localities in south eastern Australia, and is of substantial conservation interest. Males have an elaborate courtship display, apparently unique in the order, in which they repeatedly elevate the abdomen, tipped with expanded white flanges, whilst at rest on vegetation

rather than incur the extra time, costs, and risk of exploring new terrain. Information on many other groups is far more tenuous – but the caveats for butterflies, as the most informative group, are clearly of wider relevance. However, for our butterflies, three major centres of higher diversity occur along the east coast: the far northern tip of Cape York with its proximity to Papua New Guinea; the ‘Wet Tropics’ forested region between Cooktown and Townsville; and the region encompassing south east Queensland and northern New South Wales. The last may largely represent faunal mixing of the Bassian and Torresian taxa – but its diversity has also fostered high collector interest, as being easily accessible, particularly, from Brisbane and Sydney. The most diverse groups of insects commonly are those that have been here longest, although there are many exceptions to this, in which very primitive groups have scarcely diversified. Some ancient insects remain isolated single species of ‘living fossils’: the Western Australian scorpion fly noted above (Austromerope poultoni) is one such example. The tiny damselfly Hemiphlebia mirabilis (known as the ‘Ancient Greenling’) in Victoria, South Australia and Tasmania has long been considered an isolated ‘living fossil’ within the Odonata (Fig. 4.3). It seemingly has no close relatives at all and is conventionally placed in a major taxonomic division (superfamily) of its own. It now persists in a few small swampy areas and, although some authorities suggest that it has been derived from one or other of two much more widepread superfamilies, it remains both anomalous and peculiar. The insect fauna is characterised by many such oddities, from isolated single species to more diverse lineages found wholly or almost wholly in Australia. Some of the aquatic insect orders, such as Odonata (with three other families – Hypolestidae, Diphlebiidae, Cordulephyidae – also Australian endemics, and the three species of Petalura, although

Insect Species?

43

dwarfed by their Carboniferous antecedents, are the world’s largest modern dragonflies) and Ephemeroptera, contain many such taxa, but they are widespread also in terrestrial groups. The other major consideration is the occurrence of ‘radiations’ (proliferations of closely related taxa to form many very similar species – some of these taking place in parallel with radiations of major groups of plants: Chap. 8), producing complexes of daunting taxonomic complexity and often highly localised. How these are interpreted has major influences on how we enumerate insect diversity, so some comment is needed here.

Insect Species? Within many such radiations, the diagnostic boundaries between easily-defined entities (normally, species) are often very difficult to discern, and lower level populationbased or geographically discrete units are commonly designated. These are formally ‘subspecies’ (with a third formal name, so ‘Genus, species, subspecies’ augmenting the usual binomial ‘Genus, species’ combination), with the presumption that some consistently recognisable feature(s) can be used to diagnose them. Nevertheless, much of the problem of defining the richness of Australia’s insect fauna rests on our inabilities to define ‘species’ (as the main entities we wish to enumerate) by fully objective criteria. Not surprisingly, understanding the problems involved is both most advanced, and most controversial, amongst the insects we know best – the butterflies, amongst which many subspecies have been named (so that the approximately 420 full species in Australia increases to about 650 named taxa once subspecies are also included). Many local forms, some differing in adult appearance very little from others, attest to the variation that occurs – but often the causes of that variation, its consistency, and the biology leading to and accompanying it are wholly unknown, so that the validity of many butterfly subspecies is hotly debated amongst lepidopterists. In contrast, equivalent debate cannot occur for most insect groups, simply because not enough people are sufficiently interested in them and they are more poorly known, and the opinion of (often) a single specialist cannot be refuted authoritatively and becomes dogma. Although ‘species’ are the most commonly recognised units for enumerating biodiversity, it is worth here emphasising that there is no single universally accepted definition of the term, and that biologists and philosophers alike continue to debate the merits of various ‘species concepts’ across the whole gamut of life forms. The pithy summary that a species is ‘whatever a taxonomist says it is’ as a definition (as ‘taxonomic species’) is pervasive amongst many insect groups, often as a refuge to mask our lack of biological knowledge. Studies of variety, however, encompass ‘biological species’ (drawing on the widespread definition of reproductively isolated biological entities), ‘ecological species’ (occupying different ecological niches and so isolated), ‘evolutionary species’ (differing evolutionary lineages), ‘genetic species’ (within a common gene pool), and ‘morphospecies’ (the typological approach whereby species are defined largely on form and appearance). The last is by far the most common application in insects.

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It is thus not surprising that ambiguities occur within treatments by different authorities dealing with the same insects. Within-species (or within ‘species-group’) to enlarge the scope realistically with the hierarchy expressed in Table A.2 (p. 225), several different contexts for variations occur, spanning distribution and isolation, ecology, and the boundaries between related taxa. Two butterfly examples from the south east mainland illustrate some of these contexts, with the initial premises including (1) variations in the appearance of the adult insect are the major basis for naming subspecies; (2) the biological causes of those variations are usually unknown; (3) populations differ in appearance in different parts of the species’ range; and (4) the various populations either intergrade gradually or are geographically disjunct, now occupying isolated fragments of a formerly more continuous range, possibly along one or more environmental gradients. These might originally have been parts of a pattern of continuous gradual variation, or cline. These examples indicate some of the interpretative problems that may arise.

The Yellowish Skipper and Donnysa Skipper, Hesperilla flavescens and H. donnysa Hesperilla donnysa is sometimes called the ‘Varied sedge-skipper’, a very suitable name in view of the considerable variations in colour pattern the butterfly displays across its range. It is very closely related to H. flavescens, with this skipper restricted to small areas of southern Victoria and South Australia. Further confusion and debate continues over the validity of various named subspecies based on this variety. H. flavescens includes two geographically disjunct subspecies on a few sites near Melbourne, Victoria (H. f. flavescens) and the very similar H. f. flavia in South Australia. It is typically confined to near-coastal saline sedgelands, where caterpillars feed on a single species of sedge, Gahnia filum. Whether the two subspecies arose independently in parallel, or are fragments of a formerly more continuous coastal range remains a tantalising problem. H. donnysa mainly occurs further inland with its caterpillars feeding on a wide range of Gahnia sedges. However, uncertainties occur over precise separation of the two species on the adult appearance, as the two intergrade substantially, so that H. flavescens is sometimes considered an extreme yellowish form of H. donnysa, with its range and biology a subset of that of the more widespread, and possibly parental, taxon.

The Swordgrass Brown, Tisiphone abeona Tisiphone abeona is one of the most intriguing and well-studied complexes of a variable butterfly in Australia, with members of each of the coastal populations in the south east (Fig.  4.4) each relatively constant in appearance, but sufficiently distinct to have given rise to seven subspecific names, with the additional complication

Insect Species?

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Fig. 4.4  Range of Tisiphone abeona subspecies. The various named subspecies of this variable butterfly are each distributed in a circumscribed part of the south east; T. a. ‘joanna’ is a hybrid form occurring where the ranges of two parental subspecies overlap narrowly; the western outliers of T. a. albifascia have sometimes been deemed a separate subspecies, T. a. antoni, but this is not now accepted widely  

of an eighth named form (T. a. joanna) near Port Macquarie highly variable and almost certainly a hybrid maintained by meeting of populations to the north (T. a. morrisi) and south (T. a. aurelia). A further form from this area of New South Wales may represent yet another (unnamed) taxon in this group. The southern subspecies are alike in sharing a broad orange band on the fore wing, and the northern ones have more extensive creamy markings. Both series are clinal in character, changing gradually along their range, and the two Victorian subspecies are regarded widely as not distinct, despite their disjunct distributions. Similar patterns and difficulties are almost inevitable in many other insects for which at present we have seen (or distinguished) only the most obvious forms. Insect species and subordinate categories have most traditionally been diagnosed from morphological features alone, usually by people who had never seen the living animal and its environment, so that biological information was almost inevitably fragmentary. Fabricius could have had little idea of the habits of Myrmecia gulosa, of how it might differ from other bull ants (or even whether any such others existed), or how these fit into the panoply of ants in Australia. Myrmecia as now understood contains about 90 described species with only one of these found outside Australia, in New Caledonia. Within the genus, many of the species can be differentiated only on rather small features, such as the arrangement of projections (‘teeth’) on the mandibles. It is not uncommon for specialists in any insect group to find individuals that in some way fall intermediate between described species, and to refer these to a ‘species group’ designated by the name of the nearest similar species to indicate their possible affinity: the currently delimited ‘Myrmecia gulosa species-group’ includes around half the described species of Myrmecia, and an array of subspecies names. Two other examples extend the range of contexts and problems of enumerating insect species and assessing how they have evolved and spread. Brightly coloured jewel beetles (Buprestidae) have long been a focus for collectors. The genus

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Fig. 4.5  Different species within a genus may have very different distributions that may overlap in places; the example of four species of fruit flies within the genus Bactrocera (Tephritidae, Dacinae) in eastern Australia, as representatives of a group of important economic pests: all occur in the far north, and the ranges shown for B. musae, B. neohumeralis and B. tryoni all include more northerly and easterly regions occupied by other species (Information from Drew et al. 1982)  

Castiarina, by far the most diverse in the family, is restricted to the Australia/ New Guinea region, and some 480 species have been recognised in Australia. Most of these have been described from adults collected on blossom, where they can be numerous and important pollinators. Larvae occur in wood, developing within the trunks of trees and shrubs, and the biology and host plant relations of most species are still unknown. Adult colour pattern has traditionally been a major component of species separations. However, numerous minor variations in this occur, and the causes of such differences have not been comprehensively examined; genetic information on the genus is sparse. Even in groups of insects in which species are relatively easily recognisable, distribution patterns can differ markedly in related species. Figure  4.5 shows the distribution ranges published some years ago for four representative species of dacine fruit flies. Fruit flies are amongst the most serious economic pests in Australia, and their depredations on fruit crops have led to long term investigations on distribution, biology and control, as well as species recognition. Most species are now included in the genus Bactrocera, but numerous species complexes (including more than 20 ‘subgenera’) occur amongst the more than 330 species recorded from Australia and nearby parts of the Pacific. Their differing ‘ecological tolerances’ are reflected in the differing amplitudes – from tropical to cool temperate – reflected in the

Insect Species?

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Fig. 4.6  Caledia captiva range. The distribution of some major intraspecific categories in eastern Australia as assessed by karyotype differences (see text). The ‘Moreton’ form forms a cline from south east Victoria northward to its main centre on the central east coast, where it meets the northern ‘Torresian’ form; the position of the dynamic hybrid one is indicated by a dashed line (After Marchant and Shaw 1993)

distributions shown and, as climates change, together with areas of fruit cultivation, all such plots are also subject to substantial changes (p. 183). Wherever two closely related insect forms meet in distribution, and create even a narrow zone where they overlap, hybridisation may occur – as in the Tisiphone butterflies noted above – but the outcomes can be very difficult to interpret meaningfully. The grasshopper Caledia captiva is very widely distributed in grasslands in eastern and northern Australia, and was also amongst those insects first collected in 1770, although not described until considerably later. Although formally a single named species in Australia, it is in fact very complex, and illustrates some of the problems involved in approaching the question of ‘what is a species?’, and how such extensive variation may be explained properly. Found also in Papua New Guinea, the range of C. captiva spans about 35° of latitude (Fig. 4.6) and it is common throughout both tropical and temperate zones, apparently having gradually moved southward from the tropics. Over this distance, it can be subdivided into several distinct entities that differ substantially in DNA features, the organisation of the chromosomes, and extent of reproductive isolation, as assessed through comprehensive series of laboratory trials. Two main geographical groups have been referred to as ‘Torresian’ (occurring in Papua New Guinea, Northern Territory and coastal Queensland) and ‘Moreton’ (south east Queensland to Victoria) and these two meet to form a hybrid zone in south east Queensland. That zone is only about a kilometre wide, but more than 250 km long, and is of considerable evolutionary interest. The open forest to improved pasture region supports grasshoppers at densities of up to about 2,800 individuals per hectare, and chromosome characteristics have been

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assessed in samples taken at small intervals (200 m) to attempt to characterise the purported hybrid zone. Only a kilometre separates the two distinctive types, with further abrupt transitions occurring within only 200  m, and with climate studies suggesting that the zone itself may move during periods of climate stress. Many questions remain, but this carefully documented example illustrates the extensive genetic and ecological changes that can occur over very short distances and, equally, not be revealed without equivalent fine-grain study, with – in this case – the two protagonist forms only partly isolated reproductively but with a further member of the complex (‘Daintree’, Fig. 4.6) fully isolated and does not produce hybrids. More recent studies suggest that this variation within C. captiva is really a suite of clines, and that it comprises only one real species. The chromosomes of the two main forms differ substantially, with ‘Moreton’ showing a consistently variable pattern along the latitudinal gradient: more than 600 different chromosome forms (transcriptions) have now been recognised in this grasshopper! Such variations are not necessarily worthy of formal recognition by naming, of course, but indicate areas in which debate over formal status may occur – and for numerous other species for which the picture is far more incomplete, scientists may well be tempted to elevate sporadic varieties of this nature to subspecific or specific level. As another complex example, the eucalypt-frequenting stick insect Didymuria violescens includes at least ten different distinct chromosome forms, each of them uniform within a particular part of the species’ range. These have been called ‘races’ and overlap only very narrowly in places. D. violescens is very variable in appearance and morphology, but variations in appearance are not correlated with patterns of chromosome differences, so any individual cannot be allocated to population/ race simply on what it looks like. Hybridisation can occur between parapatric races, so that all ‘races’ are referred to the same species as not being reproductively isolated. However, the varying levels of reproductive incompatability imply a process of incipient speciation, with the narrow hybrid zones constituting partial genetic barriers between dynamically interacting ‘potential species’. Both these examples, and others (Fig. 4.7) such as the so-called ‘tension zones’ designated for areas where putatively different races of some other species of grasshoppers come into contact, emphasise the difficulties of hard categorisation, and a conservative approach to enumerating species has been advocated repeatedly because such forms in the transition of becoming more distinct and reproductively isolated are open to very subjective opinion on what they really represent, and much of the ‘evidence’ cannot be resolved objectively. Different chromosomal forms of plant-feeding insects (as those most commonly studied) may be linked with host plant-specificity. The complex gall-forming genus of coccoid bugs, Apiomorpha (Eriococcidae), with nearly 40 species described, predomnantly infests Eucalyptus. The gall form is often very characteristic for a given species (Fig. 4.8), although some are more variable. A. munita has diploid chromosome counts ranging from 6 to >100 in different populations, with considerable variety within each of the three defined subspecies, so that considerable caution is needed in using genetic information to categorise taxa. All three subspecies of A. munita are found only on the series Symphyomyrtus, with no overlap of host plant species across them.

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Fig. 4.7  The common grass-blue, Zizina labradus (Lycaenidae), is one of the most abundant and widespread butterflies in Australia, occurring in a variety of open grassy habitats. However, its nomenclature is still not wholly resolved across its wider range in the western Pacific region. A closely related New Zealand form, Z. oxleyi, has been claimed to be threatened by hybridisation with the introduced or invasive Australian taxon. A recent account concluded that Z. labradus is best treated as a subspecies of the widespread Z. otis, rather than as a distinct species

‘Pattern’ (here, the genetic and/or morphological peculiarity of populations of an insect) and ‘process’ (how that variety has arisen and the extent to which it represents functional or reproductive isolation) are easy to confuse in studying insect evolution. Most practising entomologists have a realistic idea of what constitutes ‘a species’ in their particular group of interest, and of how to appraise variations. However, insect taxonomy has a history of two camps of people who describe insect species. They are referred to traditionally as ‘splitters’ (those who name new species on small, sometimes individual, character differences) and ‘lumpers’ (who tend to amalgamate variety of probable specific value into the same taxon, rather than proliferate formal names). Others, of course, tend to adopt a more balanced viewpoint than either of these, but without close independent study of any insect group, there is no infallible guideline to what any given name may really mean, however useful it may be as a descriptive epithet. But much insect variety is never acknowledged formally in that way, and the characters contributing to that variety, often marking consistent variants, will continue to proliferate as new approaches to comparisons arise. How we interpret this consistently, or even whether consistency is possible across different insect groups diversifying at different rates and in different ways, remains largely an open question. Genetic studies of various insects have sometimes confirmed the validity of subspecies as discrete, and in many cases led to more intensive study resulting in some being elevated to full species on a greater array of characters than initially examined.

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Fig. 4.8  Australia has a variety of gall-inducing coccids (Hemiptera: Coccoidea). Each gall supports the development of a single individual and may persist for many months. Eriococcidae are a major family involved, with species of Apiomorpha a notable radiation on eucalypts, often with galls species-specific in appearance: female galls of two taxa are shown to illustrate this. Both are about 2.5 cm long, and male galls are much smaller: (a) the 3–4 flanged gall of A. munita; (b) the cylindrical gall of A. conica

In other cases the variation may be shown to be continuous, even if modified locally by environmental factors (such as the food plants of herbivores), or temperature, so that any separate name is spurious. So-called ‘cryptic species’, however, are undeniably numerous and reflect situations in which even experienced specialists may fail to recognise that a series of specimens confronting them includes more than one species.

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Consistent biological differences (such as restriction to particular food plants) may lead to biological differentiation without any obvious morphological change. Part of the debate over the size of Australia’s insect fauna thus devolves simply on how to define a species, and whether this can be achieved consistently and acceptably across the various orders. Species have been defined in various ways, but some clear consistent definition is obviously needed for comparisons and absolute evaluation of diversity present. This is not easy, not least because different groups of insects may be evaluated by specialists in different ways, so that differences between species are not always obvious – in evolutionary terms, they form continua which we are trying to partition and label into discrete components: to impose firm, but sometimes artificial, boundaries. Taxonomy involves ‘pigeon-holing’; but for variable entities the borders of the ‘holes’ blur, and different specialists may proffer different sized ‘pigeons’ for consideration. Suggestions that cryptic species may outnumber recognisable species severalfold provide considerable uncertainty, but some recent molecular studies of insects – using DNA extracted from tiny parts of a body, such as a single leg or small area of wing, as techniques continually gaining in efficiency and subtlety – indeed reveal massive unsuspected variety. ‘One’ South American butterfly, for example, was revealed to comprise ten recognisable species! Our challenge for the future is to interpret, enumerate, and document this variety. Bar-coding is at present receiving much attention as a means of helping to diagnose species or other ‘evolutionary units’ and, as in the above case, may reveal high levels of previously unsuspected diversity: one recently- described Australian geometrid moth, earlier hidden within the diagnosis of a previously-diagnosed species of Oenochroma, has actually been named ‘barcodificata’ as a homage to the technique! However, many scientists urge the combination of traditional and such more modern techniques in delimiting species, because of the ambiguities that arise from the latter used in isolation. For another geometrid moth, the consistent but small differences in male structures led to the Tasmanian form being named as distinct from the mainland species – but accompanying DNA data were ambivalent, even though implying this was the correct decision. But – despite advocacy to the contrary – bar-coding is viewed widely as not being an alternative to good traditional taxonomy based on structural characters: some workers seem to regard it simply (and, perhaps, cynically!) more as a ‘quick fix’ that can make any differences seem significant, and lead to subjective evaluations that are not then testable objectively. The status of a structurally-defined species, by contrast, can be re-evaluated as more material comes to hand, and the character states revised or augmented. Up to the present, a high proportion of named insect species have been described from appearance and structural features of dead specimens, often from very few individuals and no biological knowledge other than what can be inferred from incomplete knowledge of related forms. Whilst analytical techniques are improving rapidly, DNA analysis from long dead insects is still inconsistent. Often, the extent of individual variation within a species is simply unknown, and specialists may elect to designate ‘new species’ on small morphological features alone. Such classical taxonomy is thereby typological, rather than reflecting any boundaries between ­biological species, and based on the rationale that if the insect has a given suite of

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structural features it is ‘species x’, and if it does not it is ‘species y’. This may be entirely valid, of course, but inevitably has led to a substantial number of insects each being named on repeated occasions and, as the butterflies noted above illustrate, realistically interpreting natural variation is fraught with difficulty. One species of thrips (not Australian, but equivalent examples are almost certain to be found in other orders) has received 18 different names in four different genera! Such situations can arise through differences between males and females, and between small and large individuals, between individuals from different hosts, natural variations in colour and form, seasonal differences, and so on. Ideally, taxonomic evaluation should be able to appraise this variation within populations and across the putative species’ range – but this has rarely been possible in the past, so that confusion over ‘real numbers’ of species has two main sources: that many real species remain undescribed and often undetected, and that many of the species’ names available as a basis for species counts represent synonyms, also undetected. For thrips, to follow the example introduced above, it has been estimated that around 1,500 of the 7,000 species names recognised worldwide in 2004 are synonyms, because variation was not recognised or material was insufficient for detailed comparative study. An insect taxonomist rarely knows that two partially similar dead individuals are (or are not) the same species. He or she uses knowledge and experience, often acquired over many years of study of the insect group involved, to predict that a specimen belongs to a particular species, but this is not the same as knowing that it could interact with other individuals as a biological species. A purported species is at one level simply a scientific hypothesis, whose validity may eventually be proved (or disproved) by collection of more specimens or more data. Whereas ‘counting species’ is a central theme in insect diversity, the recognition and designation of individual insect species is of far more than theoretical interest alone, because different entities can, and usually do, have different biology. As suggested above, we may in some cases be counting names, rather than species! If our insect has a name that can be applied consistently (implying that the insect is recognisable, commonly not the case in Australia) that name becomes a key to retrieving information on that species, should any have been published in the past. Conversely, a wrong name can easily generate false information which sometimes has far-reaching implications. Confusion between closely-similar pest species, for example, can have important and expensive consequences. The two major species of Helicoverpa (formerly Heliothis) moths are amongst the most severe agricultural pests in Australia, but have very different life cycles and diapause regimes, so that confusion may lead to inadequate control of their depredations and substantial crop losses. In short, even closely–related species can differ substantially in their biology and for good ecological understanding it is emphatically not sufficient to categorise insects from surveys to assess diversity simply to broad categories such as beetles, flies, moths or, in many cases, even just to families and genera. These broad groupings can each mask substantial biological variety. The extent of this generalisation is not always appreciated, and accepted uncritically, but consider parallels amongst animals that are more popular and better-known. The limits to individual species knowledge that would flow from designation of various vertebrates only as ‘snakes’,

Further Reading

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‘possums’ or ‘parrots’ would be widely obvious to naturalists – simply because each of these includes massive variety of detail – yet many of the people who would ridicule reliance on such broad categorisations in the vertebrate literature readily accept terms such as ‘beetles’ or ‘wasps’, perhaps taken to the family levels as (for example) Carabidae or Ichneumonidae as impressive names for families which each include as many species as the whole vertebrate fauna, and manifest equivalent variety of biological detail. At the other extreme, of course, designating individual zebra as separate species simply on small individual differences in stripe pattern would be equally misleading in assessing species diversity! In the meantime, the somewhat uneven information available allows some comments on Australia’s insect diversity as we currently understand it. Points over which there is little debate include (1) that Australia has an enormous number of insect species; (2) most of these are endemic and many have evolved in Australia, collectively from a considerable variety of founder elements – some of them ancient; (3) we are far from being able to provide an accurate estimate of numbers or inventory of variety, and most species are still unnamed; (4) that there are too few specialists to remedy this situation in the foreseeable future; and (5) insects are amongst the most ecologically and economically important components of Australia’s biodiversity, and many lineages are increasingly at risk of loss from human activities. Of the orders present, and reflecting a wider global picture, the ‘big four’ are Coleoptera, Diptera, Hymenoptera and Lepidoptera, all with a complete metamorphosis. Beetles are probably the richest of these, with perhaps a quarter of the possibly 80,000 species described. Several other orders – the plant-feeding sucking bugs (Hemiptera) and the grasshoppers and their allies (Orthoptera) each contain well over a thousand species and are the largest exopterygote orders whilst, at the other extreme, Archaeognatha and Zygentoma (the remnant Apterygota) are sparse and amongst Pterygota – not counting the Zoraptera, as noted earlier- only Megaloptera and Mecoptera have fewer than about 30 species. Spread across about 27 orders, this wealth of species is generated and supported by the variety of environments suitable for insects in Australia, and the resources they furnish. Understanding their ‘working environments’ is central to understanding the insects themselves. Linked intricately with this, knowledge of feeding habits and interactions with other species are key aspects of assessing how insects fit in to wider communities and how their individual populations vary in time and space. Behaviour is a major determinant of capability and adaptive potential in any such context.

Further Reading Attiwill P, Wilson B (2006) Ecology in Australia. In: Attiwill P, Wilson B (eds.) Ecology; an Australian Perspective. Oxford University Press, Melbourne, pp 3–14 Barker S (2006) Castiarina: Australia’s richest jewel beetle genus. Australian Biological Resources Study, Canberra (survey and taxonomic review of the major component of Buprestidae in Australia) Braby MF (2000) Butterflies of Australia, CSIRO Publishing, Collingwood (the standard work on these insects, with much more detail on the examples noted in this chapter, and many others)

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Cook LG (2001) Extensive chromosomal variation associated with taxon divergence and host specificity in the gall-inducing scale insect Apiomorpha munita (Schrader) (Hemiptera: Sternorrhyncha: Coccoidea: Eriococcidae). Biol J Linn Soc 72:265–278 Drew RAI (1989) The tropical fruit flies (Diptera: Tephritidae: Dacinae) of the Australasian and Oceanian regions. Memoirs of the Queensland Museum 26 (Detailed taxonomic review of 209 species, as an example of richness within a native pest insect group) Drew RAI, Hooper GS, Bateman MA (1982) Economic fruit Flies of the South Pacific Region, 2nd edn. Queensland Department of Primary Industries, Brisbane Marchant AD, Shaw DD (1993) Contrasting patterns of geographic variation shown by mt DNA and karyotype organisation in two subspecies of Caledia captiva (Orthoptera). Mol Biol Evol 10:855–872 Ogata K, Taylor RW (1991) Ants of the genus Myrmecia Fabricius: a preliminary review and key to the named species (Hymenoptera: Formicidae; Myrmeciinae). J Nat Hist 25:1623–1673 (Technical account of bullant taxonomy, indicating complexities of species delimitation and recognition) Shaw DD, Marchant AD, Conteras N, Arnold ML, Groeters F, Kohlman BC (1993) Genomic and environmental determinants of a narrow hybrid zone: cause or coincidence. In: Harrison RG (ed.) Hybrid zones and the evolutionary process. Oxford University Press, New York, pp 165–195 (Overview of the detailed studies on Caledia captiva) Yeates DK, Harvey MS, Austin AD (2003) New estimates of terrestrial arthropod species-richness in Australia. Records of the South Australian Museum, Monograph series 7:231–241

Chapter 5

Environments and Habitat for Insects in Australia

Introduction: Places to Live ‘Places to live’ can be considered at various scales, from broad biogeographical regions extending over thousands of kilometres, to part of a single small plant or individual animal host. Those scales form a hierarchy, so that an environment for any given insect can be considered as a series spanning these extremes, with each subordinate level representing increased specificity and ecological specialisation. One leading European ecologist recently used the apposite simile of the Russian matrioschka dolls to illustrate this, with each successive doll inside a larger one representing finer detail of need but still depending on the protection of the enveloping covers. A moth caterpillar that can feed as a generalist herbivore on a variety of different plant species, perhaps across several different plant families could be considered to need a ‘larger minimum doll size’ than (for example) a caterpillar that can feed only on the young foliage of a single plant species that is itself restricted in distribution to a small part of a region. In general, many insects thrive on rather limited resources – one advantage of being small is that each individual does not need much food. The lifetime consumption of either of our caterpillars, above, may be only a few grams – very substantially less than that for kangaroos or cattle, which also eat for much longer than the few weeks or months many caterpillars need to develop. Such limited needs foster specialisation and fine-grain division of the environment – simply, dividing up a larger resource in some way means that more species can share it. Being small can be functionally advantageous in using key resources. The major environments, as the largest units defined and influencing biota, are each complex and very varied. Much of Australia’s distinctive character reflects its vegetation, recognised as diverse and unique from 1770, as acknowledged initially by the naming of Botany Bay, and with many of the plants differing markedly from the flora of any other region. That vegetation can be classified or categorised in various ways to display its variety, and this is done usually by a combination of structure and floristic composition – the particular plants present there. Each such

T.R. New, ‘In Considerable Variety’: Introducing the Diversity of Australia’s Insects, DOI 10.1007/978-94-007-1780-0_5, © Springer Science+Business Media B.V. 2011

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Fig. 5.1  Simplified distribution of major vegetation types in Australia (cf. Fig. 5.2). Key: black, closed forest; dense dotting, open forest; coarse dotting, woodland; unmarked, shrubland; vertical hatching, scrub and heath; horizontal hatching, herbland (After Carnahan 1977)

vegetation type – however it is delimited or defined – may also support specialised insects restricted to (or predominant in) it, and a high proportion of native insects are associated in some way with plants, many of them to individual plant species or genera. Evolution together (so, ‘coevolution’) over tens of millions of years has produced intricate associations and interdependent relationships between plants and insects in Australia. Terrestrial environments have traditionally been characterised by vegetation type, with the two axes of ‘life form and height of tallest plant layer’ and ‘percentage foliage cover of tallest plant layer’ most commonly employed to designate a series with the extremes of ‘tall closed forest’ (trees >30  m tall; dense cover of 70–100%) to ‘low open shrubland’ (shrubs 0–2 m tall; very sparse cover, 103 Chewers and miners: detritivores on plant tissue Collectors