Planet of the Bugs: Evolution and the Rise of Insects (16 page)

A New Twist on an Old Style

During the Late Carboniferous, the giant griffenflies started chasing a new kind of insect that was tasty but harder to catch than the old
net-winged ones. Neoptera, or the new-winged insects, were faster flyers, and they had an original trick made possible by tiny articulating skeletal plates, called axillary sclerites, in the membrane near their wing base. These allowed directional wing movements that were never possible among the paleopterans. When neopteran insects landed on a plant and were done flying, they could twist their wings at the base, fold them back over their body, and put them away, making
the neopterans much smaller than the older insects, which held their wings constantly outstretched, kitelike. This neat twist would open myriad new possibilities for future insect evolution by allowing front and back wings to specialize separately. It made the future evolution of stoneflies, grasshoppers, bugs, beetles, lacewings, butterflies, bees, flies, and most other modern insects possible.

FIGURE 5.4. Fossil folded front wings with eyespot-like markings of an extinct upper Carboniferous neopteran insect,
Protodiamphipnoa gaudryi
. (Photo by Olivier Bethoux. © MNHN–Olivier Bethoux.)

Some of the neopterans were fast on their feet, too. Quickly after landing, they would deftly fold their wings and run under a leaf or into cracks and crevices, making themselves tough targets for the air dragons. This was such a successful adaptation that before you could say “cockroach,” the tropical world was infested with them. Several groups of new-winged insects appeared during the Carboniferous, but the roaches (order Blattaria) were by far and away the most successful of the age. By the Late Carboniferous there were more than eight hundred species, and they made up about 60 percent of the known Carboniferous insects. They were a bit different from modern species: they could actively fly and females had an egg-laying device, an ovipositor, that resembled a tail. We still classify them as roaches, however, and in terms of species diversity, we should probably call the Carboniferous period the “age of roaches.”
¹⁷

Roaches have a bad reputation, mostly because of a few bad eggs: a few nasty pest species that overrun our houses and apartments. But please don’t base your overall impression on just those few. Modern tropical forests contain thousands of cockroach species, which enjoy a wide variety of habits. Most live on the ground in leaf litter or under logs, but many live on tree trunks or in the forest canopy. Some are blind and live in caves, while others are semiaquatic and live alongside streams or in bromeliad water tanks high in the treetops. Most are nocturnal, but some are active by day, and some even prefer bright sunlight. The night-active species may stir at different times: some in early evening, some around midnight, and others before dawn. Modern roaches are moderately omnivorous, and they play an important role in the decomposition and nutrient cycling of leaf litter and organic material on tropical forest floors. Several living cockroach species are even known to pollinate tropical plants.

Many of the early cockroaches are thought to have preyed on small soft-bodied insects or scavenged the bodies of dead insects, probably of fallen titanic air dragons. Based on the abundance of fossil fecal
droppings, many others are known to have been detritivores, and, like modern roaches, are thought to have played a valuable role in the rapid decomposition of leaf litter. The wood roaches evolved a symbiotic relationship with their gut microorganisms and became the first effective macroconsumers of dead wood. The roaches in turn were the most abundant food source for a host of predators, including scorpions, spiders, centipedes, fish, amphibians, reptiles, and the flying air dragons. So with the onset of roaches an important turn occurred in the cycling of organic molecules. More biomass from plant material escaped the geological cycle of sedimentation and rock formation, and was cycled back into the living world by small animals. The great coal age was coming to an end.

In the Late Carboniferous, around 299 million years ago, the sun rose over moist lowland rain forests of giant horsetails, seed ferns, and ancient conifers. As steam rose from the marshes, scurrying roaches folded their wings and nestled into the leaf litter. In the forest canopy, colorful old net wings basked in the early morning sunlight, absorbing heat and fluttering into the air. Giant shimmering gossamer-winged air dragons lifted to the chase. Down below, in the still-shaded shorelines, the amphibians glanced wistfully to the treetops and worried about their next meal. Along the shorelines, also basking in the morning sunlight, was another new animal, one I haven’t mentioned yet. During the Carboniferous, the lizard-like vertebrates evolved, and by the end of the period there arose large reptile species that ate meat, mostly from fish and amphibians. But you can bet that small ones liked to eat insects whenever they could get them. You can also bet that whenever there was a massive mayfly emergence, all the amphibians and reptiles would be busy lapping up their fill off the low vegetation.

As hungry reptiles patrolled the margins of the coal swamps, the Carboniferous times were waning. The world was getting drier, and while the coal swamp lands were decreasing, conifers were evolving and transforming the terrestrial landscape. Insects had evolved some of their most important traits—wings and the ability to flex them in a complex way—but now that the Permian times were approaching, some even more startling innovations were developing. During the Permian the giant fin-backed reptiles dominated the shorelines, the flying insects diversified like never before, the most gigantic insects
of all time patrolled the airways, and complex metamorphosis became widespread. But the Permian is crucial to the history of life on earth not just because of its evolutionary innovations. The end of the Permian marks the biggest change that life has ever seen: a catastrophic mass extinction greater than any before or since. Understanding that event, and why many insects survived it, may prove to be the most important key in explaining why insects rule the planet.

6

Paleozoic Holocaust

The end-Permian mass extinction had the greatest effect on the history of life of any event since the appearance of complex animals.

DOUGLAS H. ERWIN, “The Mother of Mass Extinctions”

So what did cause the greatest mass extinction in the past 600 million years, and perhaps the greatest in the history of life? The short answer is that we do not know, or at least I do not know. . . . In the wake of the Alvarez-impact hypothesis many of us seem to prefer a single dramatic cause as an explanation for such events. Our knowledge of recorded history provides precious little support for such a view, and I see little reason, a priori to expect such a neat and tidy resolution to this riddle.

DOUGLAS H. ERWIN,
Extinction

The aim of science is to seek the simplest explanation of complex facts. We are apt to fall into the error of thinking that the facts are simple because simplicity is the goal of our quest. The guiding motto in the life of every natural philosopher should be: Seek simplicity and distrust it.

ALFRED NORTH WHITEHEAD,
The Concept of Nature

Two film genres from my childhood remind me of Permian times. The first is the giant arthropod thriller. These sorts of movies are not as popular as they once were, but you’ve probably seen one or more. The classic example is the 1955 thriller
Them
, whose plot involves gigantic ants mutated by atomic radiation. It was the first giant arthropod thriller and, in my opinion, it was one of the best. Many others have appeared over the years. You can take your pick. There was the massive
Tarantula
terrorizing teenagers at drive-in movies,
¹
The Deadly Mantis
with preying mantids big enough to eat army tanks, and
Mothra
, the moth creature whose wing beats created winds strong enough to blow over office buildings in Japan. And of course there was
The Fly
, an all-time favorite about the hapless inventor who has the rotten luck to get insect parts mixed with his own when he enters his innova
tive transporting device.
The Fly
is perhaps the only example of the giant arthropod thriller that Hollywood valued enough to honor with a modern remake. Finally, there are the more recent but very popular
Alien
movies. In case you were too appalled to notice, the alien had all the key features of an arthropod: a hard external skeleton, segmented body, jointed appendages, and metamorphic development with molting, as well as extendable mouthparts resembling those of an immature dragonfly.

The second genre is the murder mystery, which was very popular when I was a kid. There were the classic novels by Agatha Christie and their movie adaptations, like
Murder on the Orient Express
and
Ten Little Indians
. Who could forget Alfred Hitchcock’s
Psycho
and
Rear Window
? The weekly television show
Perry Mason
was a favorite, and judging from the popularity of shows like
CSI: Miami
and
Law and Order
, it seems that America’s craving for murder mysteries has not diminished.

Fortunately for us, there are no insects as large as
Them
or
The Deadly Mantis
, and humans thankfully don’t have internal parasites as enormous as the Alien. But during the Permian, about 299 million years ago, insects did reach gigantic sizes—not as large as any in these movies, but still impressively bigger than any living today. The fact that these massive insects did exist raises interesting questions about why they lived and why even larger insects never evolved. The Permian also presents a number of unsolved murder mysteries. The colossal insects died during that period, disappearing forever, and the Permian saw the rise and fall of the giant fin-backed reptiles and saw the last of the trilobites. But all these creatures were not alone in their demise. The end of the Permian is marked by the greatest mass extinction in the history of life, when most of the species alive up to that point died. This event is the greatest murder mystery of all time.

Observing the Scene of the Crime and Assessing the Time of Death

The transition into the Permian’s early years was gentle. Many of the Carboniferous period’s plants and animals continued to thrive, and giant fin-backed reptiles were prominent until the Middle Permian. Among them was the massive
Dimetrodon
, which fed mostly on fish and
amphibians, but these beasts probably ate insects too, especially when young. Even a large hundred-pound
Dimetrodon
would have paused for the meal provided by a newly emerged air dragon nymph just crawled up from the water but not yet able to fly. The great fin-backed reptiles dominated the Early Permian shorelines, but in the Middle Permian years they were ecologically displaced by the appearance of new kinds of large vertebrates, including the first warm-blooded animals—the Kazanian protomammals—and the thecodont reptiles, the clan that would later, in the Mesozoic era, lead to the dinosaurs.

Over just a few million years, the protomammals diversified explosively, and what a bunch of noisy brutes they were. They could run, jump, snap, and bite like anybody’s business. They scouted out the hills first with slim, fast, dog-sized predators, but before you can say “
Dimetrodon
steaks, a dime a pound,” the protomammals diversified into all sorts of terrestrial ecological roles. The dominant large predators, dome-headed Kazanian protomammals, had bony skulls, could roar, and butted heads in fearsome territorial displays. There were also dozens of smaller species that fed on insects; the hot-blooded pursuit of arthropods had begun. So in the upland meadows, while the bugs first tippled on plant saps and the beetles first chewed on logs, it was a brave new world for the vertebrates. Never before had the terrestrial habitats seen such an assortment of these bony creatures. But after ten million years or so, the Kazanians fell by the wayside, and a whole new wave of protomammals terrorized the Permian forests: the dynasty of the Tartarian protomammals. The old dome-heads were replaced by ferocious two-tuskers. Once again a new regime of warm-bloods filled the big animal niches in the Permian meadows and forests, and once again there were lots of small ones that probably ate insects. Then after only a few more million years, the Tartarian protomammals also fell, this time in the days of the end-Permian mass extinction.
²
Overall, about 70 percent of the vertebrate families were lost.

Permian times also saw a startling diversification of insect species, one that surpassed the famed Cambrian explosion in species richness, if not in experimentation with anatomical forms. It is the geological interval with the greatest ordinal-level diversity—there were at least twenty-two insect orders, more than even now—and the old-winged insects reached a peak of diversity unlike any before or since.
³
The Permian years also saw the first of the orthopteroid insects, the
ur-crickets and ur-katydids; the first of the hemipteroid insects, the true bugs with sophisticated siphoning mouthparts; and the very first insects with complete metamorphosis, the beetles, lacewings, scorpionflies, and caddisflies. However, the insects experienced more extinction than at any other time in history. Eleven of the Permian’s twenty-two orders are now extinct: eight apparently disappeared completely at the end of the period, and three others fell into serious decline and were gone by the Early Triassic.