Planet of the Bugs: Evolution and the Rise of Insects (31 page)

At this high elevation, it is easy to imagine and appreciate the solar-panel hypothesis for the origin of insect wings. It is cold here, even though we are situated nearly on the equator. The elevation, the forest shade, and the ever-present water conspire to put a definite chill in the air by day. At night it is even more surprisingly cold. Passing the tall tree-ferns, I visualize Devonian-age insects basking themselves on the upper fronds. We have seen some of them today: machilid jumping bristletails on logs and stones, and primitive wingless diplurans under rotting logs and leaf litter—only slightly modified survivors of the ancient times.

Pausing at a stream crossing, I see taller trees arching over the rivulet; a storm has dislodged one into the stream, from its eroded banks. The water is clotted with decaying, algae-matted, woody debris. I linger here to meditate on the Carboniferous times, and to visualize
the masses of plant materials inundating the ancient waterways. At Yanayacu you can touch, smell, and feel the rotting plants, and maybe get a bootful of dark, tannin-tinted mud, suitable for fossilization—if only the insects, worms, algae, and bacteria don’t get to it first. It is easier to imagine the Carboniferous forests at this elevation, because here we are too high up for many termites, and the fallen mossy wood accumulates deeply. Lacking termites, as in Carboniferous days, the storm-wracked trees at Yanayacu, always dripping with water, are more slowly eroded by bacteria, algae, fungi, lichens, and even gigantic tropical earthworms. My coal-swamp reverie is gently broken by the flight of a damselfly along the streamway—an apt reminder of the Carboniferous insects that evolved large wings and could fly. The fair damsel pauses briefly on a fern frond until, disturbed perhaps by a raindrop, she continues her flight upstream. I break from my Carboniferous reflections and continue trekking downslope.

As we make our way along the trail, I notice further that Yanayacu cloud forest is a mosaic of past survivors. We see the modern relatives of the Permian neuropterans, lacewings, and beetles. We see the descendants of Mesozoic sawflies, and the great-great-grandnephews of feathered dinosaurs still flit and twitter in the treetops. But the Cretaceous innovations—the social wasps, ants, flowers, and especially the exposed-feeding caterpillars—greet us riotously.

Slipping briefly in the mud, instinctively catching my balance like a soggy Tai Chi master, I flash from my Mesozoic daydreams back to the present, and continue my Cenozoic reverie. We follow the watercourse now along slopes that were uplifted during the Cenozoic years. The chilled waters gurgle down toward the Amazon basin—but that is not our destination. For on the Andes’s eastern slopes, the insects will come to us. Over the course of millions of years, pulses of Amazonian insect species have migrated to the highlands and adapted to its cooler climate.

My destination is the forest at the base of the stream trail. I reach the site with considerable anticipation and excitement. Anticipation, because near here, two years earlier, my graduate student, Andrew Townsend, fortuitously sampled a specimen of a new euphorine wasp species, which belongs to the group of microscopic parasitic wasps that has become the focus of my life’s research. Drew did not realize his discovery at the time, but I later found the new wasp among
his samples when we studied them under microscopes in my remote Wyoming laboratory. My excitement doubled because the specimen was not only a new species, perhaps never seen before by human eyes, it was also a new genus.

I have returned with my research team to search for more undiscovered insects. I am hoping to find more specimens of my new wasp genus, so we can better understand its variation, but I know that the task is daunting and has little chance of succeeding. Finding one rare three millimeter long wasp in the Ecuador cloud forest is far, far more difficult than attempting to find a needle in a haystack. If only it were that easy.

We come armed with the knowledge of precisely where and when the wasp was collected, and we know a simple sampling method that might work: yellow pan trapping—the very method Drew used to collect his wasp. Bright yellow plastic bowls are placed along the forest trails. They are filled with slightly soapy water (in this case I’m adding a surfactant, the same stuff you put in your dishwasher to remove water spots), to reduce the surface tension so that tiny insects will more easily fall into the fluid. Flying insects see the Day-Glo yellow bowls as bright spots on the forest floor, and because many of them are strongly attracted to the color, they dive right in. In a dry climate like Wyoming the bowls would dry out quickly and need to be replenished. But at Yanayacu cloud forest the ever-present raindrops keep them filled. You just need to scan the bowls frequently to pick out promising specimens, or filter them out with a fine-mesh net.
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Drew placed 20 bowls and found one golden nugget. I’m here to up the ante by placing 200 along the same trails. But will the method work again? I am troubled by the thought that very likely the insect I’m searching for lives most of its life high up in the forest canopy. I may be hiking directly under thousands of them. Perhaps two years ago that one particular wasp randomly spotted a yellow dot on the floor and flew down to investigate. Perhaps decades might pass before anyone sees it again.

Rise of the Imagobionts

My worries, while realistic, proved to be unfounded. After four days of hard work we managed to find one more specimen of the new species, and by the end of the week I was very pleased to have found three of
them. The field station manager, Jose Simbaña, had also been running yellow pan traps for ten days a month, every other month, for the last year. His trap samples consisted of pint-sized jars full of tens of thousands of microscopic insects, pickled in alcohol. By day we checked my yellow pans, and by night we worked in the laboratory, sifting through Jose’s samples one spoonful at a time. We looked at so many samples under the microscope that late at night I could close my eyes and see the burned-in images of insects floating in a sample dish. At the beginning of the week, I joked that when I found my wasp, everyone would know it because I would shout “Eureka!” By the time I finally found one, I was so physically and mentally exhausted that all I could manage was a low whistle and “Here it is.” I’d estimated that we looked at more than a hundred thousand other insects for each one of my new wasp species that we spotted. Why are some insects so numerous, while others are extremely scarce?

FIGURE 10.1. A solitary male of a parasitoid wasp,
Napo townsendi
, perches on a
Dendrophorbium
leaf in the Yanayacu cloud forest in Ecuador. (Photo by Andy Kulikowski.)

Different insect species have different behaviors that affect their
population levels and relative abundance. In a forest like Yanayacu, good examples of very common insects are the fungus gnats. The maggots of these tiny flies breed in enormous numbers in the forest floor’s decaying leaves. The flies are everywhere, and they dive into the yellow pans like crazy. If you wanted to, you could easily sample hundreds of them in a morning’s work. Other insects, such as ants, are social, so again it’s not difficult to sample large numbers of them.

My new wasps, on the other hand, are solitary insects that belong to a larger group (subfamily) of microscopic wasps known as the Euphorinae.
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They are also a special kind of parasitoid whose unusual biology contributes to their scarcity: they live and feed as immatures inside the bodies of other adult insects, such as beetles, bugs, bark lice, green lacewings, ants, bees, and even other parasitic wasps—a difficult kind of parasitism that I call imagobiosis.
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Adult insects are the least abundant life stage for any insect population, and are therefore harder to locate, compared with eggs or larvae. An adult insect is densely armored and may defend itself by biting, kicking, or spitting chemicals. It is fully mobile, so if all else fails it can run or fly away. How did imagobionts evolve to overcome these obstacles?

In our previous discussions of parasitism, we considered idiobiosis, where venom is injected into the host insect, rendering it permanently paralyzed. Idiobionts tend to attack hosts that are embedded in plant tissues or wood (and are therefore barely moving in the first place) and alter them so they can’t move any more. That strategy for living and feeding worked really well when it was first developed back in the Jurassic days, and it still works well for tens of thousands of modern parasitic wasp species. But the idiobionts are pathetically slow at laying their eggs. A wasp that attacks beetle larvae concealed deep in wood commonly spends the better part of an hour or more just laying one egg. It needs to drill a hole through dense wood, insert its ovipositor, locate the host beetle, inject paralyzing venom, lay an egg, then withdraw its ovipositor from the plant substrate. Idiobiont wasps apparently could not evolve to attack mobile adult insects directly. An adult would get fed up and fly away before the wasp could get the job done. The evolution of the ability to parasitize adult hosts required koinobiosis.

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.