Splish Splat? Why Raindrops Don't Kill Mosquitoes

When a raindrop hits a mosquito, the mosquito and drop join together, and the mosquito rides the drop for about a thousandth of a second before its wings, which act like kites, pull it out of the water. i i

hide captionWhen a raindrop hits a mosquito, the mosquito and drop join together, and the mosquito rides the drop for about a thousandth of a second before its wings, which act like kites, pull it out of the water.

CDC Public Health Image Library
When a raindrop hits a mosquito, the mosquito and drop join together, and the mosquito rides the drop for about a thousandth of a second before its wings, which act like kites, pull it out of the water.

When a raindrop hits a mosquito, the mosquito and drop join together, and the mosquito rides the drop for about a thousandth of a second before its wings, which act like kites, pull it out of the water.

CDC Public Health Image Library

Imagine how tough life would be if raindrops weighed 3 tons apiece as they fell out of the sky at 20 mph. That's how raindrops look to a mosquito, yet a raindrop weighing 50 times more than one can hit the insect and the mosquito will survive.

How?

Put yourself in a mosquito's shoes — or rain boots — for a moment and step outside into a downpour of seemingly gigantic raindrops.

"They're basically plummeting comets falling all around you," says David Hu, an assistant professor of mechanical engineering at Georgia Tech. You'd think a mosquito wouldn't stand a chance. "We expected the similar thing to happen as when you drive your car through bugs — you see this bug just splattering."

Yet mosquitoes clearly survive close encounters with raindrops. So Hu's group set out to run an experiment that made the most of their skills as mechanical engineers and biologists.

"Hitting a mosquito with a raindrop is a difficult experiment," he says. "The first thing we did was drop small drops from the third floor story of our building onto a container of mosquitoes, and you can imagine that didn't go very well. It's kind of like playing the worst game of darts you can imagine."

So the team took the experiment inside. They fired jets of water drops at the mosquitoes and recorded the results with a super-high-speed video camera. They found that mosquitoes don't actually dodge raindrops — they hitch a ride.

"As the raindrop falls, rather than resisting the raindrop, they basically join together kind of like a stowaway on this comet," Hu says. "So as a result they get very, very little force."

To them it's like getting hit with a feather. They ride the drop for about a thousandth of a second until their wings catch the wind like little kites, and tear the mosquito away from the drop. The mosquitoes don't seem any the worse for wear.

"It's hard to see the expression on a mosquito's face, but they definitely survived. And most of them didn't even land — they continued flying as if nothing had happened," Hu says.

The results are published in the Proceedings of the National Academy of Sciences.

The real hazard for mosquitoes is apparently if they are flying very close to the ground. If they don't peel off from the raindrop in time, they live out that axiom about being between a rock and a hard place.

"For example, if you're standing on the ground, and a piano falls on you, you basically get smashed," Hu says. He expects that's what would happen to the mosquito.

So what did researchers learn? For the folks who are making flying robots the size of insects, as long as your robot is small enough, you don't have to worry about rain.

For biologists, here's another exquisite example of how life has evolved to survive on a planet that's inundated with fluids. And for anyone who might be tempted to kill a mosquito by swatting it in midair — it'll never work.

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