An ordinary housefly performs wing strokes so complex that conventional aerodynamics theory can't explain how it gets off the ground. So Michael Dickinson of the University of California, Berkeley, built a scaled-up set of mechanical wings that have at last revealed how flies stay aloft.
Flies flap their wings forward, then flip them over so the leading edges point to the rear before flapping them back again. Dickinson observed that the rotation of the wings speeds air over them and provides extra lift. When the wings shift to the backward stroke, they also travel through the faster-moving air from the forward stroke, which acts like a head wind. "This is a really spiffy mechanism, because the insect recaptures energy it lost to the air," says Dickinson.