We live in central Mexico and these spiders are very common here. They are incredibly fast when they move, but most of the time they are absolutely still. The one pictured is about an inch and a half in diameter and is a very young one. We have seen them much larger, up to about three inches across. If you could help in identifying it we would appreciate it very much. We just call them "Flat Spiders" because they are.
Perry & Katherine
Hi Perry and Katherine,
We believe this is a Selenopid Crab Spider. Our Audubon Guide states: “These rather flat bodied spiders, … live under stones and bark. … When disturbed, they run sidewise, crablike, and hide in a crevice. Mostly tropical spiders, there is only 1 North American genus which occurs in the Southwest.” That genus is Selenops. The Guide describes that genus this way: “These spiders are active in dim light and darkness, seldom by day. They creep forward slowly to reach prey but run sideways much more rapidly. a characteristic of this genus.”