No one had researched how these flying robots might survive rain, nor had anyone studied how living insects do so, Hu said. So he and his colleagues engineered an experiment to "smart bomb" mosquitoes with water droplets to see how they'd respond. They put mosquitoes in mesh cages, which vibrated every few seconds to prevent the mosquitoes from landing. They then dropped water on the insects with the same forces that would be present in a rainstorm.
Though raindrops are up to 50 times the weight of a mosquito, it was immediately clear that collisions were not fatal. Glancing blows sent mosquitoes spinning in the air, but they soon recovered. Direct hits resulted in the mosquitoes and water drops falling together before the insects got free and continued their flight.
To understand how the mosquitoes survived, Hu and his colleagues suspended Styrofoam pellets of various weights under water droplets, and found that mosquitoes' low mass explains their ability to survive. If a mosquito sitting on a twig gets hit by a droplet, the water will crush the insect with 10,000 times its body weight in force. But if a mosquito is hit in midair, only 10 percent of the droplet's force transfers to the insect's body. That's only about 0.02 ounces (0.6 grams) for a typical droplet, the equivalent of a mosquito being hit by a feather.
In contrast, a dragonfly that weighs more than 1,000 times that of a mosquito would absorb 90 percent of a droplet's force. The heftier dragonfly would stop the droplet rather than surfing it down like the lightweight mosquito.
"There's something special about being very lightweight," Hu said.
If mosquitoes fly too close to the ground, they do risk death by droplet, the researchers found. The insects need to leave themselves five to 20 body lengths to detach from the raindrops, or they'll hit the ground at a speed of 1,000 mosquito body-lengths per second.