Planet of the Bugs: Evolution and the Rise of Insects (9 page)

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 an
cient 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.

The centipedes are perhaps the most familiar myriapod group. There are more than three thousand species, mostly tropical, and they are active mainly at night. Centipedes have thirty or more legs, two per segment, and they really know how to use them: most can run very quickly. Unlike insects, centipedes do not have a waxy cuticle to prevent water loss. They can dry out rather easily, so they tend to stay in moist habitats near soil and avoid direct sunlight. All centipedes are predators, and they capture small animals with their fanglike front legs, which house venom glands. Most feed on other small arthropods, but some large tropical species, up to ten inches long, are capable of killing small vertebrates. Similar to the predatory scorpions, centipedes were certainly capable of surviving in the rocky intertidal zone and feeding on various other small animals long before plants colonized the land.

The millipedes, the leggiest arthropods, are called “diplopods” because they have evolved a unique body type: each segment has two pairs of legs rather than one, and contains two pairs of nerve bundles and heart valves. This shows that their segments formed when two primitive segments, each with one pair of legs, fused together. There are more than seventy-five hundred millipede species, and although they live primarily in the tropics, they can be found all around the world.

Millipedes are a lot nicer than centipedes. If you want a Silurian pet, I’d highly recommend one.
¹
They are friendly, they do not have venom or bite humans, and these days it’s not too unusual to find some of the giant African species for sale in pet shops. Like the centipedes, however, millipedes prefer to stay out of the sunlight, and so they hide in moss, tunnel in soil or under loose rocks, or live in caves. A few species are known to prey upon other soft-bodied arthropods and worms, but most are scavengers that eat decaying vegetation in addition to fun
gal or bacterial accumulations. It appears that the millipedes are yet another arthropod group that was perfectly capable of colonizing the beaches well before land plants evolved; these scavengers would have been able to feed on lots of non-plant-based organic material such as decaying green algae mats, fungi, and bacterial blooms in Silurian microbial soils.

FIGURE 3.3. A white millipede (order Polydesmida) illustrates a unique characteristic of these leggy myriapods: each segment is equipped with four legs. Polydesmids are the largest order of millipedes, with over 2,700 species known. (Photo by Kenji Nishida.)

The symphylans have escaped the notice of most people, but they are very important to the insects’ story because they most closely resemble the ancestral kind of myriapod from which insects evolved: namely, a short creature with fewer segments than millipedes and centipedes and only two unmodified legs per segment. The symphylans are quite small, only about 2 to 10 millimeters long (less than half an inch). There are about 120 known species, and they mostly inhabit the tropics. Like the millipedes, symphylans live secretively in soil, moss, and decaying vegetation and avoid the sunlight. Modern symphylans feed mainly on decaying vegetation, but like the millipedes,
they were capable of living on organic materials in microbial soils before land plants appeared.

These mysterious dwellers in the mosses have a very unusual method of reproduction. Male symphylans produce spermatophores, which they leave on top of long plant stalks. Females need to wander around and find them. Upon discovering a spermatophore, a female symphylan bites it, but instead of digesting it she stores the sperm cells inside her cheeks in special pouches. When she lays an egg, she reaches around and picks it up with her mouthparts, fertilizes it, and proceeds to glue the fertilized egg to a piece of moss.

Green Tide: Plants Colonize the Shorelines

Toward the end of the period, new, taller plants joined the myriapods in transforming the Silurian landscape. Two lines of evidence give us a good idea of what they were like. Preserved fossils from approximately 420-year-old Late Silurian sediments contain the archaic rhyniophyte plants, which are named after an early Devonian genus,
Rhynia
, discovered in Rhynie, Scotland. The oldest one,
Cooksonia
, was the very first vascular plant, and it grew only a few inches tall. Very simple and semiaquatic, the rhyniophytes lived along marginal habitats and had parts that could emerge out of the water. They did not have leaves, flowers, or deep roots, and the more advanced early Devonian species were also relatively short—about 50 or 60 centimeters long (mostly less than 2 feet). The rhyniophytes had creeping stems that ran sideways along the shore, probed tiny root hairs below into the soil, and sent shoots upward from multiple points along their top. Each vertical shoot forked once or twice, forming reproductive structures called “sporangia” at the upper tips. The rhyniophytes’ lateral stems allowed them to spread thickly over moist shorelines, since they contained vascular fluid-transporting tissues.

The second line of evidence comes from plant DNA. Molecular studies support the long-held assumption that land plants evolved from photosynthetic green algae and that the nonvascular plants—liverworts and mosses—evolved first, around the Silurian, followed later by primitive vascular plants, such as ferns. Liverworts and mosses require a lot of moisture to survive and decompose rapidly when they die, so they did not fossilize well; however, we can be sure
that the Late Silurian shorelines were full of them, as well as the rhyniophytes and a diversity of soil fungi.
²

If I haven’t said much about plants up to now, it’s because the terrestrial arthropods were able to thrive for millions of years before plants arrived and developed the capacity to survive. Arthropods had the initial advantage, because they developed their hard structural parts much earlier. More importantly, being mobile, these animals could pick and choose the time of their land expeditions. Because they’re nocturnally active and can easily avoid the sun’s harmful rays, the arthropods didn’t have to wait for the ozone layer to form before they colonized the land. They just did so under the cover of darkness.

Plants, on the other hand, need sunlight. They didn’t have the option of moving ashore at night and hiding by day. This means that plants were not able to survive on land until two things happened: they had to wait for a sufficient ozone layer to develop so they could remain safely exposed all day, and following this they had to develop structural support mechanisms. By the Late Silurian they solved the problem of structural support by evolving the complex molecules lignin and cellulose, and arranging the tough stuff into fluid-transporting bundles. Some scientists have suggested that plants must have colonized land first because they create the oxygen that terrestrial animals require, but the cyanobacteria and green algae had been producing this gas for billions of years before the plants moved inland. Ironically, they—not animals—needed elevated oxygen levels, for the ozone layer’s ultraviolet filtering effect and to build lignin and cellulose.