Planet of the Bugs: Evolution and the Rise of Insects

by Scott Richard Shaw

  But in Wyoming the migratory birds can be unpredictable. In some years they are late arriving, and in others they seem to bypass our area entirely. So I’ve found another indicator of spring, a resident animal that’s a sure sign every year: xyelid sawflies, which live at high elevations near conifer trees and willow bogs. In the early spring, color returns to the willow twigs, providing bright yellow, orange, and red twiggy contrasts against the brilliant white snowbanks melting along the hillsides. Soon the willow buds swell and burst into a profusion of furry pussy willows. Once they start producing pollen, some very tiny insects arrive to gather a high-protein meal. Among them are the xyelid sawflies, which after emerging from their overwintering cells in the soil are usually the first adult insects to become active in the Wyoming mountains.

  Xyelid sawflies are living fossils, relicts of the Triassic years. They are the most primitive living group of the insect order Hymenoptera, the lineage that now includes some of the most successful of all insects: bees, ants, social wasps, and parasitic wasps. And what curious creatures they are. The base of their antenna and maxillary palpus (a segmented feeding appendage located behind the mandible) are leglike, giving them the appearance of having extra legs near their front end. They use their leggy mouthparts to gather a meal, and then they fly up to the tops of nearby conifers, just as they did millions of years ago. The name “sawfly” refers to the females’ serrated egg-laying organ, what entomologists call an ovipositor. With it the females abrade slits into developing pollen-bearing pine cones and insert eggs into the nutritious plant tissue. After the eggs hatch, the young xyelid larvae feed on the tissue, drop to the soil, and finally dig cells in which they pupate until the following spring. This habit served the ancient sawflies well, as it allowed their larvae to live in the treetops, where they were able to evade even the tallest of the dinosaur macroherbivores: the brontosaurs.2

  Breaking the Silence of Permian Extinction: Triassic Not-So-Silent Spring

  What period of life could be more springlike than the Triassic? After the catastrophic end-Permian mass extinction came a time of rebirth. The landscape was drier than during the Paleozoic years, but the Triassic forests became dominated by new kinds of plants: mostly conifers, cycads, gingkoes, and ferns. Triassic conifers are now famous because some of them fell into rivers and were washed into coastal lowlands, where they were buried in volcanic debris. Over time these trees fossilized into colorful quartz minerals and became the petrified trees located across the southwestern United States, especially in Arizona’s Petrified Forest National Park.

  There were no flowering plants, fruit, berries, or grasses in the Triassic. Still, with the prevalence of conifers and the loss of the ancient insect orders, Triassic forests would look familiar to us. Myriad aquatic insects, including mayflies, damselflies, stoneflies, and caddisflies, fluttered along the streams. The forests buzzed and hummed as legions of new species appeared, especially among the roaches, crickets, planthoppers, true bugs, lacewings, beetles, scorpionflies, and true flies. The Triassic also saw the origin of several new insect groups, some unfamiliar and some now very familiar, such as the walking stick insects, webspinners, earwigs, dobsonflies, snakeflies, and wasps. Vertebrates were busy snapping at all these creatures: the first turtles, salamanders, frogs and of course, most notably, the very first dinosaurs had arrived. By the Late Triassic, several dinosaurs roamed the forests, including some very small bird-sized species like Saltoposuchus and Procompsognathus, and a few larger ones like Plateosaurus.

  FIGURE 7. 1. A xyelid sawfly, Pleroneura californica, a sure sign of spring in Wyoming, is a relict of an ancient insect family that arose during the Triassic period.

  You’ve no doubt heard lots about the dinosaurs’ impressive dynasty. They certainly became the dominant macroherbivores and macropredators of the Mesozoic era, fiercely overshadowing the mammals for a hundred million years or more.3 But I’ll put a twist on the story: dinosaurs were impressive characters in a big Mesozoic world already largely filled with insects. We don’t usually hear much about the both of them together, but dinosaurs must have influenced the insects, and I’m sure the insects affected the dinosaurs.

  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 carnivorous 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.4 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. During 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.)

  But the dobsonfly’s wings are not its most impressive feature. Surely the males’ jaws are, even to the eye of a young dinosaur. These jaws are long and sicklelike, and they look like the tusks of a wooly mammoth. Oddly, male dobsonflies don’t use them much for defense, but mostly for grasping a female during courtship and mating and for asserting their territory when confronted by rival males. It’s the female who can deliver a surpris
ingly powerful and painful bite with her much shorter jaws. After mating, the female dobsonfly lays her eggs on the underside of a fallen tree or other object overhanging a stream, so her young, when they emerge, can fall directly into the water. The larvae aggressively prey on other aquatic insects, as well as small fish and small amphibians. They grow into large larvae, called hellgrammites, which also deliver a painful bite. Other than some paper wasp stings and ant bites, a hellgrammite bite is one of the most intense experiences insects have to offer. I know because I’ve been bitten by one, and I suspect that as the hellgrammites crawled out of water in search of pupation chambers, the foraging dinosaurs found them daunting and learned to avoid them.

  Real Bugs Don’t Eat Quiche

  When dinosaurs visited the streambeds for a morning drink, they might also have paused to observe some tiny insects moving along the moist sand near the water’s edge. They probably wouldn’t have bothered to eat these insects, which were mostly too small, but I suppose the dinosaurs might have regretted not finishing them off in the Triassic. The first of the insect order Hemiptera, the group that entomologists give the fine distinction of being called “true bugs,” this bloodline later spawned bed bugs, assassin bugs, and giant water bugs—all of which might have been capable of biting dinosaurs before the Mesozoic was over. The true bugs are more familiar to us as the pleasantly scented stink bugs, plant bugs, and seed bugs, among others, but these are all much later species.

  The Hemiptera first arose as a lineage of semiaquatic predators and beachcombers. Like their closest cousins, the plant-feeding homopteran planthoppers, true bugs have liquid-feeding, piercing mouthparts. Unlike their cousins’ beaks, however, the beak of a true bug is on the front of its face, and it’s longer and more flexible; with it true bugs started sucking the blood out of other hapless insects. True bugs had a very auspicious start, for few insect groups have diversified so greatly in terms of the new habitats they later colonized. From streambeds they moved to the water’s surface and then into the water, where most remained fully aquatic predators; a few, however, became scavengers of green algae and plankton.