Planet of the Bugs: Evolution and the Rise of Insects

by Scott Richard Shaw

  One Small Step for Arthropods

  The story of land colonization is usually considered to be the story of the Silurian period, 444 to 419 million years ago. There is evidence that some living things may have been on land before that time. There is even a debate about what it means to be on “land.” We’ll get back to that point. Suffice it to say that the Silurian is the first age of life where we find abundant fossil evidence of both land animals and land plants. By the end of the period these groups had formed terrestrial ecosystems, at least in marginal, wet marshlands. Nevertheless, these simple ecological systems undoubtedly gave rise to all the later land-based communities of life.

  I’m surprised by how often people equate the word “animal” with the word “vertebrates.” Recently I came across a science article claiming to be about the “first land animals,” but it was about lungfish. Let me make one thing abundantly clear. The arthropods are animals, and they were the first to lift their little legs and step on land, at least by the Early Silurian. The arthropods were best equipped to make the journey. They had the necessary protective gear (external skeletons) and locomotion system (jointed legs) since the Cambrian years. Those lazy, slow-witted, slimy, lumbering lungfish ancestors of ours didn’t manage to crawl onto land until sometime during the Devonian—a full forty million years after the arthropods accomplished it. The fact that they were able to do it at all is another contingent event, requiring that some fish just happened to develop enough bony structure in its fins to possibly support its bulky weight on land. It’s another coincidence of history, without which none of us terrestrial vertebrates would be here. Again, our mere presence in this story seems nothing less than miraculous.

  But as the cartoonist Larry Gonick has adroitly pointed out, we descendents of the lungfishes are the ones who write the history books. And once again it becomes necessary to point out the very subtle human-centrist bias that we have crafted into the history of life, simply by calling the Silurian period the “age of land colonization.” We casually and nonchalantly overlook the glaring fact that vertebrates played no role in this drama. For tens of millions of years we continued to paddle around in the oceans, and now we have the unmitigated nerve to imply that the stage was somehow being set for us. Life proceeded quite nicely on land for tens of millions of years without us, and it might easily have done so forever.

  Also, by calling the Silurian the time of land colonization, we subtly distract attention from the other major ecosystem: the oceans. We glorify the colonization of land simply because it is a necessary step in the processes leading to the evolution of humans. But the real Silurian news story is the glorious diversity of life in the oceans. The Silurian marks the time of the first coral reefs. These weren’t composed of corals like the ones we have today, but of ancient rugose and tabulate coral species that later became extinct. The trilobites didn’t go away yet, either; there were still lots of those, along with huge numbers of ammonoid shelled squids and brachiopods and a diversity of fishes. These fishes were mostly jawless, but the Silurian also included the first jawed fishes, the first armor-plated fishes (called “placoderms”—some were up to thirty feet long), and the first freshwater fishes, all of which were also jawless. Before returning to the land, we need to acknowledge that the real pinnacle of biological systems of that time—the peak of Silurian diversity and ecosystem complexity—remained out there in the oceans. We should probably call the Silurian the “age of the first coral reefs.”

  Although the trilobites were declining in species richness, some of the remaining species were quite common in the Silurian coral reef ecosystems. One particularly abundant trilobite was Calymene celebra, which is now celebrated as the state fossil of Wisconsin. During Silurian times, what is now Wisconsin was located south of the equator and entirely covered by shallow seas teeming with trilobites. As a result of these ancient warm seas, the limestone formations of southern Wisconsin are layered with Silurian trilobites, mollusks, brachiopods, and corals. The Wisconsin trilobite Calymene was a bottom-feeder that had the ability to roll into a ball to protect itself from predators, a defensive behavior that may have contributed to its continued success.

  Wisconsin isn’t the only state to honor a Silurian animal. New York has declared a sea scorpion, Eurypterus remipes, as their official fossil. Sea scorpions lived from the Cambrian through the Permian periods, a span of about 250 million years, and although they originated in the oceans, some colonized brackish and freshwater habitats. Sea scorpions are quite notable as probably the largest arthropods that ever lived. Some of the largest species grew to monstrous body lengths of seven to eight feet long. These animals were not true scorpions but more like a predatory version of a modern horseshoe crab. They had a long, sharp, spinelike tail—hence the name “sea scorpion”—but there is no indication that they could sting. They did have large spiny legs for grasping prey, and some had pincerlike claws. So these were probably the first predators that could efficiently feed on the hard-shelled trilobites and brachiopods.

  More than 300 sea scorpion species have been discovered from all around the world, but the New York fossils remain particularly important. The first sea scorpion ever discovered was found in 1818 in Silurian rock layers from that state. Around 420 million years ago, the entire area between Poughkeepsie and Buffalo was covered by shallow Silurian seas, and so the rock formations there are so full of their remains that the region is called the “sea scorpion graveyard.” Without question, these creatures were among the most spectacular residents of the Silurian coral reefs.

  Far less spectacular, but far more abundant and diverse, were the brachiopods, which evolved some thirty thousand species in the ancient oceans. Their common name—lamp shells—comes from the fact that the shells of some brachiopods resemble the shape of an ancient Roman lamp. They also resemble clams, but the resemblance is only superficial, as the two shells of a clam are similar to each other in size, while brachiopods have a smaller top shell and a larger bottom one. Lamp shells peaked in diversity during the Ordovician but retained high species richness over Silurian times. Some of the brachiopods cemented their shells to surfaces to keep them in place, so they were important in building the structure of Silurian reefs. So abundant were the lamp shells in Paleozoic seas that they now are probably the most common fossils in the middle-eastern United States. The very first fossil that I discovered as a child was a brachiopod lamp shell, found protruding from a rock along the banks of the Mississippi River. The state of Kentucky has declared any brachiopod as its state fossil, not bothering to name any particular genus or species; there are just too many of them.

  One Giant Leap for Arthropod-Kind

  The coral reef ecosystems may have been the biological pinnacle of Silurian times, but since insects are fundamentally terrestrial animals, the story of land colonization must still be told, with a slightly more arthropodan bias. It may have taken tens of millions of years, but eventually species richness on land did outpace that of the oceans; the complexity of our tropical forest ecosystems has vastly outstripped the complexity of our ocean reefs ever since. The pitter-patter of those little arthropod feet echoes loudly across the ages and had profound implications in shaping life’s subsequent diversity.

  Many biologists have long assumed that plants needed to colonize the land first and to establish ecosystems for animals to occupy. That may not be the case, as some good evidence suggests. Namely, there are trace fossils of arthropod footprints, fossilized tracks, dating to sediments from the Late Ordovician. Even if terrestrial plants were present then, it’s clear from the footprints that arthropods were walking out on the open wet soils, quite separate from plants, at the earliest of times on land.

  If arthropods were strolling on the beaches more than 443 million years ago, what they were doing there? They may have been avoiding deepwater predators. We must assume that the very first animals to walk on land were arthropods that lived in the shallowest waters, in the intertidal zones. Our longtime compa
nion the moon played a significant role in the evolution of life by creating these pools and the tides that shape them. When the tides ebbed and flowed, any arthropods that could survive on the moist shorelines at low tide would have benefited greatly, simply by avoiding the big predators. As the Silurian progressed, the coral reefs presented an increasingly hostile environment. While the tide moved out, predators that breathed with gills, such as sea scorpions, cephalopods, fish, and even large trilobites, swam into the deeper waters. The little arthropods that survived along the shorelines enjoyed a peaceful safe haven, perhaps.

  FIGURE 3.1. A coiled millipede is a quintessential example of a myriapod: a long, multisegmented arthropod with lots of legs. Creatures somewhat like these were among the first animals to colonize land. (Photo by Kenji Nishida.)

  Two groups of arthropods appear to have colonized the shorelines at about the same time: the arachnids and the myriapods. The arachnids were the scorpions and the group from which spiders, mites, and their relatives are descended; the myriapods were long, multisegmented, multilegged creatures, the group from which millipedes, centipedes, and insects evolved. Let’s look at each of these animals in turn and consider how and why they might have migrated to the beaches.

  Sting Time on the Beach

  Among the oldest fossils of terrestrial animals are the first scorpions, dating from the Late Silurian. We may call the Silurian scorpions “terrestrial” because they clearly moved and foraged outside the water along the shorelines, but the prevailing opinion is that they were essentially semiaquatic. They breathed with numerous flat respiratory plates layered like the pages of a book, which are called “book gills.” These breathing plates must remain wet to function, so the Silurian scorpions must have moved in and out of the water to keep their gills moist. It was not until much later, in the Devonian, that arachnids developed similar but internalized “book lungs” and became fully terrestrial. Like many modern semiaquatic organisms, Silurian scorpions could probably venture along the shores for extended periods, as long as their gills remained wet.

  We can learn a lot about these early land colonists by looking not only at Silurian scorpion fossils but also at modern living scorpions. That’s because the living world includes a composite of organisms that evolved at various times in history. Different species evolve at different rates, depending on how they interact with their environments. Well-adapted organisms may not change significantly over long periods of time, so ones that first evolved long ago, like horseshoe crabs and scorpions, are known as “living fossils.” That’s not to say that scorpions haven’t evolved and changed over time. They have. At some point in the Early Silurian there was only 1 scorpion species, and it was aquatic. In the modern world there are more than 1,100 species, and each has unique characteristics. They are all terrestrial, and some have adapted to life in some of the driest conditions, in deserts. But others still require moist living conditions, preferring the earth’s tropical rainforests. Still, when you look at a scorpion you are seeing a body form that originated in the Early Silurian with some of the first land colonists.

  Scorpions are nocturnal. By day they hide in cracks and crevices, under rocks, and beneath other objects. If the first scorpions were active at night as well, then their pioneering steps onto land were probably taken in the moonlight, to avoid the sun’s intense ultraviolet radiation. Remember that the ancient scorpions breathed with book gills and could venture out of the water only for as long as the gills stayed wet.

  Scorpions are predatory. They never feed on plants, so these arthropods, at least, could easily have colonized the land well before plants did. Modern scorpions feed extensively on insects, which didn’t exist during the Silurian period. What did they eat? If the myriapods occupied the shorelines at the same time, then the scorpions probably ate a lot of them. But if not, there were still plenty of meal choices in the rocky intertidal zone. At low tide, numerous small animals would have been trapped in shallow tidal pools, just as they are today. Soft-bodied animals like annelid worms, small fish, and molting trilobites would have been easy pickings for scorpions, which feed with claw-like chelicerate mouthparts by ripping and tearing their prey to shreds. Scorpions also have large pincerlike claws called pedipalps, capable of manipulating prey and pulling soft tissues from hard shells, and a venomous sting capable of paralyzing small animals. Since brachiopods would have been abundant in the Silurian’s intertidal zones, they too were possibly among the early scorpions’ prey; if a scorpion could hit a brachiopod’s soft parts with its sting, it could then use its pincers to pull the animal’s body from its shell.

  It is no secret: scorpions suffer from a major public relations problem. We almost universally loathe them, probably for very good reasons. All scorpions possess potent venoms used to paralyze and subdue prey. At the very least their sting is quite painful to humans, while at the very worst it is sometimes deadly. That, coupled with their habit of moving around only in the darkness where we can’t see them coming, makes them not very much fun to be around. If you travel in the tropics, you really do need to learn to shake out your shoes in the morning, since scorpions like to hide there.

  Some scientists have suggested that humans have an instinctive fear of certain dangerous animals like snakes and spiders. We should probably add scorpions to that list, because the mere sight of one quickly sends many of us into a panic. Maybe we retain some primal, genetically programmed fear of these creatures. Consider the situation for our fishy Silurian ancestors. In the deeper waters, by the coral reefs, they had to contend with the likes of the monstrous eurypterid sea scorpions, and in the balmy shallow waters, they had to contend with the likes of the stinging scorpions. The Silurian was not a very pleasant time for our vertebrate ancestors, and once again, we were lucky to have survived it.

  Having said all those nasty things about scorpions, I’m going to give you a reason to like them. The females are really nice mothers. In fact, they may provide the oldest case of parental care. Unlike most female arthropods, which simply lay eggs and let the young fend for themselves, female scorpions carry fertilized eggs inside themselves. The eggs take many months to develop, and eventually a female gives live birth to anywhere from six to ninety tiny baby scorpions. Looking like miniature versions of their mother, they crawl onto her back, where they ride around for a week or more. The baby scorpions stay under mom’s protection until they have completed their first molt, then they wander off on their own adventures.

  FIGURE 3.2. A mother scorpion with her babies onboard. (Photo by Piotr Naskrecki.)

  Just because they are nice mothers doesn’t mean that female scorpions are necessarily nice wives, however. In addition to being dangerous, they tend to be larger than the males, who seem to show an appropriate amount of caution and respect when attempting to mate with them. During their elaborate courtship ritual, a male and female face each other, raise their tails, and move in circles for hours, or even days. Mating eventually occurs indirectly. Male scorpions produce a packet of sperm cells wrapped in a membrane: a spermatophore. When a male deems the time ready, instead of coupling with a female and transferring his sperm cells to her directly, he places his spermatophore on the ground, then attempts to lead her over it. This ancient behavior doesn’t sound very efficient, but it seems to work well enough for scorpions, and we see it preserved in some of the most primitive living insects.

  The scorpions’ reproductive behaviors may provide insight into their Silurian landfall. The spermatophore’s membrane helps to slow desiccation, but it needs to remain moist or the sperm cells will dry out and die. Since solar radiation could damage these cells, spermatophore transfer can be more safely done under the cover of darkness. This suggests that scorpions initially colonized shorelines not only to seek food, perhaps, but also to fool around on romantic, moonlit Silurian beaches. The fact that female scorpions retain developing eggs inside their body and give birth to maternally protected live young, however, suggests that the Silurian strands were still
dangerous. They may have been comparatively safer than in the deep water, but there were still predators, such as large centipedes, other scorpions, and even larger individuals of the same species, that would have eaten the scorpions’ eggs and young.

  She’s Got Legs . . .

  The myriapods, multilegged relatives of the insects, have been present in the background of our story, but we haven’t said much about them. You may remember that back in the Early Cambrian oceans, in the Burgess Shale fauna, a few of these leggy creatures scurried along in the bottom sediments. Their body design was very simple: a head up front with one pair of antennae, followed by lots of segments, each with a pair of legs. It’s the simplest body plan from which a huge range of arthropod forms can be simply evolved, by a process we’ve discussed already with the trilobites: tagmosis. By fusing segments, functional body regions can be formed. By modifying legs, an assortment of feeding appendages or mating structures can also be developed. The myriapods, with their versatile body, now become key players in our story, because they are the ancestors from which modern insects evolved.

  Three groups of myriapods are worth mentioning here. The first two are quite familiar: the centipedes and the millipedes. The third is a rare tropical group: the symphylans. All three respire tracheally, by transporting air through internal tubes. This suggests that tracheal respiration was an innovation of the first myriapods which adapted to life on land, and that the myriapods passed it along to the insects. Although the centipedes and millipedes tell us a lot about the early colonization of land, they each have specialized in their own ways and evolved into classes distinct from the insects. The tropical symphylans, on the other hand, have a simpler body plan that more closely resembles the anatomy of the ancestors from which insects developed.