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

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. Theeggs 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