A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

Why flies rub their hands together: the untold story

February 4, 1994

Dear Cecil:

How come flies, after landing on someone or something, start rubbing their "hands" together? Planning something?

Cecil replies:

The truth is even stranger than you suspect. The flies most likely are … washing up. Heartwarming, no? Here is a creature notorious for its filthy habits, yet it finds time for good grooming. An example Cecil must set before his research assistants next time they decide to use oatmeal for shampoo.

Research on fly washing is a bit sparse. The best I could come up with, with the aid of Straight Dope friend of science Ed Lisowski, was a couple of papers on preening behavior in Drosophila melanogaster, a fruit fly, rather than Musca domestica, the common housefly. But the way I figure, a fly is a fly.

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.

Additional quantity of words that have been written about fly cleaning: 5,000. Additional quantity of said words worth repeating here: 0. Thank God you've got a guy like me to filter the dull facts of life.

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