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Raindrops don’t swat down mosquitoes, researchers find

Biologists determine how mosquitoes shrug off raindrops. (Bigstock)

A mosquito buzzes toward an open window, heading for an unsuspecting victim peacefully asleep in bed. Suddenly, a raindrop weighing 50 times as much as the mosquito plummets straight down on the insect. The drop should have the impact of a bus running over a human.

Sadly for the sleeping person, the pest will survive this collision.

That’s the conclusion of a team of engineers and biologists who used a combination of real-time video and sophisticated math to demonstrate that the mosquito’s light but rugged construction allows it to shrug off even large raindrop. The findings offer little aid in controlling the pest — but might help engineers improve the design of tiny flying robots.

Aviation engineers know a lot about how rain impairs the flight of airplanes. Heavy rain increases drag, reduces lift and increases the risk of stalling. But research on the impact of water on flying animals is sparse.

To help fill this knowledge gap, mechanical engineer David Hu and his colleagues at the Georgia Institute of Technology decided to subject Anopheles mosquitoes, the primary insect that infects people with malaria, to artificial rain conditions. Hu’s team is already well known for its studies of water-animal interactions, including the physics of how wet dogs dry themselves by shaking the water off.

The team built an acrylic cage covered with a mesh top that allowed water in but kept the mosquitoes from escaping. In an initial round of experiments, the researchers shot jets of water into the cage to simulate raindrops falling about 33 feet, the height at which they achieve their maximum velocity. Six mosquitoes placed in the cage were then filmed with a camera that shot 4,000 frames per second.

It was like a game of insect pinball. All six mosquitoes were able to recover from raindrop impacts without crashing to the bottom of the cage. In one typical example, a mosquito hit by a drop tumbled 13 body lengths before separating itself from the drop and flying laterally to land on the side of the cage.

To better picture what was happening, the team subjected 20 mosquitoes to drops that were falling more slowly. The videos showed that most of the collisions were glancing blows on the wings and legs.

But even when the mosquitoes took a direct hit, they still recovered after falling no more than about 20 body lengths. Hu and his colleagues hypothesized that due to the insects’ low mass — about two milligrams, compared with a raindrop’s mass of up to 100 milligrams — the raindrop loses very little speed and momentum upon encountering the mosquito and thus imparts very little actual force to it.

The team concluded that the raindrops deform and largely bypass the much smaller bodies of the mosquitoes.

Hu and his co-authors suggest in their report, published online in the Proceedings of the National Academy of Sciences, that the mosquito’s surprising resilience is due not only to its lightness but also to its exoskeleton, the hard but flexible outer covering that protects its inner organs.

Hu says that his team’s results may have implications for the design of so-called micro-airborne vehicles (MAVs), some of which are as small as dragonflies and are increasingly used by the military for surveillance in war zones and for search-and-rescue operations.

ScienceNow, the daily online news service of the journal Science



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