From Science News:
Back when hardware meant bony plates and flesh-rending teeth, a living version of the humble screw evolved naturally in, of all places, the leg joints of weevils.
The legs of at least 15 kinds of weevils have tapering, threaded, somewhat pointed ends where their legs meet their bodies. As the leg shifts position, the threaded tip tightens or loosens along a ridge on the inside of a rounded hollow structure, researchers in Germany report in the July 1 Science. “This is the first description of a true screw and nut in an organism,”...
Neat, huh? Of course, their legs don't unscrew completely, since they've got muscles attached to them. But apparently, this screw-and-nut joint allows for some real flexibility in leg positions.
As one scientist notes, "It does confirm my notion that just about anything that is possible, insects will have evolved."
(This reminds me of the g'Keks, who are aliens with biological wheels instead of legs, in David Brin's Brightness Reef. Of course, that's science fiction. And it actually wasn't clear that they'd evolved that way naturally.)