Imagine for a moment that you have wings like an insect. One day, while you're buzzing through the air you hear the distant crack of a thunderstorm and suddenly you're assaulted from above. Massive drops of water come crashing down on you, some weighing as much as a school bus. You twist and turn trying to evade the onslaught, but eventually one of these watery orbs hits you smack dab between the wings and you go careening toward the ground.
For mosquitoes, this scenario isn't uncommon. When a storm rolls in, mosquitos have to battle raindrops that are close to their size but with a mass up to 50 times that of the average mosquito (equivalent to the difference between a human and a school bus). How mosquitoes contend with these drops of doom is the subject of a study in this week's issue of the Proceeding of the National Academy of Sciences. David Hu, a professor of mechanical engineering and biology at Georgia Institute of Technology, and his team have devised some rather unorthodox methods to determine how mosquitoes survive such watery collisions. Using a high-speed camera, Hu's team bombarded Anopheles mosquitoes with drops of water and caught the resulting action at 4000 frames per second (a typical film camera only records 24 frames per second.)
They found that mosquitoes are actually quite good at dealing with raindrops, even when receiving a direct hit between the wings. Mosquitoes are very light compared to the mass of the raindrop and this means the drop pushes the mosquito down rather than breaking over it. Because the drop's speed doesn't change very much, little force is transferred to the mosquito. Compare that with a drop hitting a larger insect, like a dragonfly; the drop would break on its back, and the resulting force would transfer into the insect's exoskeleton. What's more, the mosquito has hydrophobic hairs on its body and sprawling legs that create drag. This lets it slip out from under the raindrop before meeting a wet end.
However, Hu's team also found that the mosquito isn't completely safe from forces generated when colliding with a raindrop. As the drop comes in contact with the mosquito it accelerates the mosquito sharply downward to match the drop's terminal velocity of nine meters per second. This happens over the distance of only about a 10 mm which puts an enormous amount of pressure on the insect's body, up to 300 gravities worth (2942 m/s²). For comparison, a jet fighter pilot accelerating out of a loop de loop, experiences only about nine gravities (88 m/s²).
This rapid acceleration also produces the greatest risk to mosquitoes: flying close to the ground. When hit by a raindrop, they would accelerate into the ground with great force and without sufficient time to slide out from underneath. And this is where the practical application of Hu's research comes into play. In recent years, we've seen the invention of many exceedingly small military aircraft, known as Micro Air Vehicles, or MAVs. If these vehicles become as small as mosquitoes, they would become subject to the same dangers as flying insects, including rainstorms.