February 1st, 2010


Urban Nature Pictures 2/1; 50 more Urban Species #3: Running Crab Spider

There was a slight edge to the new vet's voice: "Who's good with spiders?" She's new, forgive her. I practically shoved my vet tech coworker aside to be the first to see the offending arachnid. (It's been a long fairly bugless winter, forgive me). A small gray spider, all 8 legs outstretched like the fingers on two hands, slowly descended on a silk line, just to the left of the new vet's desk. I didn't know she wasn't a spider fan. To her credit she merely wanted (with some urgency) for the spider to be relocated out of doors. Of course, no problem; but first: some pictures.

Weber's spider guide identified it as an "inconspicuous running crab spider." Ten syllables is a bit much for a common name, so I'll stick with simply "running crab spider," Genus Philodromus. The individual above is curled in a defensive posture, having had his fill of rough handling by the photographer. Normally this type of spider is flat against a wall, or (better for his camouflage) a tree. They spin no web, instead chasing down insect prey or seizing it caught unawares. They are not closely related to other crab spiders, like the goldenrod crab spider, or the giant crab spider.

According to Weber, a spider found in the winter is likely to be a youngster, which can even be found running across the surface of the snow. They mature in spring and produce eggs in summer. Weber implies that Adults don't survive a full year, but this one appears to be a mature male, by his size (5mm body length more or less) and his palps. Looking like little boxing gloves, the palps are sensory appendages that on the male are used in the delicate mating act. He uses them to convey a packet of sperm to the female's epigynum.* Suffice it to say: if your spider holds little boxing gloves in front of its face it's probably a male.

Why does the running crab spider come indoors? The short answer is probably "because it can." Its flattened body makes it more likely to see tiny cracks in doorways and windowsills as opportunities. Once inside the artificial tropics we provide, it has its pick of flies, moths and other household wildlife to prey on.

*I don't think I want to keep on describing spider sex, but this glossary was helpful to me, if you care to go on. It begins the definition of Epigynum: "The more or less complicated apparatus for storing the spermatozoa..." That's all I could take for now.