Planet of the Bugs: Evolution and the Rise of Insects (24 page)

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.
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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.
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Tiny but Mighty Jurassic Park Killers

While one can’t very well look at the Jurassic years without admiring the dinosaurs, I do have an ulterior motive in bringing them up: to compare those massive meals with some tiny ones in dead fallen trees—beetle larvae in rotting wood, to be precise—eaten by wood wasps. Remarkably, 180 million years ago, the dinners of these small burrowing insects became even more significant than the gigantic meals of the allosaurs, which enjoyed the grandest predator-prey relationship in the history of the planet. To understand this, we need to compare the descendants and fates of the allosaurs and the wood wasps.

For all their grandeur, the allosaurs faded away. They may have been the forebears of the Cretaceous tyrannosaurs and velociraptors, but even these dinosaurs fell into decline, and around 65 million years ago the carnivorous theropods were snuffed out entirely. Such exceptionally large predators just didn’t have the staying power to last forever, but more importantly, their ecological niche requirements were massive; terrestrial ecosystems could only support a few such stupendous predators at the same time. There was no way they could possibly diversify into hundreds or thousands of descendant species.

Consider, by way of contrast, the history of the Jurassic wood wasps. Sometime in the Early Jurassic years, a band of rebellious young wood wasps rejected the vegetarian diets of their ancestors and decided to eat beetle larvae. And so the dynasty of the parasitic Hymenoptera was born. Over the passing years these meat-eating wasps diversified and specialized to feed on a variety of multifarious insect species that had already evolved in the forests. By the end of the Jurassic there were hundreds of these parasitic wasp species, by the Late Cretaceous there were thousands, and currently there are hundreds of thousands, possibly millions, of descendant species.
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We all have a pretty good intuitive grasp of what predators are: aggressive animals like
Anomalocaris
, scorpions, meganeurid air dragons, fin-backed reptiles, tyrannosaurs, and praying mantises that stalk and eat other (prey) animals. Easy enough to grasp—but there is an easy-to-overlook subtlety here: predators require multiple prey individuals and must keep hunting to survive. This approach worked fine for thousands of species over hundreds of millions of years, but in the Early Jurassic the wood wasps broke the mold and invented an entirely novel predatory behavior when they killed and ate only one other individual animal (called the host), that was large enough to feed them to adulthood.

Although the term “parasitic” is loosely applied to a variety of unrelated insects such as lice, fleas, certain flies, and parasitic wasps, all of which live at the expense of one other animal, the parasitic wasps are different from lice and fleas because they feed extensively on the host animal, eventually killing it. This deadly behavior is so important ecologically that we use another term to describe organisms that do it: parasitoid. A parasitoid is any parasitic organism, more often than not a wasp or fly, which causes its host to die. The Jurassic parasitoids didn’t just find a new protein-rich meal, they narrowed their ecological niches to smaller dimensions than those of any previous predatory animals and in doing so allowed their descendants to live in a multitude of previously unoccupied microscopic niches. From that time onward, parasitoids dominated the diversity of terrestrial communities, and by their selective killing behaviors they shaped the richness and abundance of both the insect and plant communities. The scientific fact that the most successful bloodline of parasitoid animals is descended from a clan of log-chewing Jurassic wood wasps is well established. It’s broadly supported by independent evidence from the anatomy of living wasps, the ecology of their feeding behaviors, the fossils of Mesozoic wasps, and DNA evidence from modern wasps. But why did wood wasps in particular evolve parasitism?

This is a good point to pick up a discussion thread from the last chapter, where we introduced the xyelid sawflies. Over the Late Triassic and Early Jurassic years, sawflies became among the most successful forest insects by diversifying into myriad vegetarian sawfly clans. As new plant species evolved, sawflies successfully colonized them, and new species coevolved with the forest plant community. The saw
flies also subdivided the plants into multiple feeding niches. Some fed externally on leaves, while others learned to conceal themselves by tunneling and mining in leaf tissues or by hiding and feeding in leaf shelters constructed by tying together leaves with threads of silk. As sawflies were adapting to feed on different plant parts, so were other kinds of insects. As a result, competition for all edible plant parts increased, and sawflies responded by further varying their diets. Some specialized by tunneling into plant stems and others ultimately delved into the thicker woody tissues, evolving into the wood wasps. Their success in this endeavor was propelled by the same useful tool that promoted the first sawflies: the female ovipositor.

The Story of the Sting

We tend to think of wasps’ ovipositor, from which the stinger, or sting, evolved, as being a simple structure, like a hypodermic needle adapted for injecting eggs. It is actually far more complex. If you were to slice a wasp ovipositor in cross section and examine it microscopically, you would find that it is not one hollow tube but three or four separate interlocking shafts, each of which can be moved independently of the others.
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This allows the tip of the ovipositor to drill into hard substances, such as plant stems and wood, and be directed with great accuracy. To better understand this idea, try the following exercise. Clasp your hands together by interlocking your fingers and wrapping them tightly around your knuckles. Now extend the first two fingers of each hand, so that exactly four fingers are extended straight. This represents the form of a wasp ovipositor. Look at your fingers end-on and you will see that there is a little hollow space in the middle. This is like the tube through which the wasp egg passes. Now try moving some of your extended fingers, while holding the others in place. For example, try sliding your right fingers downward while keeping your left ones stiff. You will notice that the tip formed by the left fingers is turned to the right, even though they are not being flexed. This is how a female wasp is able to direct the tip of her ovipositor; by retracting the four shafts independently or together, she can move the tip with precision in various directions.