How do flies fly?

A fruit fly tethered inside a sophisticated miniature virtual-reality flight simulator has shown researchers that the insect's flight motor, one of nature's greatest feats of engineering, works by stretching an elastic power-storage device on every wing stroke to recapture energy that would otherwise be wasted. The finding is reported in the April 6 issue of Science.

Michael Dickinson, Assistant Professor in Organismal Biology & Anatomy, used the fly flight simulator to measure precisely both the energy expended and the thrust generated by the fruit fly, Drosophila, in flight. By tracing the changes in the wing-beat frequency and stroke length as the fly navigated through its virtual world, Dickinson was able to determine that the fly has only normal efficiency in its flight muscles but has a means to recapture at least 10 percent of the energy expended in each wing stroke for use in the next. Larger insects, he calculates, probably recover more than 50 percent of their wing-stroke power.

The most energetically costly of all forms of locomotion, flight requires up to 100 times the energy consumption of the animal at rest. In comparison, a well-trained athlete running a 100-yard dash burns only about 15 times the calories of a couch potato. Scientists have long known that to achieve flight, insects -- even at such high levels of energy consumption -- must either employ superefficient muscles or some means of recycling the inertial power of wing motion.

"Flies are the F-16s of the animal world," Dickinson said. "They are high-performance machines capable of spectacular aerial maneuvers."

To analyze this performance, Dickinson tested flies in a flight simulator in which the tethered fly is surrounded by a panoramic display from hundreds of tiny lights, like a ballpark scoreboard. The visual display changes according to the fly's own behavior as it steers through a virtual landscape.

Dickinson can also introduce the electronic equivalent of constantly changing crosswinds, which the fly must battle to stay on course. The steering torque produced by the fly is measured by a laser beam bounced off a mirror on the tether arm, while the wing movements are traced by their shadows from an infrared light. The fly is harnessed within an airtight chamber that measures the carbon dioxide it produces as it burns fuel.

"The amount of carbon dioxide given off gives us the total energy cost. It tells us how much gas is going into the tank," said Dickinson, who collaborated and co-wrote the report with John Lighton of the University of Utah, a leading expert on insect respiration.

The researchers determined the muscle efficiency by studying a fly under conditions where the elastic storage capacity doesn't matter. "This fly is so small, it flies in a different physical world than we're used to," Dickinson said. "The viscosity or 'stickiness' of the air is very important, while the inertia that the fly's tiny wing must overcome is far less so." The researchers were thus able to calculate that the efficiency of the insect's muscles in converting calories to power is only about the same as our own -- about 10 percent.

By making the fly change directions in its virtual landscape, the researchers created conditions in which the inertial costs of flapping the wings increased, allowing the researchers also to measure elastic storage.

"Our findings suggest that insects in general must be using elastic storage as a means of minimizing energetic flight costs," Dickinson said. Not much is known of the mechanism.

"The insect-wing hinge is known to contain the protein resilin, the most elastic biological material known," Dickinson said.

The research was supported by the David and Lucile Packard Foundation and the National Science Foundation.