Planet of the Bugs: Evolution and the Rise of Insects (13 page)

FIGURE 4.3. Jumping bristletails (order Archaeognatha) are among the most primitive of living insects. Unlike most insects, they have appendages (styli) on their abdominal segments. (Photo by Kevin Murphy.)

Modern entomologists divide the hexapods into two groups depending on whether the mouthparts are withdrawn into a pouch, as in springtails and diplurans, or clearly exposed, as in most other hexapods. By this criterion, the true insects include only the hexapods with exposed manibulate mouthparts: the bristletails, their relatives, and all their descendants. Perhaps the oldest undisputed fossil of a true insect is that of
Rhyniognatha hirsti
, also found in the Rhynie chert formations and aged at 396 to 407 million years old. Although based on little more than a fossil tooth, this organism is particularly intriguing because it has a double-hinged mandible with two hinge joints called condyles, just as in most modern insect species. This type of jaw could belong to a wingless insect like a silverfish or firebrat, placed in the order Zygentoma (formerly Thysanura). However, all flying insect
species evolved from ancestors with a double-hinged mandible. This has led some scientists, such as Michael Engel, to speculate that insect flight may have arisen much earlier than previously thought, perhaps as early as the Devonian. Although there are no fossil wings to validate this claim, it is a reasonable possibility. Further, if insects with such refined body plans had indeed emerged by the Late Devonian, then it leads to further speculation that the six-legged body form may have also evolved earlier than usually supposed and that perhaps insects first lived during the Late Silurian. Such thoughts must remain speculative for now, pending the discovery of new fossils that might push the date of the earliest insect back even farther in time. What we do know for sure is that true insects had evolved during the Devonian period, and the terrestrial ecosystems that they emerged in were present at least since the Silurian.

During the Late Devonian, 360 million years ago, complex forest communities of treelike plants had emerged and were widespread across moist tropical lowlands. In the deeply shaded soils below, under thick layers of accumulating leaves, there lived abundant communities of small six-legged scavenging arthropods, including springtails, jumping bristletails, and silverfish. The planet had become buggy. These crawling organisms would seemingly be content to stay in the comfort and safety provided by the soil, the leaf litter, and other mossy, humid habitats. Yet at the end of the Devonian, and onward into Carboniferous times, the world again changed dramatically. At the end of the Devonian the planet cooled somewhat, glaciers formed, sea levels dropped, and the marine reef community experienced mass extinctions. On land, the composition of plant communities drastically altered into the Early Carboniferous period. But the most dramatic change, perhaps, is the first appearance of flying insects during the later part of the Early Carboniferous. By the second half of Carboniferous times, about 320 million years ago, the earth’s forests were forever transformed into a fairyland of glimmering and fluttering wings. The next part of the story, the tale of insect flight origin, is one of the most important turning points in the history of life. Where in the world did wings come from?

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Dancing on Air

In more than 600 million years of animal evolution, there have only been a handful of novel features, such as the wings of insects, that seem in our ignorance not to be mere modifications of something that came before.

DOUGLAS J. FUTUYMA,
Science on Trial

I have an enduring obsession with wings and flying things. There are few sights more fascinating for me than watching an insect in flight and few tasks more challenging than pursuing one. For as long as I can remember I’ve been particularly intrigued by the more colorful flying creatures like butterflies and moths, but I don’t think that I’m the only one. Judging from the growing popularity of butterfly houses and insect zoos, it appears that most people gain pleasure from the sight of butterflies in flight, even those who detest other sorts of insects. There is a magical, almost fairy-tale quality to them. Is this because we’ve seen too many Disney movies with delicate little fairies set with insect wings? Or do cartoonists draw fairies with insect wings because dragonflies and butterflies occupy a magical place in our psyche? I doubt anyone would enjoy the fairies so much if we depicted them with bat wings or fish fins.

I’ve been observing insects for fifty years, but one recent June I witnessed a truly magical event, the likes of which I’d not seen before. My family and I were visiting relatives at their cottage on Canada’s Lake St. Clair. We were enjoying a Father’s Day barbecue, and by coincidence massive numbers of
Heptagenia
mayflies, which had emerged from the lake overnight, were covering all the trees, the bushes, and the cottages. I’d seen this phenomenon several times before, as a child visiting lakeshores in northern Michigan, but never anything of this magnitude. In the evening, as the sun set orange and fiery over the lake, they rose, shimmering, by the thousands, by the millions, and
engaged in a frenzied mating dance. Vast mayfly clouds soared along the shorelines as far as they eye could see, extending ten to fifty feet above the ground, to about fifty feet over the lake. The density of insects was so great that, standing below a swarm and looking up, I could hardly see the sky. Just within my field of view, several million mayflies, mostly males, were dancing in the twilight, slowly rising up and drifting back down in undulating waves. Swarming is a male tactic, a visual display for attracting virgin females from a distance.
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Dance ’til You Die

Modern mayflies are truly ephemeral creatures. Since their young nymphs breathe using platelike tracheal gills, they require very clean, cold freshwater for their survival. Because of recent human-induced waterway pollution, massive emergences of many species are increasingly less frequent. Even so, aquatic mayfly nymphs, also called naiads, spend most of their lives hidden in the bottom sediments of freshwater lakes, marshes, and streams, where they feed on algae and organic debris and develop gradually. Once a year, often in synchrony, they emerge from the water, molt, and develop wings from their nymphal wing buds. The adults then have very short lives; many live for less than one day.

Examples of paleopteran insects (a term that means “old wings”), mayflies are among the oldest surviving insects with the most ancient sort of wing design. If you get a chance to see a mayfly, take a close look. You’ll observe a relict that developed flight about 330 million years ago in the Carboniferous times. Like most insects, mayflies have four wings: a front and a back pair located on the middle and last segments of the thorax. The front wings are much larger than the back ones, and provide most of the lift for flight. All four wings are simple, however, in that they are capable of moving only up and down; mayflies don’t have the ability to flex and twist their wings at the base, as most modern insects are able to do. A mayfly’s wing is a delicate membrane overlaying a complex supporting network of many fine veins. Mayflies share this high number of veins with the earliest known flying insects from the later half of the Carboniferous (the Pennsylvanian subperiod). They have another odd feature that is considered to be very primitive: after emerging from water and developing wings,
they do not fly right away. Instead, mayflies go through a subadult molting stage with functional wings. After one day they molt again, into adults. They are the only living insects that molt after their wings develop, so if you see a winged insect other than a mayfly, you know you are looking at an adult.

FIGURE 5.1. A small mayfly,
Baetis magnus
(order Ephemeroptera). Mayflies are considered to be the most primitive living examples of flying insects. (Photo by David Rees.)

When mayflies fly, they do so mainly to reproduce. By synchronizing their swarms, however, they are able not only to attract and meet mates but also to swamp predators. Mayflies are not very strong or adept fliers. They can do little more than flutter their wings and drift
and glide in easy patterns. Birds catch them easily, and fish eat their fill as the mayflies land on water. Yet all the predators in the neighborhood can’t make a dent in a mayfly swarm. When a virgin female flies into the disco-dancing cloud, she is quickly grabbed by a long-legged eager male, who transfers a spermatophore. Immediately after mating, the female flies back to the lake, then lands and floats on its surface. If she is lucky enough not to be eaten by a fish, the mother mayfly quickly dumps her eggs into the water as she dies. The eggs sink to the bottom and the cycle of death and rebirth is repeated, just as it has been for 320 million years.
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When mayflies first fluttered over ancient marshes, the skies were an open frontier: there were no birds or other vertebrates to chase them in the air.
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But the Carboniferous freshwaters were home to many species of jawless fishes and amphibians. If mayflies returned to streams and ponds to lay eggs, many were probably eaten, so even then natural selection would have favored synchronized emergence as both a predator-swamping and a mate-finding strategy. I’d like to imagine that even Carboniferous mayflies danced in large clouds to reproduce.

Although flight allowed the mayflies to rise in the air, easily find partners, and breed in relative safety, wings also served another valuable function: they helped mayflies to disperse. Nymphs that develop in streams tend to get washed downstream as they feed and develop, and by the time they emerge into adulthood, they are some distance from the egg-laying sites. With their wings, early mayflies could have moved easily upstream, laid eggs, and colonized inland lakes and ponds, which had fewer fish than the deeper waters.

Coal Country Tour

When those first mayflies took to the air some 320 million years ago, they glittered and danced over vast lowland marshes, swamps, and tropical wet forests covered with giant horsetails, club mosses, ferns, now-extinct seed fern trees, cordaites plants, and ancient conifers.
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These forests were unlike any that exist today, yet we know them well because they left behind abundant fossil plants. The accumulations of these plant materials—the most prolific in the history of life—became most of our natural gas, oil, petroleum, and especially our coal, which
are remnants of trees that grew to heights of thirty feet and more in dense wet forest stands. During storms these large trees fell into the moist Carboniferous soils and swamps. Their remains were flooded and buried under sediments, piling up to form deep layers of carbon-rich organic debris. Over time the pressure of geological activity transformed the layers into coal. My university generates electricity from a coal-fired facility, so as I write these words I’m using power produced from some of the last vestiges of Carboniferous plant life. When you drive your car to the supermarket, you might be using some as well.

Why does most of our coal and much of our petroleum date from the Carboniferous period? The orthodox view is simply that the Carboniferous swamps provided optimal conditions for fossil fuel formation. After that time, the world became drier, and conditions were not as favorable. But is that all there is to it? Forests didn’t go away after the Carboniferous. If anything, there were even more trees, growing ever more widely at higher elevations, and growing to greater heights over the Mesozoic and Cenozoic. After the Carboniferous, the continents may have been drier inland, but they still had lots of streams, rivers, and coastal wetlands. Trees from dry highlands could easily have washed into rivers and accumulated in lowland marshes. Something other than a change in the weather must have occurred.

Let’s consider another question: what happens to a tree when it dies in a modern forest? Birds, such as woodpeckers, chickadees, and nuthatches, excavate cavities in it. When these birds move out, lots of small mammals move in and excavate even larger spaces. Other smaller animals cut tunnels in the dead wood: bees and wasps, for instance, do so not to seek food but purely to build nesting sites. The tree’s bark is attacked and colonized by various insects, such as bark beetles, flat-headed wood-borers, and gnawing bark lice. Their tunneling activity loosens the bark and allows fungi to spread under that surface. Fungal growth speeds the tree’s decomposition and provides an even more nutritious food source for insects than the wood itself. Larger species of wood-boring beetles lay their eggs on the tree, and their grublike larvae tunnel deep into the heartwood. Meanwhile, down below, bacterial and fungal growth decompose the roots, which small soil arthropods chew on. Eventually the roots are weakened and the tree falls in a storm. This totally changes the local environmental conditions, since much of the tree is now spread across moist soil. It
is even more exposed to fungi and small soil arthropods. In tropical areas, the termites move in and tear the wood to pieces. They, along with the wood roaches, are among the few animals capable of digesting wood because of the symbiotic microorganisms living in their guts. In temperate areas as well as the tropics, the carpenter ants will likely move in as well. They do not eat the wood; they simply tunnel and live inside it, but eventually they reduce a large tree to small fragments and wood dust. The pieces are mixed with soil and further reduced by soil fungi and micro-arthropods. In modern forests, there isn’t much left that might survive and fossilize into coal or petroleum. Most of the tree is recycled back into the forest’s living systems.