Planet of the Bugs: Evolution and the Rise of Insects (19 page)

Aside from their obvious protective function, the cases serve other uses. Some caddisflies tie large ballast stones to their cases, allowing them to move along the bottom in fast currents without washing away. Most build a case with a hole at each end, which allows waste to be ejected from it and water to flow through it. Many have evolved the capacity to ventilate their tracheal gills by actively pumping water through the portable case, thereby increasing oxygen flow over their gills. This has allowed caddisflies to successfully radiate into slow-moving or still waters with much lower oxygen content.

In their classic paper, “Ecological Diversity in Trichoptera,” aquatic entomologists Rosemary Mackay and Glenn Wiggins observed that in modern aquatic insect communities, caddisfly species and genera greatly outnumber that of the mayflies, dragonflies, or stoneflies. They wondered why this should be so, and they came to a perceptive and surprisingly simple conclusion, neatly summarizing 250 million years of aquatic insect evolution with this simple statement: “We view much of trichopteran diversity as an expression of ecological opportunities made possible by the secretion of silk.”
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That wonderfully versatile substance allowed caddisflies to divide the aquatic habitat into hundreds of microhabitats inaccessible to other insects without silk. Even though many of the mayflies, damselflies, and stoneflies colo
nized the waters millions of years earlier, caddisflies were able to spin and weave their way to new lifestyles impossible for the more ancient aquatic insects.

The presence of caddisflies during the Permian suggests that primitive moths (order Lepidoptera) must also have been around, even though they do not appear in the fossil record until the Jurassic period, about fifty million years later. A lot of anatomical and behavioral evidence suggests that the Lepidoptera and Trichoptera are closely related to each other: they are what we call sister groups, which by definition originate at the same time because they share a common ancestor.
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So this is one of the better documented cases of a major gap in the insect fossil record. We know that moths—or at least protomoths—must have existed at least since the Permian, but clearly they did not fossilize well for another hundred million years. If the most primitive surviving Lepidoptera are any indication, there are obvious reasons for the gap. They are microscopically small species that mine and feed in plant tissue; the most archaic group, the mandibulate moths (family Micropterigidae) feed on fern tissue in extremely moist, nearly semiaquatic environments. Because microscopic soft-bodied insects living in moist, warm forests decompose rapidly when they die, the earliest moths did not fossilize much, if at all.

The insect order Diptera, the true flies, also originated in the Late Permian years, and although they were not very common then, they somehow managed to survive the Permian extinction and live on to become some of the most common and diverse insects in the modern world. Like caddisflies, ancient nematoceran flies had aquatic larvae that lived in cool, fresh, fast-moving water. These larvae developed various suction-cup holdfast structures for clinging to rocks in fast currents, where they fed on algae and organic debris. To this day, some of the more primitive aquatic fly larvae in existence can spin silk, which they use to anchor their bodies in a current or move safely downstream.

Why were streams so popular among Permian insects? During the period, the southern supercontinent, Gondwana, experienced extensive glaciations. Continental areas were colliding and inland areas were being raised up to greater heights. In areas were glaciers met temperate and tropical climates, melting ice and snow from upper
elevations created several cascading waters, which offered a rich new frontier of streambed nutrients for insects that could adapt to the swiftly moving currents and eddies. Mayflies and stoneflies were the first colonists to follow the streams up to higher and higher elevations. Soon they were followed by species of caddisflies, nematoceran Diptera, and aquatic predatory Neuroptera. Whether the Permian was a grand disaster or a time of plenty just depends on your point of view. For insects that were able to find and colonize new niches it was a time of grand success. The aquatic mayflies, stoneflies, caddisflies, and nematoceran flies all successfully survived the Permian and have radiated extensively since then.

Meet the Beetles and Other Bugs That Bite Their Bark

From the first humble beetle (order Coleoptera) arose a vast multitude of descendants. Modern tropical forests are home to possibly tens of millions of beetle species, and some published estimates suggest that there may be as many as thirty million to fifty million, an overwhelming number that has led entomologist Mark Moffett to describe earth as the “planet of the beetles.”
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To what do these insects owe their astronomical success? They alone have developed the ultimate body armor while maintaining the benefits of flight dispersal. A beetle’s front wing is modified into a hard shell, known as the elytron, which covers the hind wing when it is at rest. When a beetle flies, its hind wing unfolds into one that is larger than the front, and flight is powered entirely by these extended back wings: an unusual arrangement called posteromotorism. The shell-like front wings are held outstretched and can only generate lift, glider-style.

During the Late Permian there were only a few groups of beetles, about six families, and they all belonged to the most primitive suborder of beetles. They were the first wood-boring insects, living in the forest undergrowth where they buzzed and flew from one fallen dead tree to another. Their hard armored bodies protected them from insect predators while they chewed into damp decaying wood to lay eggs; this environment sheltered their larvae, wood-boring grubs, from dry air and sunlight. The beetles were among the first organisms to feed on lignin and cellulose by mixing wood with fungi.

FIGURE 6.3. A beautifully preserved fossil of
Liomopterum ornatus
(family Liomopteridae) from Permian rocks of Kansas with well-developed paranotal lobes on the first thoracic segment. This neopteran (new-winged) insect family was another casualty of the Permian extinctions. Formerly placed in the Protorthoptera, these insects are now regarded as likely stem-Plecoptera. While they became extinct, their aquatic, stream-dwelling stonefly relatives (order Plecoptera) survived and flourished. (Photo by Frank Carpenter. Museum of Comparative Zoology, Harvard University. © President and Fellows of Harvard College.)

Bark lice (order Psocoptera) joined the first beetles in the deadwood. Tiny voracious insects with chewing mouthparts and gradual metamorphosis, bark lice gnaw on organic materials under the loose bark of dead trees and can congregate in massive numbers. Their modern cousins, the book lice, will feed on paper, and if undetected they can completely destroy library books. During the Late Permian, bark lice and the first beetles teamed up with wood roaches and fungi to help quickly decompose and recycle nutrients from dead forest trees and leaf litter.
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One Main Suspect?

Many insect species may have gone extinct during or near the end of the Permian, but virtually all the orders with complete metamorphosis survived, as well as many others with gradual metamorphosis, such as bark lice, thrips, and homopterans. Only one order with complete metamorphosis, the tiny and scarcely known Miomoptera, vanished then. Two other small groups, the little-known orders Glosselytrodea and Paratrichoptera, endured beyond the Permian–Triassic boundary and became extinct much later, during the middle Mesozoic era. Maybe they were not able to adapt to the Mesozoic’s environmental changes. Maybe they were exterminated by the warm-blooded dinosaurs. Maybe they were failures, or maybe they just evolved into more modern groups. Whatever happened to the Glosselytrodea and Paratrichoptera doesn’t really matter here. The key point is that they survived the Permian.

So did a lot of the other orders, which went on to diversify and are now common. Some insects, like the Homoptera, Neuroptera, Coleoptera, and Mecoptera, enjoyed substantial species richness in the Late Permian, suffered some declines, but carried on successfully into the Triassic years. Other groups, the Trichoptera, Lepidoptera, and Diptera, had low diversity but nevertheless survived the end-Permian holocaust. They are currently among the most species-rich orders. Why did they survive? The idea that low-diversity groups, like the trilobites, are particularly prone to extinction does not seem to apply here. Some of these groups could have lived on happily, provided that they had pioneered ecological niches in unaffected habitats and had an arsenal of survival skills, like complete metamorphosis. The suspect in the Permian killings must be some kind of selective agent. We are looking for a killer that could wreak havoc on coral reefs and massacre the coastal lowlands but leave the upland communities in comparative bliss.

Perhaps one factor ties together the multifarious elements of this story. We still need to consider our old friend plate tectonics, also known as continental drift. You may have already read or learned about continental drift,
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and you may recall that before the present continents were configured, there was a time during the Early Mesozoic when the northern continents were joined in a landmass called
Laurasia, and the southern ones in a landmass called Gondwana. You may recall also that prior to that time all the land areas were united in a single vast supercontinent called Pangaea. Present-day North America was wedged directly into South America, Africa, and Europe, and present-day Africa, most centrally located, was directly connected with North America, Europe, Asia, South America, Antarctica, India, Saudi Arabia, and Australia. Although many people mistakenly assume that this massive land aggregation was the starting point for continental drift, that’s not the case at all. It’s just the origin of our modern arrangement of continents. Pangaea first formed during the Early Permian, when more ancient continental configurations aggregated, and after parts later broke away, it reformed again in the Middle Triassic. It took millions of years to assemble and corresponds suspiciously closely to the end-Permian extinction event.

As islands and continents collided, previously separated areas were merged. This would have reduced marine areas by eliminating shorelines and their wetland communities, and it would have brought different communities of plants and animals together. When these groups mixed, competition would insure that some species would dominate and become “weedy” and widespread, while other, less-aggressive species that formerly survived in isolation would be driven to extinction. As larger continental masses collided and fused together, these processes would have accelerated. Volcanic eruptions might have been triggered; inland, new mountains would have been uplifted and new rivers and streams would have emerged, ripe for insect colonization. As the land area became larger, the global climate changed. Inland areas became hotter and drier. Some plants and animals, like the insects with complex metamorphosis, were better able to adapt to these shifting environments.

Can Pangaea alone explain the Permian extinctions? As compelling as those arguments sound, the current answer is “no.” In past decades we used to think that the extinctions took place over millions of years, but careful studies of Permian period sediments in China by Douglas Erwin and other scientists has narrowed the interval of the end-Permian extinction down to a mere hundred thousand years or less. That may seem like a long time, but it is way too fast for plate tectonics alone to have been the main culprit. However, I still think it’s important to recall that they may well have been a key factor in many
of the terrestrial insects’ successful diversification. Pangaea’s formation helped bring about the Permian’s arid terrestrial climate, which is usually tagged as the reason for the holometabolous insects’ rapid evolution then.

The end-Permian extinctions don’t look like a single event or a fast event caused, for instance, by an asteroid impact. Cosmic collision enthusiasts have been searching the planet for geological evidence for more than thirty years, but they have found nothing definitive. They haven’t identified an impact crater or impact debris from this time period. Moreover, the Permian–Triassic boundary layer lacks iridium, which is contrary to what is expected for an asteroid impact. Claims of the discovery of fullerenes (“buckyballs”) in the Permian–Triassic boundary layer have filled collision enthusiasts with excitement, but these reports have been contested and are unrepeated. Impact supporters have even gone so far as to suggest that an asteroid collision may have triggered the Siberian volcanic events, which in turn obliterated the impact crater. That’s a really sexy idea, but it still lacks good evidence. The philosopher Alfred North Whitehead has advised us to “seek simplicity and distrust it.” His advice seems good in this case. We will keep seeking a simpler explanation for the end-Permian extinctions, as that is the nature of science, but for now they still appear to be complicated.