A Staff Report from the Straight Dope Science Advisory Board

Where do bugs go when it starts to rain?

December 25, 2007

Here in Louisiana, autumn is the season of the love bug (also known as the march fly, honeymoon fly, telephone bug, kissy bug, or double-headed bug; scientific name Plecia nearctica). One October weekend I was hanging out at the local tennis courts and the love bugs were swarming around me. There must have been close to a thousand bugs in a very small area. Then a slight breeze came up and the bugs began to disperse. When a hard rain began to fall, they disappeared entirely.

Here's my burning question: Where do bugs go when it rains? And while we're on the topic, why do mosquitoes come out at dusk? Where are they the rest of the day?

There'a a pretty simple rule of thumb about what insects do during a rainfall: if the impact from a single drop would be enough to knock a particular type of insect out of the sky, then insects of that type will seek shelter. This means that when it starts raining, larger insects will generally continue to go about their business while smaller ones will find a cool, quiet hiding place as quickly as they can. Any small sheltered spot will do – under eaves, in crevices, etc. A stable perch is better than one that blows around with the breeze, but beggars can't be choosers – if there's nothing around but vegetation, the insects will make do. March flies are smaller than the typical raindrop, so they won't remain exposed for long once it starts coming down.

There are some insects that prefer to fly while it's raining, the classic example being the west coast beetle family known informally as rain beetles – they're large, shaggy things very closely related to scarabs. A number of these species literally will not fly unless there's a downpour, and given their short adult lifespans and the very dry weather southern California (for instance) gets in some years, such beetles may have the opportunity just once in their lives, and only briefly at that.

As for mosquitoes: mosquitoes hunt by homing in on infrared emissions from warm-blooded prey, and that's a lot easier to do when the air temperature is cooler than the prey's body temperature. (Am I the only one here who's seen movies like Predator and Westworld?) The complicating factor here, of course, is that mosquitoes themselves are cold-blooded, so if the air gets too cool, they can't stay warm enough to keep flying – meaning there's an optimum range of temperatures for them to hunt in, which tends to present itself near or after sundown. The rest of the time they lay low in the same kind of hiding spots already discussed.

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