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Mystery Solved: Why Raindrops Don’t Kill Mosquitoes

Part of a scientist’s job is to ask questions ordinary people wouldn’t necessarily think of. Einstein famously wondered “what would it be like to ride on a light beam,” for example, and the answer led him to the Special Theory of Relativity.

For David Hu and several colleagues at the University of Georgia, the current big question is perhaps a little more mundane: how is it that mosquitoes can fly through the rain without getting knocked out of the air? The average raindrop falls from the sky at about 22 m.p.h., and if it scores a direct hit, you’d think the annoying little insect would be toast. Yet they seem to survive just fine, and this was a question the Georgia scientists couldn’t let go. “There have been lots of studies of insect flight,” Hu said in an interview, “but very few about flight in challenging conditions.” So Hu and three others conducted an experiment to find out the mosquitoes’ secret. The results appear in Proceedings of the National Academy of Sciences.

The first thing they learned was that it’s hard to aim a raindrop at a mosquito. “We first tired dropping [the raindrops] from the third floor, but that didn’t work out,” he said. So the team packed scores of mosquitoes into a tall, narrow tube and dripped water from the top, a drop at a time. “Sooner or later,” he said, “you're bound to hit one.”

What happened next depended on how far up the tube the ill-fated mosquito happened to be. If it was well above the bottom, the drop, which weighed about 100 times as much as the bug, would knock the mosquito aside and keep on going. “It’s like a human being hit by a truck,” Hu said, “but there’s a big difference.” The difference is that mosquitoes, like many insects, have exoskeletons, which give form to their bodies like our internal skeletons do, but also act like a protective suit of armor. “For us,” Hu said, “this would be an eyeballs-out impact.” For the insects, it’s just an annoyance.

It is, at least, when the bug is up in the air. If it happens to be sitting on a solid object — the bottom of the tube, for example, or a leaf or twig out in the real world — that’s another story. “It’s bad to be hit by truck,” Hu said, “but it’s worse to be hit by a truck when you’re standing against a brick wall.” If a mosquito is caught between a drop and a hard place, the force is something like 10,000 times its body weight. Even an exoskeleton won’t help in that kind of situation.

It’s also dangerous to get hit when the insect is close to the bottom: the scientists observed that the mosquito needs to be between five and 20 body lengths above the ground for it to recover from the impact. “If it’s struck low,” Wu said, “it hits with the speed of a falling drop, and it’s killed.”

The team also learned that mosquitoes don’t even try to evade raindrops — not surprising because the drop is falling 10 times faster than the mosquito can fly. “The duration of an impact is just a millisecond or so,” Wu said. “It’s very hard to react.”

At some point, a non-scientist is bound to wonder what the point of all this might be. After all, mosquitoes have been around for 170 million years or so, which suggests they can’t be easily killed by something as common as raindrops. In part the answer is pure intellectual curiosity, and in fact, Wu’s group is now looking into how mosquitoes navigate through fog.

In part, though, this research could also help guide the emerging technology of insect-size flying robots, which are now under development in many labs for possible surveillance and rescue applications. If you want a tiny robot to fly in bad weather, it’s useful to know how tiny animals do it.

Finally, there’s a question Hu and his colleagues haven’t even bothered to ask: what does this have to do with climate? The answer may be nothing — but there is a possible connection. One effect of a warming climate is that disease-bearing insects, including mosquitoes, could begin to expand their range into regions that were once too cold to support them. Another effect is that rainstorms are likely to become more intense. One might hope that the rain could keep the mosquitoes in check by knocking them out of the sky.

Based on the new research out of Georgia, however, that’s not looking too likely.