Planet of the Bugs: Evolution and the Rise of Insects (22 page)

FIGURE 7.2. A beautifully preserved fossil dragonfly,
Protolindenia wittei
, from Jurassic rocks of Bavaria. The aquatic insect order Odonata (dragonflies and damselflies) is one of only two paleopteran (old-winged) insect orders which survived the Permian extinctions and flourished in the Mesozoic era. (Photo by Frank Carpenter. Museum of Comparative Zoology, Harvard University. © President and Fellows of Harvard College.)

People are still debating what the earliest dinosaur was like, but a popular notion is that the South American “rabbit crocodile,”
Lagosuchus
, might be the very first one—or certainly an early model suitable for illustrating the dinosaurs’ origins.
Lagosuchus
wasn’t, of course, a rabbit or a crocodile, but an honest-to-goodness dinosaur with the group’s defining feature: a long scimitar-like thumb-claw that could swing in two directions. Like crocodiles, they were long, slender, graceful, and they had an extended narrow snout and lots of sharp teeth; like rabbits, they had long, powerful hind legs, which allowed them to run and jump. By some accounts they could climb trees and probably hop from tree to tree.

According to the orthodox view, the very first dinosaurs were car
nivorous predators. Because of their sharp teeth and flexible thumb-claw, which was good for slashing and ripping, we can safely assume that
Lagosuchus
, for instance, was a meat eater. Then along came
Plateosaurus
, the first of the plant-feeding dinosaurs. The plateosaurs could reach up into the lower branches, grab them with their thumb-claws, and pull them low enough that, reaching with their long necks, they could eat the tasty leaves. Yes, bloody red meat and green leafy salad, that’s what the dinosaurs craved and ate. That’s what all the good books tell us. What more could they want?

It’s probable that dinosaurs craved dietary diversity. Maybe they had enough sense to improve their nutrition by consuming a wide variety of small tasty items, like soft-bodied insects that would be fully digested and not show up well in dinosaur coprolites. Virtually all living vertebrate animal groups feed extensively on insects. Fish do—at least freshwater fish eat lots of bugs—and so do amphibians, reptiles, birds, and most mammals. Early mammals were insectivorous, and so were early primates, especially lemurs. Many human cultures even include insects as part of a broader diet because they are an excellent source of protein, fat, calories, several trace minerals, and B vitamins.
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How can we suppose that only dinosaurs, among all the major vertebrate bloodlines, ignored them? Small predatory dinosaurs and the young certainly wouldn’t have turned up their noses at an insect meal. But even the plant feeders must have eaten lots of bugs, either accidentally or intentionally. Some dinosaurs might have preferentially selected plant parts with the most edible insects because, like bacon bits in a salad, they would have provided more nutrition than just conifer needles and cycad fronds alone.

Let’s think for a moment about the kinds of insects modern vertebrates like to eat. Mostly they fall into one of two categories: relatively large insects, like fat wood-boring beetle larvae or shrimp-sized caterpillars, or much smaller ones that can be located easily in large numbers. These might be swarming species that are super-numerous at certain times or gregarious species that live in large groups. During the Triassic, plenty of juicy wood-boring beetle grubs were chewing in fallen trees, large cicadas were tippling on forest vegetation, and other insects were feeding in leaf litter and soil or tunneling in plants. Dinosaurs had eyes and ears, of course, so they could see the insects moving
on surfaces and hear them chewing through decaying logs. And their nimble thumb-claw was a perfect tool for digging grubs from rotten wood. Surely the early small dinosaurs hunted for beetles there, just like woodpeckers and long-clawed insectivorous primates. Also, even though the social insects didn’t exist in the Triassic, plenty of swarming species would have provided the first dinosaurs with seasonal feasts. While at rest, they need only have used their tongues to lap these insects off foliage. Then again, the earliest dinosaurs certainly had the ability to jump at flying insects and snatch them from the air. The homopterans, which by the end of the period had more species richness than any other insect order and were super-abundant, also provided a likely meal. Because of their piercing mouthparts, planthoppers are literally stuck in place while they are drilling for food. For the dinosaurs, they would have been easiest to find.

Insect diversity must have shaped dinosaur diversity, not only because various small and herbivorous dinosaurs likely depended on bugs for protein, but also because the carnivorous dinosaurs ate an assortment of tiny amphibians, reptiles, and mammals, all of which were largely insectivorous. In return, dinosaurs likely shaped the evolutionary patterns of some insects. Although the Triassic ended long before the advent of alkaloid-bearing flowering plants, ancient species had various secondary chemical defenses, which some Triassic plant-feeding insects might have adopted to fend off dinosaurs. Moreover, these insects might have evolved aposematic warning coloration—bright yellow, orange, and red—since modern insects with chemical defenses tend to evolve this kind of protection when visually searching predators feed on them. Others might have evolved cryptic colors that resembled plants, wood, or soil. And some might have followed a different route altogether, developing behavioral escape mechanisms; it’s certainly possible that the mayflies’ and cicadas’ mass synchronized emergences adapted and were fine-tuned in response to intense dinosaur predation. In time, Triassic insects exploited the dinosaurs: there emerged dung-feeding beetles that harvested dinosaur droppings as well as blood-feeding and, possibly, parasitic insects, which might have fed directly on them.

The Dinosaurs’ Buggy World

Several important insect orders survived the Permian, eleven of which thrived during the Triassic years and still survive today: Ephemeroptera (mayflies), Odonata (damselflies), Blattaria (roaches), Orthoptera (crickets), Plecoptera (stoneflies), Homoptera (planthoppers), Neuroptera (lacewings), Coleoptera (beetles), Mecoptera (scorpionflies), Trichoptera (caddisflies), and Diptera (true flies). More than 90 percent of the Late Triassic insect species belonged to these eleven orders. Moreover, at least eight new orders first appeared during the Triassic. Seven of them seem more or less familiar because they also still exist: Phasmatodea (stick insects), Embiodea (webspinners), Dermaptera (earwigs), Hemiptera (true bugs), Megaloptera (dobsonflies), Raphidioptera (snakeflies), and Hymenoptera (wasps). But one order of new insects might legitimately be considered the most distinctly Triassic because it lived only during the period. These are the giant titan insects of the order Titanoptera.

The titans were large orthopteroids with sharp chewing mouthparts, and they looked a bit like oversize katydids. They had spiny front legs suitable for grasping victims, and they are thought to have preyed on other insects. The largest of the titans had wingspan of thirty-six centimeters (more than one foot wide) and were large enough to catch and eat small vertebrates like salamanders and frogs. The males had large, reticulated sound-producing areas on their front wings, and during the mating season, they must have produced loud songs as they called for mates and defended territory in the forest vegetation. No one knows why the titans disappeared after the Triassic times, but their extinction corresponds closely with the diversification of small dinosaurs and the appearance of the first birds. So maybe the titan insects were just too big to be that noisy and get away with it. Perhaps they were the first casualties of the emerging brigade of insectivorous feathered dinosaurs.

The phantom insects of the order Phasmatodea (also known as Phasmida)—or as we commonly call them, the stick insects and the leaf insects—evolved a highly successful strategy for surviving in a forest filled with hungry dinosaurs: crypsis, the development of forms and colors that provide camouflage. Some lost their wings and developed long, thin bodies resembling sticks, while others evolved green
wings that resemble leaves. All of these insects chew on plant tissues, and they further avoid predators by mostly feeding at night. Phasmatodeans didn’t evolve crypsis because they wanted to look like plant parts. Rather, crypsis is the result of natural selection enforced by visually searching predators. Those phasmatodeans with the most convincing forms and colors survive to reproduce, while those with less-effective colors tend to get eaten more often. Notably, stick insects first evolved in dinosaur-ridden tropical forests even before the birds appeared. Maybe the wild diversity of phasmatodean body forms in the modern world is, in part, a legacy of dinosaur feeding habits.

FIGURE 7.3. A very cryptic stick insect from Ecuador, an example of the insect order Phasmatodea, which began its successful rise in the Triassic period. (Photo by Angela Ochsner.)

If the dinosaurs tilted a curious eye at stick insects from time to time, they surely must have seen another mystifying creature. Dur
ing the Late Triassic, layers of white silk started to cover tree trunks, rocks, and leaf litter. These weavings were the handiwork of the elusive order Embiodea, also known as webspinners. The lively web-spinners evolved their own strategy for totally avoiding dinosaurs. They developed very small body sizes, invented a whole new way of making silk, enveloped their world with the material, and stealthily went into hiding, living peaceably for 220 million years by chewing on lichens and decaying vegetation. Instead of spinning silk from their mouth or anus, the webspinners spun it from a novel silk gland in their front legs while they walked. Because of this unusual habit, I like to point out that they are the insects most like Spiderman: they are the only ones to use their “wrists” like Peter Parker. Heck, even spiders don’t do that. Spiderman really should have been called the “Webspinner” or “Embiidman.”

Scurrying in the leaf litter, with long pincerlike structures on their tail ends, the earwigs joined the webspinners. These are the “skin-winged” insects of the order Dermaptera—and the origin of their name is obscure. Legend has it that during the Middle Ages, earwigs used to crawl into people’s ears or wigs while they were sleeping. That may well have been the case, as earwigs are nocturnal scavengers that feed mostly on decaying vegetation then look for dark hiding places in the morning. These pinching little insects are so annoying that I’m going to speculate that even the dinosaurs didn’t like them much.

I imagine that a morning in the life of the little Triassic dinosaurs looked something like this. After a nice relaxing sleep, they rose early as the sun’s rays slanted through a dense forest cover of cycads and ferns and got rid of their earwigs. After quickly looking around, they probably started the day by trotting down to a nearby stream for a refreshing drink of cool water. Then maybe after picking a few newly emerged aquatic insects off the streamside vegetation, they began to forage and tear apart some rotten, fallen trees by the water’s edge in search of yummy beetle grubs. There were plenty of grubs to satisfy the heartiest appetite, but sometime during the Late Triassic the dinosaurs started encountering the pupae of yet another new insect in the pulpy streambed logs, large and tasty ones that would not go down so easy because they could bite back. These pupae of the dobsonfly are notable in being the only pupae that have functional mandibles, with which they are able to defend themselves. When the adult dobsonfly
emerges from its pupation chamber, it has very large wings; hence the scientific name Megaloptera, which means large-winged insect.

FIGURE 7.4. This webspinner from Thailand,
Eosembia auripecta
(family Oligotomidae), is an example of a successful but cryptic insect order that evolved during the Triassic, alongside little dinosaurs. This tiny female is only about 1.8 centimeters long. (Photo by Janice Edgerly-Rooks.)