Later experiments with Robofly, whose wingspan measures 25 inches, revealed a second source of lift, which comes as a fly rotates its wings between strokes. A rotating object (such as a tennis ball hit with backspin) pulls air over its top surface, which reduces air pressure above the object, and pushes air in the opposite direction below, increasing air pressure there. This rotational force, which is also generated as a fly flaps its wings, can supply the insect with up to a third of its entire lift.
A fly can also generate lift from its own wake. When it sheds a vortex from its wings at the end of each stroke, the vortex drifts slowly away, still spinning. As the insect brings its wings back through the next stroke, Dickinson found, the wake pushes against the wings and lifts them up.
As Dickinson decrypted the physics of insect flight, he found himself drawn into the company of people who build robots. Recently he helped his Berkeley colleague Fearing create the Micromechanical Flying Insect, a blowfly-like device less than an inch long that's being developed with funding from the Office of Naval Research and the Defense Advanced Research Projects Agency. So far, though, the robotic blowfly has only flown on a tether, and with just one wing. The main limiting factor, says Dickinson, is the battery, which is currently too large and too weak to make extended flight a reality.
Dickinson's interest in robotics is as a biologist; he doesn't aspire to be the next Orville Wright. He looks at flying devices as an opportunity to evaluate his notions about how animals function. "In biology, it's rare that you have the opportunity to test your ideas by building something," he says.