Planet of the Bugs: Evolution and the Rise of Insects

by Scott Richard Shaw

  Birds of a Feather, Butterflies of a Scale: Classifying Insects

  Classification is the tool we have invented for assembling living diversity into hierarchical groups. Species are grouped together with other related species into genera. For example, if you live in eastern North America you are probably familiar with Bombus pennsylvanicus, the Pennsylvania bumblebee, while in my mountainous area of Wyoming the common big bee is Hunt’s red-tailed bumblebee, Bombus huntii. These are distinct bee species, with different distributions, feeding habits, and nesting habits. Most people would generalize and simply recognize either one as a bumblebee, that is, a member of the genus Bombus. But even with this common example, the situation is far more complicated. We have about ten species of bumblebees in Wyoming, more than thirty in North America, and many more worldwide.

  Groups of similar species and genera are classified into family groups. Bumblebees, honey bees, carpenter bees, and others are assembled into the bee family Apidae, consisting of hundreds of species worldwide. Among insects, family groups can vary quite a bit in size and diversity. One of the largest, the parasitic wasp family Ichneumonidae, is estimated to contain fifty thousand to sixty thousand species, which is more than all the kinds of vertebrate species combined. Several other insect families have comparable hyperdiversity, such as the Curculionidae (weevils), the Carabidae (ground beetles), the Noctuidae (noctuid moths), and the Tipulidae (crane flies). My research specialty is the parasitic wasp family Braconidae, which does not have a generally accepted common name, but consists of more than fifteen thousand named species and is estimated to contain well over fifty thousand total species. On the other hand, a family group can be very small: the wasp family Pelecinidae consists of just one wide-ranging species from Canada to Argentina.

  Regardless of species diversity, groups of related families are assembled into larger groups called orders. The largest and commonest of the insect orders are very familiar, even if you don’t know their scientific names. Four of the insect orders are hyperdiverse, each with thousands of described species and probably millions of existing unnamed species. These four biggest orders are the Coleoptera (beetles), the Hymenoptera (wasps), the Diptera (flies), and the Lepidoptera (butterflies and moths). There are many other insect orders that are comparatively smaller but still very large, consisting of hundreds or thousands of species, and you are probably familiar with many of them, such as dragonflies, grasshoppers and crickets, termites, roaches, lice, true bugs, and, especially if you like to fish, the mayflies and caddisflies. Lots of other interesting but rare insects are classified into small and obscure orders that most ordinary people never encounter, such as the webspinners, icebugs, zorapterans, and twisted-wing parasites. The smallest and most recently discovered order of insects is the Mantophasmatodea, also known as African rock crawlers and gladiator insects, with two living species named in 2002. There are also several orders of insects that are now extinct, but were formerly quite diverse in the Paleozoic era, hundreds of millions of years ago.

  All these buggy orders are assembled into a much larger group called the class Insecta (the insects). For the most part, these are all the small creatures that people commonly call “bugs,” creatures with six legs and three body parts: the head, thorax, and abdomen. The insects in turn are classified into a larger group called the phylum Arthropoda (arthropods). This includes not only the insects but also their near relatives with hard external skeletons and jointed legs, familiar creatures such as spiders, ticks, mites, millipedes, centipedes, and scorpions, as well as less familiar beasts like the extinct Paleozoic trilobites. Lastly, the arthropods are classified in the much larger kingdom Animalia (animals).

  Six-Legged Menagerie: Understanding Insect Diversity

  Why should one group of animals have become so astronomically diverse in comparison with all other kinds of animals? How are we to understand the ecological success of insects in particular? First, insects have evolved small body sizes, which are limited by aspects of their anatomy, physiology, and ecological interactions with other creatures. Small size has promoted insect species diversity by allowing bugs to divide the world into exceedingly small niches. Plant-feeding insects, for instance, may partition one plant into many different niches. You will find different kinds of insects feeding on leaves, boring in stems, mining under leaf surfaces, chewing in flower buds or seed pods, living under bark, or boring deep into the heartwood or below ground into roots. A young beetle may spend its entire immature life living and feeding inside a single tiny plant seed, while the adult can live in a totally different niche.

  FIGURE 1.2. Fairyfly wasps (family Mymaridae) are insect-egg parasitoids that occupy minute ecological niches. Tinkerbella nana is one of the smallest known flying insects. (Photo by Jennifer Read.)

  While many common insects, such as the monarch butterfly, have a wide distribution and a fairly broad niche, the vast majority of insect species are extremely small and have very tiny, often localized niches. The tiniest insects, microscopic “fairyfly” wasps too small to see without magnification, develop inside the single egg of another insect. The specialized narrow niches of parasitic insects are also particularly impressive. There are bird lice that live only in the pouches of pelicans and mammal lice that live only in the fur and nostrils of sea lions. In fact, most different kinds of birds and mammals have specialized lice species living on their bodies, except for bats. But another group of insects has evolved to fill this ecological niche. Remarkably, there are bloodsucking parasitic flies that live in bat fur, so while vampire bats might drink your blood, their own blood is being fed on in turn. In Wyoming and other boreal parts of North America, there is a flightless parasitic beetle species, Platypsyllus castoris, the beaver parasite beetle, which lives only in the fur of living beavers. Neoneurus mantis, an insect I discovered in Wyoming, is a tiny wasp that develops parasitically inside the abdomens of one species of mound-building Formica ant. When I first found them in 1990, they were flying in association with only three particular ant colonies. Over the past two decades, two of those colonies have declined in numbers, and the parasites have disappeared. So over the past several years, I’ve only been able to find this tiny wasp in the nearby mountains in June, at only one particular ant nest.

  FIGURE 1.3. Bat flies (family Streblidae) are highly specialized, blood-feeding bat parasites. This one lives on vampire bats at Palo Verde National Park in Costa Rica.

  A second reason for their vast diversity is flight, which has allowed winged insects to expand their niches into the air. For as long as 150 million years, insects were the only animals that could fly, and that gave them great advantages in terms of their ability to escape predators and to disperse and colonize new areas. The eventual evolution of winged pterosaurs, birds, and bats didn’t drive insects out of the air. It simply provided the selective forces to drive additional diversification and specialization. But wings are also very useful for things other than flight. Insects are cold-blooded animals, and since blood flows through the veins of their wings, insect wings work very well as little solar panels for warming up on cold mornings. Insect wings are also canvasses for multifarious colors and patterns that play important roles in insect behavior, especially in courtship and mating, and facilitating mate recognition in diverse and complex environments. Wing colors can also enhance survival by either crypsis (camouflage) or aposematism (bright warning coloration).

  Third, but certainly not least, insects have evolved elaborate methods of development, including complex metamorphosis with young developmental forms (larvae) that are stunningly different from adults. That was a remarkable innovation, which allowed adult insects to avoid competing with their own offspring for food. A young insect larva, such as a caterpillar of a butterfly, becomes a feeding machine that can concentrate its activities on eating and growing, often in a concealed or protected environment. In contrast, adult insects with complete metamorphosis can concentrate their activities on courtship, mating, and egg production. Man
y feed on completely different resources from their young, or do not feed at all. The fact that more than 75 percent of all modern insect species have complete metamorphosis attests to the usefulness and success of this behavior.

  An inquiring student of insects might seek more sophisticated answers in the study of insect evolution over the past four hundred million years. An advanced course on this topic might cover the history of insects based on fossil remains, and relationships inferred by studies of anatomy, behavior, genetics, and molecular sequences. One would probably learn that the most ancient insects evolved on land in association with the most ancient land plants, in the Late Silurian or Early Devonian periods. By the Carboniferous period, the time of the great coal-forming swamps, insects had evolved wings. The innovation of flight allowed the first great radiation of insect species, including giant dragonfly-like insects and many other ancient species that no longer exist. By the Permian, insects were diversifying into increasingly modern forms, including species we would recognize as bugs, lacewings, and beetles. The development of complete metamorphosis in the Permian was a key innovation, perhaps, that allowed many insects to thrive and diversify in an unstable and changing world. Other insects, and many other creatures, were not so lucky.

  At the end of the Permian, our world experienced a massive extinction episode. In the oceans many ancient life forms, including the trilobites, disappeared for all time. On land several kinds of life went extinct as well, including some of the ancient insects that had flourished for millions of years. But others survived and diversified, especially those with complex metamorphosis. And so it was through the Mesozoic times, when the insects coexisted with the large dinosaurs. Sometime in the Early Cretaceous period flowering plants evolved, and insects adapted by rapidly evolving diverse new species. Along with the flowers came the insect associates: butterflies, bees, ants, and social wasps. Another major extinction event, perhaps triggered by a massive asteroid impact sixty-five million years ago, brought to a close the time of the big dinosaurs. The little feathered dinosaurs, which we now call birds, survived and flourished, as did many flowering plants and insect groups. And so it has gone through the Cenozoic: the ongoing coevolution of flowering plants and insects has generated awesome biological diversity and complexity in tropical forest ecosystems.

  The story of insect evolution, as revealed by their rich fossil history, though illuminating, is not completely satisfying. We might still wonder about the origins of the first insect. I began this chapter with a quote from Barry Hughart, who pointed out that “all events have an end and a beginning.” To gain a full understanding of something as complex as insect diversity on earth, we need to “understand correctly what comes first and what follows.” We need to delve even deeper into the insects’ prehistory, into the time before the Silurian myriapods heroically occupied the land, with their external skeletons and legs. The myriapods inherited these features from sea-dwelling Cambrian arthropods, which first developed body armor and mobility in response to increasing oxygen levels and predator activity in the oceans. So our story of insects must begin there in the earth’s ancient oceans, more than a half-billion years ago.

  2

  Rise of the Arthropods

  About 600 million years ago . . . all hell broke loose in organic evolution.

  DAVID RAUP, Extinction: Bad Genes or Bad Luck?

  Sometimes the destination isn’t as important as the drive.

  CHRISTINA APPLEGATE, Up All Night

  If you want to experience the geological ages of animal life on earth, then I highly recommend the scenic drive from Shoshoni to Thermopolis, through the awesome Wind River Canyon in Wyoming. The exposed rocks are a window on the past opened by the formation of the Rocky Mountains. As crust was thrust upward it tilted on its side, depositing the oldest rocks at the top of the canyon. So the drive from Shoshoni initially follows a path of Precambrian rocks devoid of animal fossils. A road sign near the canyon’s entrance marks the start of rocks from the Cambrian period, when life finally showed the inclination to crawl out of the bacterial slime, and the first animals with skeletons evolved. It was the time of the first arthropods, and is especially well known for the rapid diversification of the ones commonly known as trilobites. The scenery is stunning. This is the only highway I’ve ever seen where the ages of animal life are marked by signs.

  In a half-hour or so, you drive through all the ages. After passing through the Cambrian, the “age of invertebrates,” you enter Ordovician time, the “age of fishes.” Soon you are at the Silurian age, the time of the first land plants. Then on to the Devonian time, the so-called age of amphibians. The limestone layers of Silurian and Devonian time suffered extensive erosion in this area of Wyoming, so these layers pass in the blink of an eye. They are followed by the Carboniferous, the time of coal-forming swamps. Next you are on to the Permian, and as you approach the foothills and exit the canyon, you leave the Paleozoic era and enter the Mesozoic, the “age of reptiles,” or as we now say, the “age of the dinosaurs.” Take a brief moment to mourn the passing of the trilobites. But at the same time, you can rejoice in the appearance of the dinosaurs. They are celebrated at the end of the drive at the excellent Dinosaur Center in Thermopolis. Be sure to budget enough time to soak in the sulfurous hot springs teeming with bacteria and to also visit that impressive museum. Finally, as the road levels and the sagebrush prairies open to the valley, you enter Tertiary times, the first years of the Cenozoic era and the onset of the “age of mammals.” Lament, if you must, the passing of the big dinosaurs. But rejoice in their passing as well, because it finally, finally allowed us mammals to make our move on this planet.

  By now, the implicit human-centrist bias in some of that history I just recounted should be obvious. By noting the times of the proliferation of mammals, reptiles, amphibians, and fish, we are merely observing some tenuous history of events leading to the origin of the human species. How ridiculously unlikely it seems that we should be here at all, and how those labels for the geological periods distract from the real pattern of life’s diversity. I’ll be picking apart this human-centrist mythology bit by bit as we proceed. For now, it will suffice to examine the Cambrian.

  The Cambrian period has generally been called the “age of invertebrates.” That’s certainly not because anyone sought to glorify our invertebrate ancestry. It’s simply an observation that we didn’t initially see any of our vertebrate ancestors in fossils from Cambrian layers. Notice that we didn’t call it the “age of arthropods” or the “age of trilobites,” either of which would be apt. Calling it the “age of invertebrates” is a bit like calling it the “age of no humans.” The name subtly derides the success of arthropods by noting the absence of vertebrae rather than touting the evolution of exoskeletons. But subsequently, we did discover our likely vertebrate ancestor in Cambrian times, and what a humbling event that was. A small creature called Pikaia was discovered in the 515-million-year-old Burgess Shale fossils of Canada. Pikaia was a mere one-and-a-half-inch-long, wormlike creature that burrowed in bottom sediments. She was soft-bodied but did have an internal supporting structure: a primitive notochord, the ancestral structure of a vertebral column. Pikaia is now regarded as the most likely common ancestor of fish, amphibians, reptiles, dinosaurs, birds, and mammals. But she was such a modest ancestor that no one lobbied for a renaming of the Cambrian as the “age of Pikaia.”

  When viewed from a great temporal distance and with a nonhuman eye, the history of life appears differently. It’s certainly clear that there was no inevitable or rapid progression toward humans. A nonhuman space traveler might rewrite our biological history far more succinctly. The first three billion years or so might simply be called the “age of bacteria.” The time from the Cambrian to the present (the last half-billion years or so), the time of multicellular animal life, could simply be called the “age of arthropods.” Since the onset of animal complexity, the arthropods have been the singular successful group, both in diversity and ab
undance. The rise of insect diversity was ancient enough that the last three hundred million years or so could be dubbed the “age of insects.” The last ten thousand years, during which human civilizations have arisen, is but a miniscule and insignificant blip compared to the vast spans during which first bacteria, then arthropods, then insects in particular, have dominated this planet’s landscapes.

  The other remarkable thing about the Cambrian is simply that we survived it at all, and by “we” I mean not just humans but the entire lineage of vertebrate animals. There it was, Pikaia, our humble wormlike ancestor, tunneling along in bottom sediments. It is quite rare in Cambrian fossils and perhaps was never a very abundant creature, even in those days. In the waters above, along cruised animals like Anomalocaris, a three-foot-long, nightmarish predatory arthropod with long, spiny feeding appendages. Anomalocaris paddled along in the Cambrian seas, picking off whatever small animals it could catch—no doubt feasting on lots of trilobites. From time to time, Anomalocaris doubtless swooped down to pick off a tender Pikaia for dinner. Luckily for us humans, most of the abundant Cambrian arthropods, trilobites, also fed in the bottom sediments, peacefully, along with Pikaia; however, there is evidence that some of the Cambrian trilobites may have been predatory. There are fossils of trilobite tracks intersecting with worm burrows and resting, which suggests that some trilobites may have preyed on soft worms in the sediments. Trilobite body forms and mouthpart styles are certainly diverse enough to suggest that they had evolved a comparable variety of feeding habits. But still, if Cambrian trilobites had become extensively predatory, then it’s exceedingly unlikely that we would be here to piece together this story.