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Why You Can't Kill a Mosquito with a Raindrop

By Elizabeth Preston
Jun 5, 2012 4:01 AMNov 5, 2019 12:20 AM


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Compared to a spindly mosquito, the mass of a raindrop is like a bus bearing down on a human. Yet the delicate insects thrive in wet, rainy climates. To find out how mosquitos live through rain showers, researchers pelted them with water drops while filming them at high speed. They saw that the insects' light weight, rather than being a liability, might be the key to their survival.

David Hu is a professor in both the biology and mechanical engineering departments at Georgia Tech. He's previously studied how water striders take advantage of fluid dynamics to skate across the surfaces of ponds. Andrew Dickerson, a graduate student in Hu's lab, has used high-speed video to find out how dogs and other animals shake water off of themselves. And in their newest study of animals getting wet, the team asks why a rain shower doesn't flatten every mosquito around.

The researchers trapped mosquitos in small mesh cages and sprayed them point-blank from above with jets of water. This Supersoaker-esque blast was similar to raindrops falling from the sky at terminal velocity. To get detailed video of collisions, they also hit mosquitos with drops falling at a slower speed.

The first thing they saw was that mosquitos made no effort to avoid the water. And they seemed to know what they were doing, because all the insects that got hit survived.

Going to the tape, the scientists saw that the consequence of getting hit by a raindrop depends on what part of the mosquito's body takes the blow. Since the insects are so lanky, 75% of hits happen on the legs or wings. This can throw a mosquito into a brief tumble or even a barrel roll, but it recovers without much trouble.

Direct hits to mosquitos' bodies are a different kind of carnival ride. The speeding raindrops glom onto the insects and propel them downward. Mosquitos captured on camera sometimes fell as far as 20 body lengths while being pushed by a raindrop. For a human, that would be a 12-story drop and a quick ending to the story. But mosquitos are able to pull away sideways from the raindrops and continue on their way, unharmed.

The only danger seems to come if mosquitos are flying close to the ground when they're hit, leaving themselves too little time to escape. The authors note that one unlucky bug was driven into a puddle and "ultimately perished."

To crunch some numbers—and find out why no mosquitos were being crunched—the researchers turned to substitute bugs that were simply Styrofoam balls of different sizes and weights. Although a raindrop isn't any bigger than a mosquito, the insect is extremely lightweight compared to the water. When the heavy drop hits the airy mosquito, it's almost like hitting nothing at all. And this, the researchers found, is what keeps the mosquitos alive. By offering barely any resistance, a mosquito minimize the force of the collision. The raindrop doesn't even splatter when it hits.

Of course, a bus hitting a human is pretty damaging no matter how little resistance the person put up. Mosquitos have the added advantage of a hard exoskeleton to help them resist the blow.

There's another reason this impact is survivable, David Hu explained in an email: Even though the force of the collision is 100 times the mosquito's mass, it's still only equal to the weight of a single feather. ("If we were in a comparable situation," he added, "we would not survive.")

If the impact didn't kill us, the acceleration would. Humans being hurled downward generally black out around 2 or 3 G's. But a mosquito suddenly driven toward the ground by a raindrop experiences an acceleration of 100 to 300 G's. The authors note that "insects struck by rain may achieve the highest survivable accelerations in the animal kingdom."

Although not especially useful to people trying to kill mosquitos or survive vertical bus collisions, the research could prove very handy to engineers designing insect-sized robotic aircraft. To fly successfully through rainstorms, these aircraft might adopt some of the mosquitos' technologies. A low mass would minimize the force of collisions. And sprawled legs, the authors write, could give tiny aircraft enough torque to pull away sideways from a falling drop. Mosquitos also have water-repellent hairs that may help them separate from stuck-on raindrops; aircraft could achieve the same thing with hydrophobic coatings.

Now if they would only design the miniature robot planes to attack the mosquitos, we'd have some real excitement.

Andrew K. Dickerson, Peter G. Shankles, Nihar M. Madhavan, & David L. Hu (2012). Mosquitoes survive raindrop collisions by virtue of their low mass PNAS : 10.1073/pnas.1205446109

Images courtesy of the laboratory of David L. Hu.

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