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  • Richard Conniff writes about behavior, in humans and other animals, on two, four, six, and eight legs, plus the occasional slither.

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Why Raindrops Don’t Kill Those Damned Mosquitoes

Posted by Richard Conniff on November 18, 2011

I’ve always been puzzled by the mystery of mosquitoes flying through rain without getting splattered.  The conventional wisdom in the past was that they somehow dodged the raindrops, which we should have realized was an impossibility.  Now a physicist has come up with a better explanation.  (Note:  I find the analogy of a falling boulder hitting a falling human somewhat disturbing.):

Newswise — Mosquitoes, which thrive in hot, humid climates, are as adept at flying in rainstorms as under clear skies. That’s puzzling: Why aren’t the bugs – which each weigh 50 times less than a raindrop – battered and grounded by those falling drops?

In fact, say David Hu, an assistant professor of mechanical engineering and biology at the Georgia Institute of Technology, and his graduate research assistant Andrew Dickerson, mosquitoes are hit by raindrops. Hu, Dickerson, and colleagues measured the impact forces of drops on both free-flying mosquitoes and custom-built mosquito “mimics” (small Styrofoam spheres of mosquito-like size and mass), and captured the interactions using high-speed video.

The researchers found that because the bugs fly so slowly (a maximum of 1 meter per second) compared to the drops (which fall at velocities ranging from 5 to 9 meters per second), “they cannot react quickly enough for avoidance, and likely cannot sense the oncoming collision anyway,” Dickerson says. But, he adds, “under low-wind conditions, the insects fly slowly enough that frontal impacts are infrequent, similar to us running in the rain. Instead, transverse impacts on the body and wings dominate.”

The mosquitoes’ low mass and speed – and thus low inertia – means that the raindrops are largely unaffected by the collisions. Thus, the drops don’t splash on the bugs. “The most probable impact is one that rotates the mosquito instead of pushing it vertically downward,” Hu says.

Indeed, Hu and company’s video analysis shows that, after pushing past the mosquitoes, falling drops have lost very little speed. “Consider this analogy,” Hu says: “A falling boulder hits a slowly falling human. The human, unless hit square-on, will be pushed aside quickly, and continue falling at a speed similar to pre-impact. Should the same boulder hit the earth, the boulder will break into many pieces.”

Hu discusses the findings and their implications for the development of flapping micro-aircraft in a talk at the APS Division of Fluid Dynamics Meeting, which will take place Nov. 20-22, 2011, at the Baltimore Convention Center in the historic waterfront district of Baltimore, Maryland.  The talk, “How mosquitoes fly in the rain,” is at 2:10 pm on Sunday, Nov. 20, in Room 309.


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