The team constructed what it calls a "flight arena," a 20-centimeter-high 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 10 meters, the height at which they achieve their maximum velocity. Six mosquitoes placed in the cage were then filmed with a high-speed video camera that captured 4000 frames per second.
It was like a game of insect pinball. All six mosquitoes were able to recover from drop impacts without crashing to the bottom of the cage. In one typical example, a mosquito hit by a water drop tumbled a distance of 13 body lengths before separating itself from the drop and flying laterally to land on the side of the arena. 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 rather than on the insects' bodies. The impacts caused the mosquitoes to pitch, yaw, or roll depending on where they were hit.
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 2 milligrams, whereas a raindrop can be 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. To test this hypothesis, the team constructed insect "mimics" made of small Styrofoam spheres the same weight and size as the mosquitoes. When the researchers released the spheres into the arena, they were momentarily suspended in mid-air as water droplets collided with them from above.