Planet of the Bugs: Evolution and the Rise of Insects (12 page)

It’s tempting to think that tagmosis was the driving factor in the evolution of six legs. It seems likely that as with the trilobites, the hexapods would be more successful in part because having fewer segments and appendages would simplify molting. No doubt mastering the molting process and developing a simple body form were key factors in early insects’ success, but it can’t be as simple as that. Indeed, some myriapods mastered the molting process despite the difficulties of having numerous appendages. Just consider the millipedes’ long-term success. They can have hundreds of legs, but they have survived for hundreds of millions of years. Some of the centipedes are fast on their feet as well. So although improving molting efficiency by reducing the number of body segments and legs could be an important factor in the origin of insect form, it can’t be the only one. It’s also important to consider that the evolution of six-legged form and speed was accompanied by the simultaneous evolution of very small body size. Some of the ancient Silurian and Devonian myriapods were quite large (some millipedes were up to fifty centimeters long). The most ancient hexapods, in contrast, were only a few millimeters long. Over the Devonian, arthropod body sizes underwent very serious downsizing.

The reasons for this trend in reduced arthropod waistlines should be clear enough. The ancient myriapods faced some very serious predators: scorpions, centipedes, and spiders. Natural selection by such macro-arthropod predators would select for smaller arthropods and faster arthropods with fewer legs, since predators tend to hunt preferentially for larger prey, selecting them out of the environment. But being very small has several other advantages. It allowed insects to get deep down into the moss and right into the soil, into moist environments where they could hide and more safely complete their molts. Moreover, small body size also provides a breathing advan
tage. Smaller animals have more surface area relative to the volume of cells in the body. Therefore the very smallest insects can breathe directly through the cuticle, because they are so small and live in a very moist environment where a thick skeleton is no longer needed. But the ultimate advantage to microscopic body size is that fewer resources are needed for survival. Small animals can grow and reproduce more rapidly than large animals; therefore, they evolve faster, and they can occupy much smaller ecological niches.

There’s one other advantage, perhaps, to evolving small body size and migrating deep into insulating blankets of moist soil. The Devonian period, with the evolution of plants with leaves, saw the first accumulations of leaf litter, and consequently the first wildfires. Charcoal deposits from the Devonian document that fires emerged with the first forest communities. With each fire the dry layers of leaves would be burned away, and communities of larger arthropods would recolonize the area. Small arthropods living in the deeper moist soil would be insulated from such local catastrophes.

Springtails Vault to Devonian Superstardom

Probably the most important fossil beds of Devonian-age hexapods are the Rhynie cherts of Aberdeenshire, Scotland. The fossils at Rhynie were formed between 396 and 407 million years ago, when present-day Scotland was a low-lying marsh located in the tropics. Hot spring activity at the Rhynie marsh produced crystal quartz called “chert,” which preserved some very small organisms with remarkable clarity; fossil plants from the area, for instance, are preserved in cellular detail. The Rhynie cherts also contain the oldest-known fossils of mycorrhizal fungi, but Rhynie is famous for another reason: among the fossils are the oldest examples of well-preserved hexapods,
Rhyniella praecursor
.
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Rhyniella
is also the oldest example of a soil hexapod that continues to thrive in the modern world: the order Collembola, known as springtails.

Apparently the first hexapod group to successfully colonize the finest niches available in microbial soils, springtails get their common name from the fact that they possess an unusual forked taillike structure on the abdomen that allows them to pole vault up to twenty times their body length and spring themselves to safety when disturbed. Spring
tails are extremely small, ranging in size from only one to ten millimeters long, and they can be exceedingly numerous: their populations commonly number in the thousands per square meter of soil surface. There are records of as many as a hundred thousand springtails per square meter of soil surface in some remote island shorelines, where there may be fewer predators. Some springtails prey on bacteria, nematodes, tardigrades, rotifers, and protozoa, but most of them scavenge on organic plant materials. Despite their small size, they still account for high levels of arthropod biomass, and because springtails are among the commonest soil micro-arthropods associated with decomposing organic materials, they doubtless contributed in a serious way to the processing of Devonian soils. Their feeding speeds the recycling of nutrients from decaying plants, and their activities improve the physical characteristics of soils, improving nutrient flow and drainage.

FIGURE 4.1. Springtails (order Collembola) are the most diverse six-legged arthropods inhabiting forest soil and leaf litter. Despite their microscopically small size, they are often numerous and important nutrient recyclers, and sometimes they are quite beautiful. (Photo by Kenji Nishida.)

Springtails share the same indirect reproduction methods with their myriapod ancestors—male springtails produce spermatophores, which they post on stalks in their moist, mossy environment—and
their mating rituals may seem comical to us. Some males spend a lot of time posting stalked spermatophores in a circle around a female, in high hopes that she will pick up at least one, only to see her pole vault away, out of the circle of love. Other male springtails have evolved clasping antennae that allow them to grab onto a female and ride around with her, hopefully until she becomes more receptive. These primitive mating methods would seem to limit the springtails to moist environments, yet modern springtails have evolved to survive under some surprisingly extreme conditions. Some have glycol antifreeze in their blood, and they are the only hexapods known to live along the shorelines of Antarctic islands. On the other extreme, some have evolved to survive in desiccating deserts. They can dry out but rehydrate when it rains.

Because springtails, along with other kinds of ancient wingless hexapods, are microscopically tiny, people rarely notice them. If you want to find them for yourself, there is actually a very simple sampling method, called a Berlese funnel, which takes advantage of their dislike of dry conditions. You can easily and cheaply fashion one of these at home. All you need is a large plastic funnel (like you might use for adding antifreeze to a radiator), some window screen, a jar, some alcohol, and a desk lamp or utility light. Cut a round piece of screen and place it in the bottom of the funnel. Set the funnel on top of a large jar with some alcohol, and add to the funnel a sample of moss, leaf litter, soil, or any such material likely to contain microscopic arthropods. Usually a scoop of any material from the forest floor will work well. Place the light above the wide end of the funnel, close enough to cast some heat and light on the sample, but not close enough to start a fire. As the moss and soil dries, the tiny arthropods will migrate downward to the bottom of the sample. When they get to the screen (if the mesh is not too small) they will fall through, into the alcohol jar. This simple method is one of the easiest ways to see a diversity of springtail species, and may be the only way most of you will see the other kinds of rarer hexapods discussed in the remainder of this chapter.

A Tale of Tails

Other kinds of primitive hexapods survive in the order Diplura (the diplurans). The name literally means “two tails” and refers to the two
prominent taillike cerci that extend from the end of their abdomens. The diplurans are considered to be closely related to the springtails (they share a unique mouthpart structure—the mandibles are withdrawn into a pouch in the head), but unlike the collembolans, they are scarce and rarely encountered. The only place I have commonly seen living diplurans is in the moist cloud forests of eastern Ecuador, where they, along with numerous springtails, inhabit the dense mosses and
soil layers that coat tree trunks and large tree branches, often high above the ground. Moreover, the fossil record of Diplura is very poor, which is hardly surprising since they are soft-bodied and live only in very moist and fungus-ridden microhabitats. Nevertheless, we know quite a bit about their anatomy and biology based on studies of several living species. Less specialized than that of a springtail, their body form is about as simple as it gets for hexapod arthropods: a head with multisegmented antenna and mandibles but no eyes, a thorax with six legs but no trace of wing development, and an elongate multisegmented abdomen ending with a pair of cerci. Diplurans come in two forms, depending on the shape of the terminal cerci. Some have long multisegmented cerci that look like antennae coming off the tail end, and they use them as back-end “feelers.” These species all seem to scavenge organic debris and fungi in soils and mosses. The other kinds of diplurans have cerci whose segments are fused into forceps-like pincers. Many of these species appear to be predatory and some are known to capture small prey, such as springtails or small insects, with their pincerlike “tails.” Like other kinds of very primitive hexapods, dipluran adults continue to molt their exoskeletons after they reach sexual maturity and adulthood, and some species are known to molt up to thirty times.

FIGURE 4.2. The two-tailed diplurans (order Diplura, family Campodeidae) are reclusive hexapods living in moss and leaf litter, which are seldom seen by humans. (Photo by Kenji Nishida.)

Among other primitive six-legged animals thought to have evolved in the Late Devonian are the jumping bristletails, or, simply bristletails. As the common name implies, they have long bristly tails, and they can jump by arching their body. Their scientific name, the order Archaeognatha,
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means “ancient mouth” and refers to their very simple jaw, which is hinged on only one weak point, at the lower head. These jaws are sometimes called milling mandibles, presumably because they can only gently grind food. Bristletail remains have been found in the Devonian Gilboa forest of New York, but several species of these ancient hexapods survive even today in moist forest soils and along shorelines near the oceans. Unlike most modern insects, they are unusual in having abdominal appendages called styli, which may be remnants of ancient leg parts. These living fossils have a few tricks of their own. They use their own feces to glue their bodies to the ground when they molt. This apparently allows them to more successfully pull themselves from their old skeleton during the molting process; how
ever, if the glue fails, they can’t get out of the old skeleton and they die. It’s an indication that early insects needed to evolve efficient molting methods.