Why don't mosquitos get killed by raindrops?

Mosquitos ride raindrops instead of resisting them
Photo by Tim Nowack, Andrew Dickerson and David Hu
featured in ScienceNews article by Susan Milius on
July 14th, 2012.

Sometimes I wonder why I never thought of such an obvious question as the one in this title!  If we humans are hit by something that weighs a few hundred or a thousand pounds, we don't survive. A raindrop can weigh 2-50 times as much as a mosquito, and yet plenty of them survive rainstorms.

In a paper on PNAS (reference below), Dickerson et al. calculated that the impact force of a raindrop on an unyielding surface is 50,000 dynes, about 10,000 times the weight of a mosquito. Should be lethal. They also calculated that a stationary mosquito in a rainstorm should get hit, on average, every 25 seconds.

The authors put mosquitoes into a small acrylic cage. In one experiment, they simulated the terminal velocity of raindrops (6-9 m/s) by using a pump to generate a jet that broke up into drops and jetting streams. They managed to observe six mosquitoes, and found that they tumbled with the jet. The mosquitoes were accelerated to a velocity of 2.1 m/s within a duration of 1.5 milliseconds. All six mosquitoes survived the experiment and flew around after a brief rest.

Using high resolution and fast films, they used slower drops to examine the response to drops. They were able to observe the dynamics of the interaction when the impacts were on the wings or legs (N=13) and on the body (N=4). The mosquitoes rolled when hit on the legs and wings, but if the drop made a direct hit on the insects center of mass, it pushed the mosquito downward, accelerating it within downward several (5-20) body lengths. The mosquito was always able to separate itself from the drop and recover its flight. As the authors point out, it is "imperative that a mosquito does not fly too low during rain or it will suffer a secondary impact with the ground." They also did experiments on "mosquito mimics."

The key to survival is that the low mass of the mosquito causes a falling drop to maintain almost all of its speed after impact, thus resulting in a low impact force to the mosquito. The paper contains some nice and understandable physics of impact.

The work has implications not only in biology and ecology, but also for the construction of insect-sized flying robots.

Reference: Dickerson, A.K., Shankles, P.G., Madhavan, N.M., and Hu, D.L., "Mosquitoes survive raindrop collisions by virtue of their low mass." Proceedings of the National Academy of Sciences, June 4 online 2012. doi:10.1073/pnas.1205446109.