Flying through rain is a dangerous proposition for insects, since their small size is not very different from that of the average raindrop. And yet, mosquitoes, most notably, can stay airborne in a rainstorm -- no doubt a reflection of their determination to be summer's most annoying pest. Just how mosquitoes manage to fly in rain, however, was unclear, until recently
, when a team of researchers at the Georgia Institute of Technology discovered that mosquitoes' secret to surviving raindrop collisions is directly related to their low mass and strong exoskeleton.
The scientists explored mosquito flight in rain using a combination of high-speed videography and force and velocity experiments. The subjects of their experiments, Anopheles mosquitoes, which normally inhabit moist environments where rainfall is common, were filmed flying in a small cage, where they were subjected to a shower of water from above, designed to simulate rainfall.
Videography revealed that mosquitoes are able to survive direct-impact collisions with raindrops at terminal velocity (the maximum speed reached by an object in free fall). When struck, a mosquito is pushed downward, tumbling in contact with the drop. After falling a distance of several body lengths, the mosquito separates from the drop and is able to resume flying. Indirect hits, such as on the legs or wings, which the team described as glancing blows, produced different effects on flight and recovery, depending on whether the blow altered the insect's pitch, yaw, or roll.
From the video footage, the researchers were able to see that raindrops deform following impact. They also noticed that the drops appeared to experience very little change in momentum despite impact, which further suggested that only a relatively small amount of force had been transferred to the colliding mosquito. This differs from a direct-impact scenario on a stationary surface, which entails a large transfer of force and causes a raindrop to burst and produce a splash.
Based on their observations from the video footage, the scientists suspected that the apparent minimal change in drop momentum during a collision was related to the fact that mosquitoes have only a small mass. The team’s suspicions were confirmed by experiments with “insect mimics”—Styrofoam spheres of comparable mass to mosquitoes—which were used to measure the actual change in velocity associated with drop impact with a mosquito (momentum is the product of an object’s mass and its velocity).
To gain further insight into the mechanism by which mosquitoes survive raindrop collisions, the researchers also performed force experiments, perhaps the most revealing of which involved compression tests to determine mosquitoes' threshold to force. The maximum compressive forces sustained by mosquitoes that still allowed the insects to fly measured about 3,000 to 4,000 dynes (a dyne is the force needed to accelerate a free mass of one gram one centimeter per second per second). This far exceeded the 200–600-dyne forces imparted by a colliding raindrop, indicating that the strength of the mosquito’s exoskeleton also plays a role in the insect’s ability to survive drop collisions.
Raindrops can be as many as 50 times heavier than a mosquito, which the researchers indicate in their PNAS paper is comparable to the weight ratio between a bus and a person. They also calculated the frequency with which a stationary mosquito is likely to be struck by a raindrop in a heavy rainstorm, which worked out to about every 25 seconds.
The study is of particular interest for the development of micro-airborne vehicles, which have applications in surveillance and search-and-rescue operations. But perhaps more remarkable is the knowledge it has given us concerning the elegant solutions evolved by nature to problems that are very significant for the life-forms affected, but that we often are inclined to overlook.