Planet of the Bugs: Evolution and the Rise of Insects

by Scott Richard Shaw

  Koinobiosis, you may recall, is a style of parasitism where more exposed, abundant, and mobile hosts are injected with eggs, and where larval development is delayed until the host becomes much larger. The more modern and common style of parasitism practiced by thousands of wasp species in this planet’s forests, it allows koinobionts to successfully find and eat a much broader array of insect species, especially young caterpillars feeding on plants. Since they tend to attack insects that are exposed or only shallowly concealed in plant tissues, koinobiont wasps evolved very fast egg-laying abilities. One of these wasps can run up to a caterpillar, stab it with its saberlike ovipositor, and inject an egg into the caterpillar’s body within several seconds. This rapid attack set the stage for adult parasitism.

  The imagobiont euphorine wasps have evolved even faster egg-laying abilities. They fly or run at their adult host; slip their razor-sharp ovipositor through its membrane, between its exoskeletal plates or into its anus; and blast an egg into it, all in just a fraction of a second. An imagobiont larva develops immediately, often killing the host, and emerges in a few weeks.

  The Million Bug March on the Forest

  Why would an insect bother to live inside another adult insect, when there are so many more less-mobile and easier-to-attack species of caterpillars and other soft things to eat? After all, by fifty million years ago an exceptional variety of immature insects were feeding on plants, probably more species than had lived at any previous time; however, the idiobiont and koinobiont wasps were equally successful, so much so, that they would have constantly competed for those plant-feeding hosts and hammered them with parasites. We see that struggle today, in a forest like Yanayacu, where any given caterpillar might yield as many as ten or fifteen different parasitic species. But when imagobionts came along and started attacking adult insects, no other species was competing for the available food inside them. As a result of invading this entirely novel adaptive zone, the adult-parasitoids diversified rapidly.

  When organisms adapt to colonize previously unoccupied niches, these unique opportunities sometimes allow large numbers of new species to evolve—a phenomenon called adaptive radiation. The Cenozoic earth provides several nice examples: the radiation of mammals into niches formerly occupied by the dinosaurs, of flowering plants and pollinating insects, of plant-feeding caterpillars on new species of flowering plants, of parasitoid wasps and flies on caterpillars and other insects, and of the epiphytic plants, especially orchids and bromeliads, into the structural complexity of forest canopies. The birds, mammals, and amphibians radiated in response to insect diversity and the availability of new niches: for example, the evolution of water-collecting concave leaf bases in bromeliads provided new habitats for frogs and aquatic insects. The rise of the insects, along with fruiting plants, promoted diversity in a particular new mammal group, the bats. Evolution of bats and other mammals, and the birds as well, provided niches for the radiation of many new parasitic insects, particularly species of bird and mammal lice, as well as fleas. Bats don’t have lice, but the niche was occupied anyway by another new group: parasitic bat flies. The history of the Cenozoic teaches a simple lesson about the nature of tropical ecology, and the nature of life in general: diversity promotes diversity. When species live together and interact, multifarious new behaviors evolve. As time goes on, more species evolve, and the more complex and interesting biological diversity seems to become.

  Adaptive radiation isn’t unique to the Cenozoic Period. Looking back over the history of life, we’ve seen some great examples of how natural selection drives it, and how new organisms diversify to occupy vacant ecological niches. Precambrian times showed us the expansion of microbial life into nutrient-rich oceans. In the oxygen-rich Cambrian we saw the radiation of multicellular respiring life, which filled the oceans with various exoskeletal animals. The Silurian showed us the invasion of land in response to ozone enrichment and the filtering of harsh solar radiation. Silurian plants and animals enjoyed an adaptive migration into shoreline niches, creating the earth’s first terrestrial communities. Devonian times gave us the expansive evolution of land plants inland and upland away from shorelines, and diversification of the insects with plant communities. The Carboniferous showed us the rapid ascent of insects with wings and the invasion of the air. Permian times were marked by the explosive evolution of insects with complex metamorphosis into numerous, previously unoccupied niches. It also showed us the single biggest setback in the history of life. But life, especially insect life, proved itself resilient, at least over the long term.

  It’s tempting to look back over the history of insects and view their diversity as steadily rising. If we were to plot this over time, starting from the origin of the first true insect about four hundred million years ago to the present day, our trend line would rise from one to seven million species, to pick a modern but conservative estimate—the number might well be higher, perhaps as high as thirty to fifty million, according to Terry Erwin. That would be a fair approximation of insect species accumulation, but the actual pattern wouldn’t be perfectly linear, because not all species evolve into new ones at the same rate, and as we have seen, sometimes extinction occurs. The end-Permian event impacted insect diversity more than the end-Cretaceous event did, so the decline in species at that time would have likely been the biggest dip of all. We can’t be sure about the exact numbers of species during each geological period, just as we are not sure about the current number, since the fossil record is too incomplete. But that trend line from one to seven million gives a good coarse view of what insects accomplished over four hundred million years.

  Tragedy of the Un-Commons

  It is tempting also to imagine that we live in the very time when insect variety, as well as that of other life on earth, has achieved its highest levels. Yet we must consider that the actual peak of living diversity was reached earlier in the Cenozoic, maybe two hundred thousand years ago, before humans extensively changed the planet. To assess our impacts on insects, we need to briefly reconsider the story of us. By twenty thousand years ago, we were probably starting to affect species diversity through fire use and hunting. With the onset of agriculture, around ten thousand years ago, we altered the natural landscape even more. This was soon followed by cities and warfare, and the resulting depletion of natural resources no doubt took some toll on small plants and animals, especially insects with narrow distributions. Over the last four hundred years or so, the advent of the industrial revolution and medicinal advances accelerated human population growth, further increasing our influence on the natural world. Over the last hundred years we have gradually become aware that humans are causing extinctions, or the threat of extinctions, of many animals and plants, not just by traditional hunting practices but merely by expanding our own habitats into theirs.

  We are currently living in the midst of another extinction crisis, one especially due to the rapid destruction of tropical forests, where the largest proportion of biodiversity is found. Since the late 1980s this has been dubbed the “biodiversity crisis,” a phrase popularized by Edward O. Wilson. We now realize that much of the earth’s biological resources are at severe risk. Insect species, especially uncommon and rare ones living in the tropics, are particularly at risk of extinction, since many have narrow distributions and occupy small, highly vulnerable niches. One published study in the 1990s estimated that the rate of extinction for microscopic insects may be as high as one to two species lost per hour. For the most part, these are very small species, many of which have never been studied or named.

  The estimated loss of species is such that over our lifetimes, a large portion of this planet’s biological diversity may be lost forever. Sure, there have been many extinction crises in the deep past, as well as many smaller episodes. But for the first time in nearly four billion years of life, a massive extinction event is being caused by the global spread of one single species, Homo sapiens. The tropical forests may be the most severely affected, but our influence is
so pervasive that we alter virtually all habitats on our planet.

  One might try to rationalize this situation by thinking that there are millions of insects, so why would it matter if any of them are lost? Some might think, who cares if one fly or bee goes extinct, if that fly or bee belongs to a species that has not been cataloged? But what if that fly—or bee, or wasp, or beetle—were a specialized pollinator of an epiphytic plant? The fall of that one insect species might cause a subsequent loss of plant species, and also of other insects that depend on them. This pattern of the removal of one keystone species stimulating the loss of others is called a cascade effect, a real phenomenon that has been documented in the ecological literature.

  We should remember that each living species is unique. Each fills a distinctive ecological role, and each encodes, in its genes, some qualities not found in other species. Some of those characteristics could potentially benefit future humans economically. Insects can be sources of oils, fibers, waxes, scents, even food. Possibly millions of species produce silk, but only a few have been domesticated and used commercially. Undiscovered insects might produce medicines that could cure human diseases. We cannot easily fly to another planet to find new, useful species, if they exist, so instead of asking whether or not we should care about the loss of insects, we should ask a more relevant question: how can we justify the current mass extinctions when we are wiping out potentially beneficial resources that can never be replaced in our lifetimes? If we could stop destroying habitats, many insects could survive, and many new ones would evolve in the distant future. Yet it would be unwise to simply wait around for all this to happen; we should instead conserve as many of the existing species as possible, in the hope that we’ll eventually discover their potential benefits.

  Setting aside the utilitarian perspective, living species are also intrinsically interesting simply because they exist. Yesterday I took a short hike in the Yanayacu cloud forest with six students, and in less than ten minutes we found a new species of the Ilatha wasp that I have been studying, one that attacks toxic Altinote butterfly caterpillars on a yellow aster plant called Munnozia. In the space of an hour we found eighteen specimens of the new wasp, and we spent fifteen minutes watching and photographing a female as she probed and laid eggs into a caterpillar. In the end, whether or not this particular species ever benefits humans really doesn’t matter. It is unexpectedly fascinating, and, moreover, discovering a new species can be a powerful teaching tool for recruiting future generations of biologists. Now more than ever it is essential to educate young students about the diversity of life, especially since living species are going extinct faster than biologists can name new ones.7

  It seems very ironic that, on the one hand, so many people are enthusiastic about the prospects of exploring space and other planets for new life forms, but, on the other, are blissfully unaware of the unexplored life here on earth. Where is the joy in sending a space probe to Mars and scraping the dust for remnants of ancient but now extinct microbes, even if they did once exist? It would be great to learn that microbes once lived there, but this discovery could wait a few decades. Wouldn’t it be just as exciting to send robotic probes or, even better, well-equipped human missions to the forests of the Amazon basin to discover strange and unknown life forms? We would be able to examine seemingly limitless new species that might go extinct in fifty years if we don’t act now to discover them. The public would be amazed at the “alien” creatures residing on our own little world, still the only place in the entire universe where we know for sure that life exists.

  Astronomers, astrophysicists, and cosmologists find the universe’s trillions of galaxies to be wonderfully complex and interesting, and they see the galaxies as the glorious outcome of the big bang. Crack open a cosmology textbook and you will probably find that the final chapters are about galactic diversity, and rarely about living creatures. As a biologist, however, I am compelled to see the living universe as the big bang’s most awesome outcome. The anatomy and physiology of a single insect is in many ways more complex than the structure and function of a star. Likewise, the assortment of living things in a tropical forest, and the intricacy and complexity of their ecological webs, is more fascinating and elegant than the physical structure of any galaxy.

  Astronomers and the public would be shocked if, one by one, galaxies began to wink out and disappear forever, especially if they were to vanish so rapidly that half would be gone in the next hundred years. What if we discovered that humans were responsible for this? Would the world stand by and allow us to extinguish galaxies, if we knew a way to stop destroying them? Everyone should be alarmed because we are daily losing our biological treasures, the living legacy of four billion years of organic evolution. Conserving a large portion of our living species isn’t that complicated. All we need to do is stop eliminating habitats and start studying this planet’s biology with a renewed vigor. If, during the next fifty years, we could promote biological education and boost research funding, we could save millions of species. The good news is that we don’t need to build a spaceship and fly to distant stars just to discover new forms of life; we only need to book a ticket to Ecuador, Brazil, Peru, Indonesia, or dozens of other tropical countries and take a walk in the forest.8 Or, if we continue destroying habitats and accelerating global climate change, we could expunge most of the remaining tropical forests along with myriads of species. The choice is ours.

  As I finish this chapter, I am looking out over the Yanayacu Valley with a clear view above the dappled forest canopy, to the furthest ridges of the eastern Andes. It is not raining at the moment, the sky is brilliant azure, ethereal mists are rising from the treetops, and billows of white clouds are balanced on the farthest, highest peak, like massive cotton balls. As far as the eye can see, in any direction, lies the forest canopy. Viewed from above, it looks like endless fields of discolored broccoli clumps. The forest in the farthest distance appears more uniformly green, but in the foreground I can see the patchiness of the trees, each crown of which is distinct, with its own shape, colors, and community of epiphytic plants. The leaves of many tree crowns are greenish but also visible are shades of yellow, white, brown, pink, orange, and red. I am looking at hundreds of different tree species forming an intricate botanical mosaic. On the nearest trees, outside my windows, I can closely appreciate the voluminous epiphyte load of diverse bromeliads, mosses, and lichens. This valley alone must contain more living species than inhabit all of the eastern United States. Just beyond my sight, over the farthest peaks, lies the species-rich Amazon basin of Peru and Brazil.

  As the sun sets, the swallows fly back to roost in the rafters above my room, and the stars begin to emerge in the quiet and misty night sky. As darkness falls, hundreds of moths begin to fly, and bats replace the insectivorous swallows. A storm rolls in over the distant Amazon, putting on a psychedelic heat lightning show over the furthest ridge. But above the Yanayacu Valley the sky is still clear. The Milky Way reveals itself with startling clarity and brightness. Some of the students sit quietly in the highest rafters, admiring the stars and meteors. Looking toward the stars, I can’t help but wonder: does life exist elsewhere?

  Postscript: The Buggy Universe Hypothesis

  No reasonable mind can assume that heavenly bodies which may be far more magnificent than ours would not bear upon them creatures similar or even superior to those upon our human Earth.

  GIORDANO BRUNO, De l’Infinito Universo e Mondi

  While in Nantucket he had chanced to see certain little canoes of dark wood, like the rich war-wood of his native isle; and upon inquiry, he had learned that all whalemen who died in Nantucket, were laid in those same dark canoes, and that the fancy of being so laid had much pleased him; for it was not unlike the custom of his own race, who, after embalming a dead warrior, stretched him out in his canoe, and so left him to be floated away to the starry archipelagoes; for not only do they believe that the stars are isles, but that far beyond all visible horizons, their own mild
, uncontinented seas, interflow with the blue heavens; and so form the white breakers of the milky way.

  HERMAN MELVILLE, Moby Dick

  We have every reason to believe that there are many water-rich worlds something like our own, each provided with a generous complement of complex organic molecules. Those planets that circle sun-like stars could offer environments in which life would have billions of years to arise and evolve. Should not there be an immense number and diversity of inhabited worlds in the Milky Way?

  CARL SAGAN, The Search for Extraterrestrial Life

  I’m beginning to wonder if we haven’t been making an erroneous assumption about the history of life. When we learned that humans were the highly unlikely result of a long series of contingent and fortuitous events, perhaps we jumped to the false conclusion that evolution is never progressive and no kinds of animals are predictable. But just because one thing is unlikely doesn’t mean that all things are equally unlikely. Instead, some events are more likely or less likely than others, as any insurance salesman can tell you. That’s why we are required to purchase automobile and homeowner’s, but not volcano, asteroid, or terrorism insurance. There may be only one human species, but there are tens of millions of insect species. This fact alone suggests to me the likelihood that insects are more probable creatures than anything else.