June 5, 2012, 10:22 a.m.
Mosquitoes thrive in rainy climates, even though a typical raindrop can weigh up to 50 times as much as the insect. Scientists have thus long pondered how mosquitoes can fly through a rainstorm without getting killed by such collisions, the impact of which is comparable to a human being hit by a bus. The short answer is that the mosquitoes are so light that they simply hitch a ride on the raindrop without any significant force being transferred to them.
Of course, the laws of physics still apply. If a mosquito is sitting on a branch or on the ground, the impact of the raindrop will probably kill it. Similarly, if the insect hitches a ride on the raindrop and is still aboard when it strikes soil or water, it will most likely die or end up drowning. But as long as it could get free before impact, the mosquito will most likely survive, researchers from the Georgia Institute of Technology reported Monday in the Proceedings of the National Academy of Sciences.
Researchers already know a lot about the relationships between insects and water. Flying insects, especially, have a dense layer of wax-coated hairs that repel water. To cope with these hairs, pesticide makers have to produce a fog of very small drops that will adhere to the insects. But virtually nothing was known previously about collisions between insects and raindrops, said biologist David L. Hu of Georgia Tech. Such interactions have become increasingly important, he noted, as roboticists have begun developing insect-sized flying robots, known as micro-airborne vehicles.
To learn more about such collisions, Hu and his colleagues set out to film them using high-speed cameras. They enclosed the experimental mosquitoes in an acrylic cage about 2 inches square with a mesh top to keep the mosquitoes in but allow water drops to enter. To simulate rain drops, which have a terminal velocity of about 9 yards per second, they tried dripping water off the third floor of a building onto the cage at ground level. But, he told Science News, "It's the worst game of darts you can imagine. You have no hope of hitting them." Instead, they began spraying the mosquitoes with water from a jet.
They found that the impact of the raindrops immediately accelerated the insects to the same speed as the droplets, which required an acceleration of 100 to 300 g (100 to 300 times the force of gravity). Previously, the greatest known acceleration among insects was in jumping fleas, which accelerate at 135 g. The mosquitoes effectively become trapped in the droplets, tumbling head over heels as the droplets continue downward. After the drop has traveled the equivalent of about 13 body lengths, however, the mosquitoes escape and fly to the walls of the acrylic chamber, where they rest for a brief spell. They then begin flying again with no apparent harm. If the mosquitoes are too close to the floor when they are hit, however, they can strike the ground with disastrous impact. The bottom line is that mosquitoes shouldn't fly close to the ground in a rainstorm.
The insects are able to survive, the team said, because their exoskeleton allows them to support large loads. Glancing blows also reduce the force of the impact. But there is a limit, however. When the speed of the droplets is increased by a factor of 10, the mosquitoes no longer survive.