Free Planet of the Bugs: Evolution and the Rise of Insects by Scott Richard Shaw Page A

moved and foraged outside the water along the shorelines, but the prevailing opinion is that they wereessentially 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 atea 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,