While looking for snails in the backyard this afternoon, I noticed this multi-legged, whitish creature crawling around on the underside of one of the rocks I had turned over. I grabbed the camera and took some pictured of it.
I turned out to be a symphylan, an arthropod in the class Symphyla, distant relatives of insects. This page has a discussion of the position of symphylans in the evolutionary scheme of arthropods.
This particular individual was ~4.5 mm long from the front of its head to the tips of the appendages (cerci) at the rear end. It may very well be Scutigerella immaculata, apparently a common species, but don't take my word for it; I don't know much about these creatures.