Why do clouds of gnats always hover around a fixed point in mid-air?

Dear Cecil:

Can you explain why gnats often gather together in large groups in mid-air, hover around an imaginary fixed point? Even if you wave your hand through a group of them and confuse their sense of locus, they return to regroup around the same point.

Cecil replies:

As you undoubtedly know, Alan, “gnat” is an imprecise term referring to several species of small flies, most often the fruit flies of the family Trypetidae. The swarming is part of a mating ritual common to many insect species wherein the males hover en masse and await the females. It’s believed the insects orient themselves over or near some easily recognizable topographical feature — a white object against a field of green, for example, or a tall bush — rather than some “imaginary fixed point.” Their uncanny persistence in doing so may be attributed to the fact that they have to propagate the species (and get in what giggles they can) in an extremely short time — gnats only live for a few weeks. Keep all this in mind next time you’re tempted to spoil some little fly’s fun by waving your hand through his orgy. Creep.

Send questions to Cecil via cecil@straightdope.com.

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