Planet of the Bugs: Evolution and the Rise of Insects

by Scott Richard Shaw

  Later in the Mesozoic era, one group, the water skaters, learned to walk on water more efficiently than any other animal. They are the only insects that moved back downstream and recolonized the open oceans; other true bugs moved from the waterside back into the forests. While some became specialized plant feeders alongside their relatives, the planthoppers, others, like assassin bugs, became violent predators of other forest insects. Some of these assassin bugs were blood feeders who feasted on small mammals, and they would have bitten a dinosaur, given the chance.

  Back in the forest, up in the conifers, emerged the snakeflies (the insect order Raphidioptera). I don’t suppose the dinosaurs down below would have noticed these hidden dwellers in the treetops: they probably minded their business and stayed out of the dinosaurs’ range. Most people also never see snakeflies, as they are rare worldwide, but oddly enough they’re another Triassic insect that lives on and is common here in Wyoming. We call them snakeflies because the adults have a long neck and prey on other insects, and when they attack they strike their head forward like a snake. Modern snakeflies feed almost entirely on homopteran insects like aphids; their Triassic ancestors probably did much of the same thing, moving about in the treetops and mowing down herds of defenseless planthoppers. The female has a long, curved, taillike ovipositor with which she lays her eggs on plant surfaces, where the larvae hatch and feed like the adults. Between its long neck and ovipositor, the snakefly’s profile looks more like a brontosaur than a snake. Given the context in which they evolved, maybe we should call them brontosaurus bugs, because over the duration of the Jurassic years, while Wyoming was still a coastal lowland tropical forest, the snakeflies fed in treetops alongside nibbling brontosaurs.

  The xyelid sawflies, first of the insect order Hymenoptera, also evolved up in the Triassic treetops. I mentioned them early in this chapter not just because they’re a sure sign of spring but because of all the Triassic insects, they alone would blossom into a massive dynasty of millions of species. “Hymenoptera” means “membrane-winged,” but there is a legend about this name. In Greek mythology, Hymenos was the god of marriage; some say the name Hymenoptera derives from the joining, or marriage, of hymenopteran wings during flight by a row of tiny hooks called hamuli. These wings were helpful because they allowed the sawflies to move from their pupation chambers in the soil up into the treetops. But their sawlike ovipositors proved all the more useful, as they allowed the sawflies to place their young deep inside plant tissues, where they were safe from snakeflies that fed only on the surface.

  From those ancestral sawflies flitting about in the treetops ultimately arose diverse bloodlines of social insects: bees, ants, and social wasps, as well as nest-provisioning stinging wasps and an extraordinary array of parasitic wasps.5 Over the long run, the ovipositor was more important than wings in the rise of the wasp dynasty. Many modern wasps have lost their wings, but the ovipositor has become modified for multiple uses: for placing eggs into plant tissues, onto other insects, or deep inside other insects’ bodies. Perhaps most significantly, the ovipositor has become modified into a stinger used for both paralyzing prey and defending against predators. It sure didn’t look like much in the Triassic years, but by the Cretaceous that little sawfly dagger would transform into a toxin-filled hypodermic needle fully capable of inflicting pain on even the largest and fiercest dinosaur.

  Perhaps the most unique feature of sawflies and wasps is something that’s not externally visible: the sex of each individual is determined by whether or not an egg is fertilized. An unfertilized egg develops into a male. If an egg is fertilized, it always develops into a female. When wasps mate, the male transfers sperm cells into the female’s sperm storage organ, the spermatheca, where they remain viable for a long time. Fertilization does not take place during mating, but rather is done by the female at the time an egg is laid, allowing her to choose the sex of each of her progeny. Some ecologists suggest that this allows female wasps to assess the quality of available food and selectively place female eggs on the best items. Also, when males are scarce, it allows females to lay eggs that develop only into males—a very useful trait allowing these insects to exist in highly dispersed populations. This curious method of sex determination may also have profoundly impacted the evolution of social behavior in wasps, ants, and bees. Oddly enough, it created a situation where sisters are more closely related to each other than they are to their own daughters. A worker bee or ant, for example, actually passes along more of her own genes to the next generation by helping the queen (her mother) raise more of the queen’s offspring, than if that same worker had her own daughters.

  The two largest wasp family groups, Ichneumonidae and Braconidae, each contain more species than all of the vertebrates combined, and in tropical areas the social insects surpass the vertebrates in total biomass. Wasps are keystone predators and parasites of most plant-feeding insect populations; consequently, they greatly influence the diversity and abundance of forest plant species. Ecologically, wasps are also important as herbivores, seed dispersers, pollinators, and nutrient-recyclers. They probably haven’t had the good press they deserve, but the wasps are the grandest success story of the Triassic years, even more so than the dinosaurs.

  The Late Triassic, about 201 million years ago, was a tough time for mammals nuzzling around in the leaf litter, searching for a meal. But it was a prime time for dinosaurs, and a glorious one for bugs. It would probably have been difficult to spot a dinosaur in the Triassic forests, just as it’s difficult to spot large vertebrates in modern tropical forests, but those areas were saturated with insects. While the shorelines teemed with aquatic species, the leaf litter crawled with an assortment ranging from bristletails to beetles. The plants, still mostly conifers, cycads, ferns, and gingkoes, were chewed or sucked upon by myriads of orthopteroid insects, thrips, planthoppers, and a few primitive moths and sawflies. All of these were attacked by a host of predatory insects, including the titans, lacewings, scorpionflies, and snakeflies.

  As the sun set each evening, the titans’ baritone rasping was the loudest noise in the forest. But down below the dry leaf litter rustled as small dinosaurs prepared to settle in for a long night’s sleep. Some of those dinosaurs started ruffling their feathers, which they had evolved to keep warm on long cool nights. For the first time in almost 150 million years the vertebrates were poised to start chasing the insects into the air. The Jurassic times were approaching, and the songs of the titans were about to be replaced by birdsong.

  8

  Picnicking in Jurassic Park

  Reverie is normal in Wyoming at sunrise.

  ROBERT T. BAKKER, The Dinosaur Heresies

  Early on this sunny winter morning, as I write these words, I’m pondering two things: a riddle and the view from my window. At first glance, the two seem totally unrelated, but nothing could be further from the truth. In fact, the view first caused me to ponder the riddle, and, as you will see in a bit, both are woven into the story of the Jurassic period, 201 to 145 million years ago, when insects and dinosaurs diversified like never before, the birds first appeared, and vertebrates finally chased insects into the air.

  First, the riddle: what is the largest organism an entomologist has described and named? An entomologist, you will recall, is a biologist (like me) who happens to study insects. More precisely, I examine insect evolution and classification and am involved in the discovery, description, and naming of new species: a branch of the science known as systematic entomology. The largest species I’ve named, a megalyrid wasp from Papua New Guinea, is slightly less than sixty millimeters long, if you count its ovipositor along with its body. Most of my new species measure only a few millimeters in length. Keep in mind while you ponder my riddle the fact that we bug hunters typically discover very small organisms. Now I’ll go on to describe the view from my window.

  The University of Wyoming Insect Museum is situated on the fourth floor and has broad windows facing directly north. On any given day they pr
ovide an impressive panoramic view above and beyond Laramie’s northern limits. What lies beyond the city is of particular interest here: at first glance, apparently nothing. The Laramie Valley is a treeless short-grass steppe. The terrain is open, windswept, barren, and seemingly hostile; for miles nothing obstructs the horizon, which, although distant, is sharp and clear. The air is cold and crisp, the brownish tan grasslands illuminated by the sun and starkly contrasting with the crystal blue Wyoming skies. It’s no problem to see all the way to the northwestern horizon and easy to imagine you can see beyond it, clear to Como Bluff, Sheep Creek, and the town of Medicine Bow. That’s where our story continues, because out near Sheep Creek more than a hundred years ago, an unusually large creature was discovered—the very organism that’s the object of our current riddle. How did this exceptional animal come to be in the windswept basin north of Laramie?

  How you react to this puzzle may depend first on how you define the word “large.” If you mean heavy or massive, then adult Goliath beetles of central Africa and larval South American Hercules beetles would be among the largest.1 If instead you measure body length, then you might select the Australian titan stick insect (Acrophylla titan), which grows up to ten inches long and has wings that spread equally wide. Although longer than any other living insect, including the Goliath and Hercules beetles, an individual titan stick insect does not weigh nearly as much. To complicate matters further, one could measure size from the other dimension: width. The huge atlas moth from India (Attacus atlas) can have a wingspan, from tip to tip, of nearly twelve inches. The great owlet moth from Central and South America (Thysania zenobia) sometimes has a wingspan that exceeds that of the atlas moth, but the total surface area of its wings is less. All these insects are notably large in their own way, but all are tropical animals found nowhere near Wyoming.

  Most of you have probably remembered our previous discussion of gigantic Carboniferous and Permian insects and already realized that the answer to my riddle must lie in considering not only living organisms but fossilized ones as well. How about the most majestic of all Permian insects, the nearly three-foot meganeurid air dragons? One of the larger species was indeed discovered much closer to home, in Permian rocks in Kansas and Oklahoma. But alas, the organism I have in mind is even larger—orders of magnitude larger than the meganeurid air dragons. I suppose now would be a fair time to reveal a crucial piece of information. I never did actually say that the organism was an insect, only that an entomologist named it. No rule of the International Code of Zoological Nomenclature says that you have to be an entomologist to name an insect, or that an entomologist can’t describe organisms from any other animal group. These days an entomologist seldom does otherwise than discover insects, but in the past biologists worked more widely among diverse groups of organisms. Once upon a time, in Pittsburgh (of all places), a famous entomologist, William Jacob Holland, christened a dinosaur from Wyoming Diplodocus hayi. This dinosaur is not only the largest organism an entomologist has ever described, it’s one of the largest land creatures that ever lived.2

  FIGURE 8. 1. The South American Hercules beetle (Dynastes hercules) is the largest of the horned rhinoceros beetles. A large male, such as this one from Ecuador, can measure nearly seven inches long, and their larvae are among the heaviest insects known. (Photo by Angela Ochsner.)

  Long-Necked Nibblers and Long Neck Biters

  For most people, Diplodocus and its better known cousins, the brontosaurs, are the quintessential Jurassic dinosaurs: small headed, long necked, humungous bodied, with extended whiplike tails. When I was a young child the popular dogma held that both lived in swamps, but we now know them for what they were: the heaviest land creatures that ever lived. If anything, they shunned the wetlands. They trotted through the tropical Jurassic forests, raising up on their hind legs and craning their serpentine necks well up into the tall trees to forage for their daily meals. A fully grown adult brontosaur (now correctly called the Apatosaurus) could measure up to seventy-five feet long from mouth to tail tip, and it might have weighed a whopping twenty-five tons. To keep up their healthy weight, the brontosaurs had to eat an enormous amount of vegetation—by some estimates, up to a ton every day. These estimates never mention trace items, such as insects. Like any large browsing animal, brontosaurs would have selected the most-tender vegetation—the newest leaves with the most nutrition and the lowest amounts of toxic chemicals—and this vegetation, of course, would attract plant-feeding insects as well. The brontosaurs probably consumed around a couple pounds of insects along with their daily salad, assuming they just randomly nibbled on the creatures. For all we know, they might have found insect-infested foliage tasty and ate all they could find. Still, a ton of salad with a few insect vitamin pills is a huge meal to digest. Along with consuming forest vegetation the brontosaurs paused from time to time to swallow a few big, smooth rocks. These gizzard stones rolled around in their massive stomachs along with powerful digestive enzymes and helped them more rapidly digest their excessive meals. Just a few rocks a day and the benefits were enormous: brontosaurs needed to spend less time chewing and so could spend more time just stripping the trees of tender leaves and swallowing. These polished stones still litter the windswept plains near Como Bluff and Sheep Creek, a mute testimony to the dietary prowess of the most gargantuan feeding machines that ever wandered the earth’s forests.

  Allow me to rephrase that last point. The brontosaurs and diplodicines might well have been the heaviest animals to ever roam the woodlands, but the Jurassic years spawned some even more impressive feeders. Perhaps now would be a good time to debunk a few of our myths about the Jurassic. You’ve probably read Jurassic Park, seen the movie, or viewed one of its sequels. While the movies did much to improve public understanding of dinosaurs as warm-blooded, fast, and possibly intelligent, they unfortunately contributed to several major misperceptions about what life was like during the Jurassic, the most notable of which relate to the large predatory dinosaurs. Despite what the movies portray, there were no tyrannosaurs or velociraptors in the Jurassic. Sure, those animals did exist, but they lived in the Late Cretaceous—tens of millions of years later. Although they may have existed around the same time, Tyrannosaurus and Velociraptor probably never ran into each other, because they lived in different parts of the world: Tyrannosaurus in western North America and Velociraptor in Mongolia. And while the velociraptors might have had big brains and hunted in packs, they were not nearly as sensational in reality: fossils indicate that they were only about 25 percent as large as the creatures depicted in the movie. A full-grown tyrannosaur could have stepped on a velociraptor with impunity, and if they ever did run into each other, the velociraptors probably quickly fled in the other direction.3

  This Jurassic confusion is totally unnecessary. One Jurassic predator was, in fact, as ferocious and impressive as any tyrannosaur or velociraptor: the Allosaurus. Some paleontologists speculated that allosaurs might have been scavengers, but let’s give them some credit where credit is due. Adults were between twenty-seven and forty feet long and might have weighed one to two tons. They had razor-sharp slashing teeth and claws and large jaws that could disarticulate like a snake’s to take huge bites of meat, up to a hundred pounds at a time. You don’t need teeth and claws like that to nibble on dead carcasses. Although the allosaurs might not have been as big as the tyrannosaurs, they hunted meals that were larger than anything any tyrannosaur or velociraptor ever encountered. They were the most abundant large predatory dinosaur in the Late Jurassic of North America, and the wealth of allosaur bones in the same fossil beds suggests that, like Velociraptor, they were social and hunted in packs.

  Imagine the real Jurassic park: near a river basin thick with ferns and cycads, a herd of brontosaurs serenely browses on the high tree branch foliage of the bordering coniferous forest. Suddenly, out of the thick, shadowy undergrowth rushes a pack of snarling allosaurs, and, momentarily, confusion reigns. The dominant bull brontosaur is quick to defend his
herd, adroitly whipping his huge tail, hurling a juvenile allosaur thirty feet into a fern thicket. Bellowing an alarm call, the huge brontosaur reels to the right, rears back, then stomps forward into the pathway of a charging allosaur, which screeches in agony: the brontosaur has crushed and broken the largest toe of the allosaur’s right foot. The bull kicks out again but misses, and the wounded allosaur, favoring his left leg, hobbles a hasty retreat. But in the chaos of the moment the damage is done. A young brontosaur, separated from the herd, bellows a high-pitched squeal as a seasoned veteran of the hunt, the matriarchal female Allosaurus, lunges from behind and rips a fifty-pound chunk of flesh from its right flank. The allosaur pack retreats to the forest to lick their wounds, while the bull brontosaur leads his surviving herd to the comparative safety of a ferny vale a half-mile upriver. The mortally wounded young brontosaur, staggering from blood loss, collapses to the ground. The allosaurs have suffered three broken ribs and a broken toe but they have won the day, gaining enough meat to feed the pack for a month while they recover from their war wounds.4

  Tiny but Mighty Jurassic Park Killers