Something ran out from under a leaf. Tiny, like a springtail, but it didn't hop. Whenever I tried to pick it up with my skinny paintbrush, it raced backwards, as fast as it had been running forward.To discover what it actually is, read the post at Wanderin' Weeta - an excellent nature blog.
Under a magnifying glass, it waved big, angular pincers at me, like a crab would. I captured it, finally, in a drop of water, then photographed it with the camera and the microscope.