Flies rubbing front legs

[marmaduke]

What are flies doing when they rub their front legs together, [edited by AnswerBank]

[Clanad]

We know that flies are cleaning themselves (as opposed to plotting) because when they get dirty they clean themselves more, starting first with the dirty part and concluding by dusting off their forelegs. The smart fly--granted we're not talking about a huge intellectual range here--keeps its legs clean because they contain an abundance of taste and tactile receptors, the better to savor the flavor of whatever rank thing the fly lights on next. Interestingly, flies clean more when in the presence of other flies, no doubt in an effort to impress the babes. Girl flies, perhaps predictably, spend more time preening than boy flies.

(Source :  The Straight Dope Q & A)