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All the past these mountains saw All the years of toil and strife Lives unknown that went before They gave us this: Our present life. –T. D. A. Cockerell, 1927 15EEpiloguepilogue

ton also allows insects to defend themselves with potent WHY SO MANY INSECT SPECIES? toxins without poisoning themselves. This question has occupied entomologists for centuries, and 2. Metamerism and repetitive pairs of appendages allowed some of the explanations we offer are unfortunately little dif- specialization of some appendages while retaining the ferent from previous ones. New knowledge of phylogeny and original functions of others. Much the way gene duplica- discoveries of fossils over the past several decades, however, tion has allowed the divergence of redundant elements, have improved our understanding. Simply put: there is no redundant appendages led to divergent adaptations. The single answer to the evolutionary and ecological success of forelegs of various insects, for example, have become rap- insects, but is due instead to various intrinsic features of torial or fossorial while the mid and hind legs retained the insects. ability for walking. Even more significant are the mouth- parts. Virtually every element within the major sets of paired mouthpart appendages – the mandibles, maxillae, and labium – have been modified in some form to suit AGE diverse diets optimally (Figures 8.2, 12.24, 13.9). Thin adult Insects are among the earliest terrestrial animals, and the mouthparts that siphon fluids, for example, evolved in recent discovery that Rhyniognatha from the Early Devonian Palaeodictyopterida, Thysanoptera, Hemiptera, anoplu- was an insect (perhaps even a pterygote!) indicates that they ran lice, Lepidoptera, many Diptera, bees, a few extinct evolved in the Silurian. As terrestrial ecosystems evolved, Mecoptera, and even a few Coleoptera and Trichoptera. insects were on the scene ready to exploit the new The mouthpart elements in each kind of proboscis resources. Major radiations of insects can be related in the evolved differently, illustrating the unique capacity in fossil record, for example, to the evolution of vascular plants, insects to develop different solutions to the same prob- to angiosperms, and (for ectoparasitic and blood-feeding lem. The evolution of mouthparts probably facilitated the groups) to birds and mammals. evolution of phytophagous diets in insects (Labandeira, 1997). 3. Flight greatly improves the ability to escape from preda- DESIGN tors and allows aerial reconnaissance of food and mates. Consider, for example, an early apterygote and pterygote Design is traditionally the main explanation for the evolu- feeding on the sporangia at the tips of a Carboniferous tionary success of insects. No single innovation accounts for plant. The apterygote must climb among the branches the success of insects, but rather there has been a cascade of and down the plant, the pterygote can flit among innovations, each of which was refined later in the evolution branches and plants, more easily finding sporangia and of insects. better selecting the most productive and nutritive ones. 1. The first of these was clearly the jointed, arthropod The ability to fold wings over the abdomen – neoptery – exoskeleton, which provided insects with the physical pro- allowed insects to invade small, cryptic spaces while pro- tection and skeletomuscular strength to live on land. tecting their best means of dispersal. Associated with this is the tracheal system of respiration, 4. The origin of the larva allowed holometabolans to acceler- which with an exoskeleton allowed animals with such ate the development of immatures, to facilitate diapause large ratios of surface area to volume to reduce desicca- during periods of stress, and to exploit diets different from tion in the terrestrial environment. A protective exoskele- the adult. A separation of adult and larval diets may have

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allowed the less mobile and more vulnerable immatures LOW RATES OF NATURAL EXTINCTION to invade cryptic spaces and also escape predators and Evidence for this comes primarily from the fossil record. parasitoids that attack the adults. Insects show minor to negligible effects on mass extinctions It must be stressed that, again, no one of these innovations that decimated major groups of organisms, the best evidence are responsible for the success of insects. Flight probably for which is from the Cretaceous-Tertiary extinctions. The would not have evolved if metamerism, serial appendages, only significant example for the extinction of major mono- and an arthropod exoskeleton did not evolve first. Neoptery phyletic insect groups are the Palaeodictyopterida, Caloneu- then further improved wings. rodea, and Miomoptera, presumably at the End Permian Event, which was a crisis that was unmatched in evolutionary history. The combination of low extinction rates and high specia- CAPACITY FOR HIGH SPECIATION RATES tion rates, beginning well before 400 million years ago, resulted in a present day accrual of insect species that is Evidence from the fossil record indicates that speciation rates unprecedented in the 3 billion years of life on earth. The low in insects can be considerably greater than what is known in extinction rate itself is related to several highly adaptive another terrestrial group of animals, the mammals. For designs of the insect body plan, each of which evolved in example, two major lineages of phytophagous insects, the response to changes that occurred over thousands to millions Lepidoptera and phytophagan beetles, comprise approxi- of years. The environmental changes that have taken place mately 250,000 Recent species. Because the great proportion over merely the past century, however, are proving to be the of these feed on angiosperms, it is reasonable to estimate that greatest evolutionary challenge to all organisms, even to ones over the past 100 million years there has arisen on average as adaptable as insects. two or three species every 1,000 years in these groups. This is a gross underestimate because it does not take into account the total diversity of extinct lepidopterans and phytophagans over the last 100 million years, which is impossible at present THE FUTURE to estimate, nor the higher numbers of total Recent species (described and undescribed). Note that this is not the actual People commonly think that because no species of mosquito, time it takes two populations to diverge into separate species roach, or other pest has been eradicated, that all insects will (evidence from the Hawaiian fauna shows that speciation easily thrive amidst the environmental destruction we are probably takes on the order of 100,000 to 300,000 years, pos- inflicting on our planet. There is probably no single greater sibly shorter), but it does reveal the remarkable rate of prolif- misconception about insects. Some insects are indeed tena- eration of insects. cious pests, but more than 99.99% of insect species are highly Traditional explanations as to why insects are intrinsically sensitive to the typical threats of pollution, pesticides, inva- capable of such high speciation rates have typically cited sive species, and habitat loss (Collins and Thomas, 1991; their short generation times, the reasoning being that with Samways, 1994; Pullin, 1995; Deyrup, 2001). A small percent- short generation times more mutations accrue and allow age of insects are endangered, and some have become faster genetic divergence. However, the excellent fossil record extinct, though the number is without doubt a gross underes- of proboscideans shows that these mammals have a high timate. The International Union for the Conservation of speciation rate, and their generation times are on the order of Nature (IUCN) has listed that over the past 100 years 600 about 50 years. Rather, it is the stunning rate of increase in plant and 491 animal species have become extinct as a result insects that probably provides the genetic grist for their evo- of humans, only 72 of which are insects. Forty red-listed lutionary mill; this is a quotient of the reproductive capacity insect species alone are from Hawaii and other remote of a species and average generation time. It has been deter- islands, faunas of which can collapse under the effects of mined, for example, that if Drosophila melanogaster lays 100 introduced rats, pigs, goats, mongoose, ants, and vespid viable eggs per female, and that half of these produce females wasps. Bermuda, for example, is a relatively depauperate that do the same, the exponential rate of increase is so great island, and an intensive study of its fauna found 258 species (taking into account the deaths of each generation) that by of Diptera, 17 of which are probably endemics and 7 of these the end of the twenty-fifth generation the ensuing mass of are possibly extinct because they have not been seen in years flies would be much larger than the earth itself. Obviously, (Woodley and Hilburn, 1994). None of those seven species there is a tremendous amount of mortality, but the actual was IUCN listed. Documenting the hemorrhaging of insect populations of fruitflies vastly outnumber (and probably out- biodiversity is fraught with difficulties. The large, flightless weigh) terrestrial vertebrates, and thus generate higher Lowd Howe Island stick insect, Dryococelus australis, for genetic variation. example, was recently discovered alive, some 80 years after it CY501-C15[646-650].qxd 3/2/05 12:44 PM Page 648 quark11 Quark11:Desktop Folder:CY501-Grimaldi:Quark_files:

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was thought to be extinct. If a creature as conspicuous as this evades notice, what about the great majority of insect species, many of which have not even been named yet? The death of the last passenger pigeon at the Cincinnati Zoo in 1914, for example, is well known, and with it were purport- edly of two species of lice, Columbicola extinctus and Cam- panulotes defectus (these lice were later found on other species of pigeons). No one except phthirapteran specialists laments the loss of a louse, but every species has some salient aspect of biology, and the lice have taught us more about cospeciation than any other group of insect. For many insects extinguished by humans, we do not notice their demise until well after the fact. The giant St.Helena earwig is another island casualty; however, it is known only from its remains (Figure 15.1), a dodo among insects. It succumbed probably to rats introduced to the 15.1. Dodo of the Dermaptera, Labidura herculeana: the giant earwig island several centuries ago. The Antioch katydid, Neduba (shown here actual size), known only from the remote South Atlantic island of St. Helena and last seen alive in 1965. Introduced rats and extincta, was endemic to coastal sand dunes around San mongoose may have extirpated it. Photo: P. Naskrecki, NHM; length of Francisco, but the species only became known 40 years after entire insect 62 mm (2.5 in.). the last collection of specimens. Coastal sand dune habitats, so coveted for housing, are in fact highly endangered habitats, and three butterflies from North American carrion beetle, Nicrophorus americana the same area as Neduba extincta have followed it. One of (Figure 15.3), so perplexing. Another carrion-feeding insect these, the diminutive extinct butterfly Glaucopsyche xerces, or that has a similar fate, which may even be extinct, is the “Xerces blue” (Figure 15.2), is the symbol of the Xerces Soci- large, red-headed European blowfly, Thyreophora cynophila ety, the only organization dedicated to the conservation of (Pape, 2000). Fifty years after its naming in 1794, it could no invertebrates – a stark contrast to the many wealthy societies longer be found and is now considered extinct. The Califor- dedicated to megafauna. Another three butterflies, some nia condor clings to survival, but Pleistocene fossils indi- tiger beetles, and other insects also endemic to coastal sand cate they occurred as far east as New York state, perhaps dunes of the western United States are endangered. Among following the loss of the megafauna to indigenous and them is the “Delhi Sands fly,” a large, long-tongued fly in the European people. Maybe Thyreophora is the California Mydidae (cf. Figure 14.16) and an important pollinator. The condor of flies. proposal for its official protection caused an uproar among Insects are unique for many reasons, and an additional people who have been outraged that, of all things, aflycould one is that no other group of animals is the systematic target have federal protection. Such people, of course, are com- of such wholesale poisoning. The environment is laced every pletely ignorant of the spectacular diversity of flies, that some year with a half-million tons of insecticides in the United are in fact important pollinators, and that the loss of pollina- States alone, and the amount continues to rise as the most tors has a domino effect on rare plants (Buchmann and Nab- destructive pests evolve greater resistance (Pimentel et al., han, 1996). 1992). The notorious chlorinated hydrocarbon DDT (banned Coastal sand dune habitats harbor many threatened in many countries, but not all) was developed in 1939, and by species, but the situation is much worse for the coastal 1946 resistance of target insects was reported. Still, 50,000 forests of southeastern Brazil near São Paulo. This forest con- tons of it was used annually in the 1950s, until it was noticed tains remarkable proportions of endemic animal and plant that it caused long-term endocrine problems in vertebrates species, including insects, and because it is tropical, the over- (Matsumura, 1985). Now, hundreds of insecticides are in use, all diversity is exceptional. Brazilian coastal forest, unfortu- though most of them function in the same way (as insect nately, comprises less than 1% of what it was 150 years ago neurotoxins). Despite the effort, no pest insect has been (Lovejoy, 1985), making it almost certain that thousands of eradicated; in fact, over 500 pest species of insects and mites insect species have gone extinct there, but we’ll never know are resistant to at least one insecticide (McKenzie, 1996). the actual numbers. Some species appear resistant to the complete chemical For some insects the causes of endangerment or extinc- arsenal, like the green peach aphid, Myzus persciae; the dia- tion are unclear. There is probably as plentiful a supply of mondback moth, Plutella xylostella; the Colorado potato small mammal and bird carcasses as ever (i.e., rats and beetle, Leptinotarsa decemlineata; and the malaria mosqui- starlings), which makes the dramatic dwindling of the toes in the Anopheles gambiae complex. Insecticides merely CY501-C15[646-650].qxd 3/2/05 12:44 PM Page 649 quark11 Quark11:Desktop Folder:CY501-Grimaldi:Quark_files:

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15.2. The extinct lycaenid butterfly, Glaucopsyche xerces or Xerces Blue, which was endemic to coastal dune habitats of northern California. A loss of habitat through development led to its extinction. AMNH; length 13 mm. CY501-C15[646-650].qxd 3/2/05 12:44 PM Page 650 quark11 Quark11:Desktop Folder:CY501-Grimaldi:Quark_files:

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ral habitats is causing unprecedented extinctions, and no doubt it is an even greater spasm than the End Permian Event, which apparently had particularly dramatic effects on marine organisms. At the present rate of discovery, it will take centuries before most insect species are given a name and known only on the basis of preserved specimens and pub- lished descriptions, and that is simply too little too late. The behavior, life histories, ecological interactions, and biology of most insects in our own yards and city parks are largely unknown, let alone the millions of species in remote regions. We will never know the full extent of what we are losing. Exploration to fill huge gaps in the fossil record of insects is another need for fully understanding insect evolution. The periods of our greatest ignorance about insects are scattered throughout the fossil record. The Paleocene is represented by very few deposits but is crucial to understanding the impact of the end-Cretaceous extinctions on insects. The Jurassic is known essentially just from Europe and Asia, so deposits from other continents would clarify pre-Cretaceous (pre- angiosperm) diversity. The Late Triassic is well represented, but the Early Triassic is essentially unrepresented for insects. Early Triassic deposits would be valuable for gauging the impact of the massive end-Permian extinctions on insects. The Early Carboniferous is completely unknown for insects, but the sudden diversity of winged insects in the earliest part of the Late Carboniferous attests to their existence well before this time, probably even into the Devonian. In fact, we consider the discovery of Early Carboniferous insects the most significant problem for insect evolution, and it may be one of the few ways – perhaps the only way – to resolve the controversy over the origin of wings. Devonian remains 15.3. The North America burying beetle, Nicrophorus americanus, are extremely sparse, with fragments known from just three was abundant at one time but now clings to survival in a few isolated populations. Its dramatic decline since the 1930s is poorly understood deposits, but this period is clearly crucial to understanding but may be related to pesticides. AMNH; length 21 mm. the early history of hexapods. We can think of no more inspiring prospect than future select for individuals that have naturally occurring mutations generations exploring the millions of Recent and extinct in enzymes that detoxify the chemical, and these individuals species of insects – an essentially endless reservoir of discov- reproduce to form whole resistant populations. The actual ery that requires very modest budgets compared to other casualties unfortunately are the parasitoids and predators of scientific pursuits, like space travel. Unlike Recent species, the pest species, as well as harmless species caught in the however, fossils locked in the earth are under less threat since cross fire, because they do not have the array of detoxifying excavations for strip malls, housing developments, and enzymes typically found in phytophagous pest insects. The hydroelectric dams can actually uncover fossils that would end result is that we will increasingly see fewer species of but- be otherwise unreachable (these activities can also destroy terflies and wild bees but more successful strains of superbug entire deposits). The most pressing problem is the protection pests, an insidious and toxic experiment of artificial selection of living diversity, not just megafauna, and if we are to pro- on a global scale. Perhaps the solution is to treat insecticides vide solutions, then entire habitats and ecosystems must be and pest species the way antibiotics are encouraged to be protected. Only as insightful and determined stewards can used against resistant microbes: sparingly and intermittently. we continue to appreciate the greatest evolutionary success The wholesale destruction and poisoning of earth’s natu- story in the history of life on earth.