Why do flies "rub" their front legs when resting?

Question

I've noted that every time a house fly is resting on a surface, it appears to "rub" or "scrub" its front legs in a repetitive movement. And they do it continuously. Why?

Answer 1

Flies "rub" their legs to clean themselves! They rely on their sense organs (antennae, compound eyes, bristles, etc) to navigate the world around them. Flies pick up small dirt particles as they fly about, so they make an effort to clean not just the legs, but the whole body.