Lightweight insects can ride water droplets, as long as they separate in time

By Susan Milius

Web edition: June 4, 2012
Print edition: July 14, 2012; Vol.182 #1 (p. 9)

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A mosquito survives being walloped by a raindrop by riding along on its descent to the ground, scientists have found.

Credit: Courtesy Tim Nowack, Andrew Dickerson and David Hu/Georgia Tech

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A raindrop hitting a mosquito in flight is like a midair collision between a human and a bus. Except that the mosquito survives.

New experiments show how the insect’s light weight works in its favor, says engineer David Hu of the Georgia Institute of Technology in Atlanta. In essence, the (relatively) huge, fast drop doesn’t transfer much of its momentum to a little wisp of an insect. Instead the falling droplet sweeps the insect along on the downward plunge. As Hu puts it, the mosquito “just rides the drop.”

The trick is breaking away from that drop before it and the insect splash into the ground. Mosquitoes that separate themselves in time easily survive a raindrop strike, Hu and his colleagues report online June 4 in the Proceedings of the National Academy of Sciences.

Such studies help reveal how animals evolved to take advantage of flight, says biologist Tyson Hedrick of the University of North Carolina at Chapel Hill. Mosquito tricks may also inspire engineers designing swarms of tiny flying robots, or interest physicists and mathematicians studying complex fluid dynamics at this scale.

Plenty of lab work has investigated how flying animals recover from disturbances, but there’s little work on raindrops because those collisions are very hard to study, Hu says. To mimic raindrop speed of about 9 meters per second, he and his colleagues tried dripping water off the third floor of a building toward ground-level mosquitoes. “It’s the worst game of darts you can imagine,” he says. “You have no hope of hitting them.”

Finally, Hu sprayed streams of water from a pump at caged lab mosquitoes and then refined the process by spraying mosquito-sized beads. His team found that mosquitoes hit with water survived using an insect version of tai chi: Move with the blow instead of resisting it. A raindrop can reach 50 times the mass of a mosquito, and after colliding, “the mosquito becomes a stowaway,” Hu says.

The wild ride comes with danger. Mosquitoes hitchhiking on water experience acceleration 100 to 300 times the force of Earth’s gravity, the researchers found. The previous champs for surviving acceleration had been jumping fleas, at a mere 130 times Earth’s gravity.

Such studies suggest insects are making tradeoffs, Hedrick says. Mosquitoes’ small mass might allow them fly through raindrops but leave them more vulnerable to other menaces, such as wind. Larger and heavier horseflies “should have no problem with wind but might be more disturbed by raindrop impacts,” he says.

Scientists who work in the field know how readily mosquitoes can survive wet weather. “I’ve worked in the field many rainy nights,” says entomologist Nathan Burkett-Cadena of the University of South Florida in Tampa, “and received zero respite from mosquitoes during even heavy rains.”

Water drops slamming into insects can knock them partly or completely off course, this laboratory video shows.

Credit: Courtesy Andrew Dickerson and David Hu/Georgia Tech