CY501-C01[001-041].qxd 2/14/05 4:05 PM Page 1 quark11 Quark11:Desktop Folder: 1 DiDiversityversity and Evolution and Evolution cockroaches, but this also brings up a very important aspect INTRODUCTION about fossils, which is their proper interpretation. Evolution begets diversity, and insects are the most diverse Fossil “roachoids” from 320 MYA to 150 MYA were actually organisms in the history of life, so insects should provide pro- early, primitive relatives of living roaches that retained a found insight into evolution. By most measures of evolution- large, external ovipositor and other primitive features of ary success, insects are unmatched: the longevity of their lin- insects (though they did have a shield-like pronotum and eage, their species numbers, the diversity of their forewings similar to modern roaches). To interpret roachoids adaptations, their biomass, and their ecological impact. The or any other fossil properly, indeed the origin and extinction challenge is to reconstruct that existence and explain the of whole lineages, it is crucial to understand phylogenetic unprecedented success of insects, knowing that just the relationships. The incompleteness of fossils in space, time, veneer of a 400 MY sphere of insect existence has been peeled and structure imposes challenges to understanding them, away. which is why most entomologists have avoided studying fos- sil insects, even beautifully preserved ones. Fortunately, there Age. Insects have been in existence for at least 400 MY, and if has never been more attention paid to the phylogenetic rela- they were winged for this amount of time (as evidence sug- tionships of insects than at present (Kristensen, 1975, 1991, gests), insects arguably arose in the Late Silurian about 420 1999a; Boudreaux, 1979; Hennig, 1981; Klass, 2003), includ- MYA. That would make them among the earliest land animals. ing research based on DNA sequences (Whiting et al., 1997; The only other terrestrial organisms of such antiquity are a Wheeler et al., 2001; Whiting, 2002), so an interpretive scaf- few other arthropods, such as millipede-like arthropleuri- folding exists and is being actively built. Entomologists are dans and scorpion-like arachnids, and some plants. But age beguiled by the intricacy of living insects, their DNA, chem- alone does not define success. Various living species belong istry, behavior, and morphological detail, as the electron to lineages that are hundreds of millions of years old, like micrographs throughout this book partly reveal. But, ignor- horsetails (Equisetum), ginkgo, horseshoe “crabs” (Limulus), ing fossils relegates us to a small fraction of all insects that and the New Zealand tuatara (Rhynchocephalia), all of have ever existed and seriously compromises our under- which, and many more species, are vestiges of past diversity. standing of insect evolution. The living coelacanth (Latimeria), as another example, is the Fossils provide unique data on the ages of lineages, on sole survivor of a 380 MYO lineage, and the very synonym for radiations, and on extinctions (Figure 1.1). Social bees, for “relict.” Not so for the insects. While there are some very sig- example, occur today throughout the world’s tropics. How- nificant extinct insect lineages, such as the beaked Palaeodic- ever, based on diverse fossils in amber from the Baltic region tyopterida, most modern insect orders appeared by 250 MYA, – an area today devoid of native advanced social bees aside and many living insect families even extend to the Creta- from the western honey bee, Apis mellifera – they were unex- ceous about 120 MYA. Some living insect families, in fact, like pectedly diverse in the Eocene 40–45 MYA (Engel, 2001a,b). staphylinid beetles and belostomatid water bugs, appeared Ants and termites existed for 50–100 MY before they became in the Late Triassic approximately 230 MYA. By comparison, diverse and abundant (Grimaldi and Agosti, 2000; Dlussky 120 MYA only the earliest and most primitive therian mam- and Rasnitsyn, 2003), indicating that sociality per se is insuf- mals had appeared, and not until 60 MY later did modern ficient for ecological dominance (rather, highly advanced orders of mammals appear. Perhaps the most recited exam- societies in huge colonies make certain ants and termites ple of evolutionary persistence concerns 300 million years of ecologically dominant today). Tsetse flies (Glossinidae) 1 CY501-C01[001-041].qxd 3/10/06 6:36 PM Page 2 Quark07 Quark07:QUARK BOOKS:CY501: 2 EVOLUTION OF THE INSECTS 1.1. A fossil plant hopper of the living family Issidae, in Miocene amber from the Dominican Republic. Fossils are the only direct evidence of extinct life so they contribute unique insight into reconstructing evolutionary history. M3445; wingspan 8 mm; Photo: R. Larimer. occurred in Europe and North America in the Oligocene and Fortunately, the voluminous and scattered primary litera- latest Eocene, 30–40 MYA, far outside their range in Africa ture on fossil insects is now summarized in several compen- today. Giant odonatopterans – griffenflies – cruised the Per- dia. The treatise by Carpenter (1992) is a catalogue of fossil mian skies, their size possibly enabled by the high oxygen insect genera described up to 1983, illustrated with repro- content of the atmospheres at the time (Dudley, 2000). When duced drawings of the type species for many genera. Since fossils provide insights like these, the greatest sin of omission 1983 about 500 families and 1,000 genera have been added to arguably is avoidance of the fossil record, despite the chal- the insect fossil record. Carpenter’s treatise is nicely comple- lenges to studying fossils. Such avoidance is certainly not for mented by the volume by Rasnitsyn and Quicke (2002), since a shortage of insect fossils. the latter reviews major fossil insect deposits, insects in The insect fossil record is surprisingly diverse and far ancient ecosystems, and the fossil record and relationships more extensive than most entomologists and paleontologists within orders, particularly of extinct families. The volume by realize. Hundreds of deposits on all continents harbor fossil Rasnitsyn and Quicke, though, uses names of insect groups insects (Rasnitsyn and Quicke, 2002; Chapter 2). Also, the from Laicharting (1781), which no one else uses or even manner in which insects have become fossilized exceed that of probably all other organisms except plants (Chapter 2). Insects are commonly preserved as compressions in rock (particularly their wings), but they are also preserved as exquisite three-dimensional replicas in carbon, phosphate, pyrite, and silica; as original cuticular remains from Pleis- tocene and Holocene tar pits, bogs, and mammalian mum- mies; as remains of their galleries and nests; and as inclu- sions in chert, onyx, gypsum, and of course amber. Insects are the most diverse and abundant fossils in ambers around the world (Grimaldi, 1996), though fossil resin records only the last third of insect evolutionary history. More recent exploration of fossilized plants has revealed a wealth of insect feeding damage (Scott, 1991; Scott et al., 1991; Labandeira, 1.2. A common halictine bee, visiting a flower in Vancouver, Canada. Flowering plants, and therefore much of terrestrial life, depend in large 1998), including specialized relationships between insects part on insect pollinators. Nearly half of all living insects directly interact and plants. with plants. Photo: R. Swanson. CY501-C01[001-041].qxd 2/14/05 4:05 PM Page 3 quark11 Quark11:Desktop Folder: DIVERSITY AND EVOLUTION 3 1.3. The diversity of life shown as proportions of named species. recognizes, and their systems of relationships (based almost now known. We have adopted Hennig’s approach here, draw- entirely on fossil evidence) often conflict with phylogenies ing fossils into the fold of the spectacular Recent diversity of based on expansive evidence from living insects. Short insects, but in a much more comprehensive treatment and reviews of the fossil record of insects include Wootton based on original study of many fossils. (1981, for Paleozoic insects only), Carpenter and Burnham (1985, now rather dated), Kukalová-Peck (1991), Ross and Species and Adaptive Diversity. The daunting number of Jarzembowski (1993), Willmann (1997, 2003), Labandeira Recent species of insects is well known to naturalists (1999, 2001), and Grimaldi (2001, 2003a). The volume by (Figures 1.2 and 1.3). Though there are nearly one million Hennig (1981) attempted to synthesize the geological record described (named) species, the total number of insects is of insects with relationships of living insects, but the evi- believed to be between 2.5 million and 10 million, perhaps dence he drew from was very limited compared to what is around 5 million species. In an age of such technological CY501-C01[001-041].qxd 2/14/05 4:05 PM Page 4 quark11 Quark11:Desktop Folder: 4 EVOLUTION OF THE INSECTS sophistication and achievement, it is remarkable that there is have evolved warning, or aposematic, coloration either to an error range for estimates of insect species in the millions. advertise their venomous or toxic defenses or to mimic such Despite this fundamental problem, without a doubt the species (e.g., Figures 13.88, 13.90). No group of animals pos- diversity of any other group of organisms has never been sesses the chemical repertoire of insects from pheromones to more than a fraction of that of insects. The enduring ques- toxic defensive secretions (Eisner, 2003). Only plants are as tion, of course, is: Why? The arthropod design of an exoskele- diverse in their chemical defenses, and in many cases phy- ton with repetitive segments and appendages preadapted tophagous insects sequester host plant toxins for their own insects for terrestrial existence, and wings further refined use. this design by vastly improving mobility, dispersal, and Our time traveler to 330–240 MYA would also have noticed escape. Judging just from Recent species, though, a more no chorusing frogs or song birds, not even dinosaurs.