Planet of the Bugs: Evolution and the Rise of Insects (2 page)

Preparing to fly, the beetle extended its rigid, shell-like front wings outward and unfolded its membranous hind wings. This style of wings is, in fact, the beetles’ key innovation, evolved some 260 million years ago in the Permian period. Only the hind wings power beetle flight; the modified armored front wings allow the delicate hind wings to be put away, hidden and protected, when not in use. These hard front wings, unique to beetles, also protect the beetles when they’re not flying and give them a more streamlined body profile, which enables them to crawl into cracks and crevices, under loose bark, in leaf litter, and among woody debris. While the beetles seem to fly clumsily with the unfolded hind wings, they are efficient enough to allow the beetles to disperse widely in search of mates and food, and to colonize new habitats.

Some of those ancient Permian beetle ancestors also evolved a very useful feeding behavior, retained even today by the modern long-horned wood-boring beetle. Their immature forms, larvae, developed the habit of boring deep into the woody trunks of dead trees to feed there on the fungal growth and decaying plant tissues. This sheltered them from increasingly adept warm-blooded predators throughout the Permian, and on through the age of the dinosaurs, the Mesozoic era from the Triassic through the Cretaceous periods. It may have also served to buffer them from environmental change. At the end of the Permian, about 252 million years ago, the ancient continents collided to form the supercontinent of Pangaea. Coastlines and marine habi
tats were severely disrupted, perhaps triggering a mass extinction of species greater than any other extinction event so far. Terrestrial habitats became hotter and drier than before, but this only seemed to stimulate the beetles’ success. While there were only 5 families of primitive beetle-like insects in the Late Permian, by the Late Triassic (around 220 million years ago) at least 20 families consisting of 250 species of true beetles had evolved. Over the course of the following Jurassic period, despite living among hungry dinosaurs, beetle numbers continued to skyrocket; at least 600 species in 35 families have been identified in middle-Mesozoic fossils.

Back along the trail at San Ramon, the beetle flexed its wings again and took a short, buzzing flight to a nearby yellow flower. After a few moments, it slowly began to chew, lazily indulging in a high-protein pollen meal. The sight of insects feeding on flowers is so commonplace in our modern world that we tend to forget how unusual it really is, geologically speaking. The ancient Permian proto-beetles didn’t visit flowers because they didn’t exist yet, at least not in the sense that we understand them. The flowering plants that currently dominate the landscape, known by botanists as angiosperms, did not evolve until the Early Cretaceous period, around 126 million years ago. Perhaps sometime during that period an ancient beetle first visited a flower, maybe in the shadow of a
Tyrannosaurus
or
Triceratops
dinosaur, and found the pollen tasty. Over the Late Cretaceous, this time in tandem with the flowering plants’ early diversification, the richness of beetle species skyrocketed again, particularly among many of the plant-feeding beetle groups that survived the period and now dominate our modern world: leaf beetles, weevils, scarabs, click beetles, metallic wood-boring beetles, and, notably, the long-horned beetles, the family into which our time-machine beetle is classified.

Maybe, 66 million years ago, some Cretaceous beetles were busily feeding on pollen from ancient magnolia blossoms when a sound from above caused a nearby
Tyrannosaurus
to glance briefly skyward and see a massive asteroid hurtling toward the earth—a catastrophe which brought the time of the giant dinosaurs to a close and marked the end of the Cretaceous. Global winter ruled for a time, killing off not only dinosaurs but also perhaps many kinds of small marine organisms. But deep in rotting tree trunks and elsewhere, the larvae of many beetles survived, completed their metamorphoses, and emerged into
a brave new world without giant dinosaurs. Over ensuing millions of years, some of those survivors lived on, evolved, and diversified to become the most species-rich animal group in the world today.

Our beetle flew into the variegated green of the primeval forest, perhaps to seek others of its kind, and in doing so, to replicate its many messages from the past. Its gentle buzzing was lost among the sounds of dripping water and the increasing rainfall. This beetle is gone now, but many others remain. We can only estimate their numbers. Studies by Smithsonian entomologist Terry Erwin indicate that there are millions, perhaps tens of millions, of different kinds of beetles in our tropical forests, most of them still unnamed. And that’s just one major insect order. Many other kinds of insects are hyperdiverse, such as the moths, butterflies, true flies, wasps, and true bugs. This hyperdiversity is the rich historical legacy of the Cenozoic era. Over the past 66 million years, as the insects have continued to diversify along with the flowering plants, our tropical rain forests have evolved into the most biologically complex and diverse ecosystems ever to arise. Why are there so many different kinds of insects, and why do they dominate terrestrial ecosystems? Science has unlocked an extraordinary number of mysteries, and the story of the insects’ rise can now be read over hundreds of millions of years of earth’s history. The messages are written there in the rocks, the forests, and the insects, for those who choose to read them.

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The Buggy Planet

It is for me a stunning fact that while the physical surface of the earth has been thoroughly explored, so that virtually every hilltop, tributary, and submarine mount has been mapped and named, the living world remains largely unknown. As few as ten percent of the species of insects and other invertebrate animals have been discovered and given scientific names.

EDWARD O. WILSON,
The High Frontier

      
All things have a root and a top,

      
All events an end and a beginning;

      
Whoever understands correctly

      
What comes first and what follows

      
Draws nearer to Tao

BARRY HUGHART,
Bridge of Birds

Earth is a very buggy planet. Nearly one million distinct living species, different kinds of insects, have been discovered and named so far. From A to Z, they overwhelm us with their diversity: ants, birdwing butterflies, cockroaches, dung beetles, earwigs, flies, grasshoppers, head lice, inchworms, June beetles, katydids, ladybugs, mantises, net-winged midges, owlflies, periodical cicadas, queen termites, royal palm bugs, sawflies, thrips, underwing moths, velvety shore bugs, webspinners, xyelid sawflies, ypsistocerine wasps, and zorapterans. But that is just the tip of the iceberg, the door to the hive. Most of the insect species haven’t even been given a name, and scientists estimate that the number of different kinds of insects living in tropical forests is perhaps in the tens of millions.
¹
Whether you adore them or abhor them, their diversity and ecological success is impressive.

Insects are so successful that it’s not much of an exaggeration to say that they literally rule the planet. Our egos allow us to think that we humans rule earth, with our cities, our technology, and our civili
zations, but we seem to be doing more to destroy the planet than to improve it, and we are like one superabundant pest species run amok over the globe. If humans were to suddenly become extinct, the living conditions for most species would be greatly improved with only a few exceptions, such as human body lice and crab lice. On the other hand, if all the insects became extinct, in the words of Edward O. Wilson, the famous Harvard entomologist, “the terrestrial environment would collapse into chaos.”
²
Human civilizations have only recently developed over the last several thousand years. Insects have successfully coevolved with terrestrial ecosystems over the last four hundred million years. They are ecologically essential as scavengers, nutrient recyclers, and soil producers, feeding on and utilizing virtually every kind of imaginable organic material. Six-legged detritivores consume dead plants, dead animals, and animal droppings, greatly increasing the rates at which these materials biodegrade. Insects, as both predators and parasitoids, are keystone organisms that feed upon and reduce populations of other kinds of plant-feeding and scavenging insects. They are also their own worst enemies: most kinds of insects have populations that are kept in check by the feeding activities of other insects.

Over the past 120 million years, insects have coevolved and explosively diversified in tandem with the angiosperms—the dominant forms of plant diversity in modern ecosystems. They are essential as pollinators and seed-dispersers for most of the flowering plants, whose communities would be vastly diminished if all plant-associated insects were eliminated. We often tend to think of plant-feeding insects in general as pests, but I like to point out that only a miniscule small fraction (less than 1 percent) of the total number of insect species are actually significant pests. In fact, most of the plant-feeding insects should be considered beneficial for two reasons. First, they reduce the reproductive output of particular plants by putting stress on them. That sounds bad if the plant is an agricultural crop, but in a natural setting, such as a tropical forest or a mountain meadow, that plant feeding has a very desirable outcome. It prevents particular plant species from becoming superabundant and weedy, allowing vastly more species to coexist in much smaller spaces. Plant-feeding insects are a driving force in the evolution of plant community species richness, and so the extraordinary plant diversity of tropical habitats is
largely due to insect diversity, not despite it. Second, but of no less importance, the majority of plant-feeding insects are themselves edible to other kinds of wildlife. Many insects are a fundamental and nutritious food source for most kinds of vertebrate species, including fish, amphibians, reptiles, birds, and most mammals, including primates and even humans. Not many organisms totally depend on humans for their continued existence, but a large part of living plants and terrestrial animals depend partly or entirely on insects for their survival.

Whether or not they rule the planet, insects certainly have largely overrun it. They can be found in abundance in virtually every kind of terrestrial habitat, from tropical rain forests to deserts, in meadows and prairies, from sea shorelines to alpine tundra and Andean páramo. Aquatic insects not only inhabit mountain streams, rivers, waterfalls, seepages, lakes, ponds, swamps, and salt marshes, but they even occupy mud puddles, sewage ponds, craters in rocks, tree holes, pitcher plant leaves, and bromeliad leaf bases more than a hundred feet above the forest floor. Semiaquatic insects exploit the force of surface tension to skate across still ponds and lakes, while the ocean water strider, genus
Halobates
, has been seen walking on the ocean surface hundreds of miles at sea. Clouds of millions of African migratory locusts have flown across the entire Atlantic Ocean to land in the Caribbean Islands. The insect macro-societies, ants and termites, are essential soil movers in the Amazon basin, where their biomass outweighs the biomass of vertebrates. But sheer insect abundance is not strictly a tropical phenomenon. Even near the Arctic Circle, the combined weight of biting flies and midges outweighs that of the mammals.

Insects and their relatives have evolved and adapted to some of the most extreme conditions on the planet. Stoneflies have been recorded at an elevation of 5,600 meters in the Himalayas, while subterranean species of beetles, crickets, and cockroaches have adapted to life in caves deep underground. Some aquatic stream beetles breathe across the surface of an air bubble and can stay underwater indefinitely. Brine flies, shore flies, seaweed flies, and deer flies have developed extreme tolerance for high levels of salt and live in salt marshes and salt flats and along ocean shorelines. Springtails have evolved antifreeze compounds in their blood, and some are among the most abundant organisms on sub-Antarctic islands. At high elevations worldwide, species of icebugs, springtails, snow scorpionflies, and some flightless
tipulid flies are active on the frozen surfaces of snow fields and glacial ice. Living chironomid midge larvae have been dredged up from the depths of Lake Baikal in Russia, where they have adapted to a low-oxygen environment by evolving hemoglobin-like blood pigments. The adaptability of water boatmen bugs is remarkable: some inhabit salty water below sea level in Death Valley, California, while others live high in the Himalayan Mountains. Some swim in frigid water under ice, while others thrive in hot springs at temperatures up to 35°C. The Yellowstone hot springs alkali fly develops in the edges of scalding hot water pools with temperatures up to 50°C. Other fly larvae living in arctic ponds are known to survive winter cold temperatures as low as −30°C. One of the most impressive organisms is the South African chironomid midge fly,
Polypedilum vanderplanki
, which has adapted to extreme drought conditions by evolving cryptobiosis—a suspended-animation condition where larvae become dehydrated and tolerant to the most extreme conditions. It has been reported that these dehydrated fly larvae can tolerate immersion in boiling water as well as being dipped into liquid helium.