It can be applied to other body parts on insects, like mandibles. My oldest friend in the world -- we've known each other since 2nd grade -- has an AB in entomology, as does his wife, and they can carry on "normal" conversations using all kinds of strange words. (I often wonder what kind of vocabulary their daughter will have; they're all musicians, too.) Since a lot of taxonomy is based on shape, position, and orientation, this kind of terminology is common.

On to the etymology: L porrectus ppl of porrigo, porgo 'to reach out, extend, offer' (< por- 'forward' + rego 'to keep in a straight line').