In the civilian realm, an MAV could be used to examine so-called dull, dirty, or dangerous environments where a human can't or shouldn't go. Who really wants to rush into a collapsed mine searching for survivors? Or creep through crawl spaces or down chimneys doing routine inspections? A robotic MAV could easily and quickly accomplish the initial reconnaissance and then indicate whether any human intervention is needed.

Flying indoors is tricky, though. The MAV must fly with agility at low speeds without smashing into walls, ceilings, and other objects; hover for sustained periods; take off and land vertically; and consume little power. Fixed-wing flyers aren't up to the job because they can't hover, and they have to fly relatively fast to generate lift. Rotary-wing MAVs can hover, but they require a lot of power. Nor can they fly close to walls: the air pushed down by the rotor bounces off the wall and interrupts the downward flow of air through the rotor, usually with catastrophic results.

Insects, on the other hand, are the culmination of more than 300 million years of evolutionary flight experience. They can hover, fly slowly, maneuver aerobatically, and do it all in an astoundingly power-efficient way. A 100-milligram fly in motion consumes just 3 joules per second. Gram for gram, an airplane consumes more than twice as much power, a helicopter five times as much.

With the fly in mind, I've spent the last seven years trying to reverse engineer insect flight, collaborating with biologists and engineers, analyzing insect behavior and flapping-wing aerodynamics, and building electromechanical flapping mechanisms.

Only one other group has made a sustained effort to design and build an insectlike MAV. In 2001, a team at the University of California at Berkeley led by Ronald Fearing, a professor of electrical engineering and computer science, working with the leading U.S. expert on insect flight, Michael Dickinson, produced a 25-millimeter-long proof-of-principle demonstrator based on MEMS technology.

But neither the Berkeley group nor we in the United Kingdom have built a flying prototype. A number of factors are holding us back, including weight reduction and a good power source, but the key issue is flight control. It is not good enough to have "something flying": if you throw a brick, it will fly, at least briefly, but so what? The great draw of insect flight is its extreme agility, and this amazing capability can be achieved only by appropriate flight control. Unraveling the secret of insect maneuverability requires first understanding the underlying aerodynamics and mechanics of insect flapping.

Flight control dates back to the Wright brothers. These days, it has reached an apotheosis in modern fighter aircraft, such as Lockheed Martin Corp.'s latest F-35 [see photo, "Calculated Flight"], including an impressive version capable of short takeoffs and vertical landings. But the rules governing flight for the F-35 require about 1.1 million lines of code; it uses another 4.5 million lines for tasks like weapons targeting, communications, and mission control. The flight software runs on three shoebox-size computers, each with a pair of PowerPC processors. MAVs obviously don't have the space, or the cooling fans, to accommodate such onboard computers.

Neither do insects, of course. Studies suggest that the fly's flight control commands originate from a few hundred neurons in its brain (out of the brain's total of about 338 000 neurons). A neuron can be thought of as the brain's smallest computational unit, each one like a switching transistor, with its binary on-off states. Obviously, then, flies are not executing millions of calculations to solve forbidding differential equations in midair. But they still must obey the same laws of physics as the F-35, so whatever they are doing must be functionally equivalent to solving those equations in real time.

For an F-35, we take measurements from a few sensors—a device called a Pitot tube for measuring airspeed, an altimeter for computing rate of climb, a set of gyroscopes for detecting rotations, and vanes for sensing sideslip and angle of attack. The aircraft's computers use the sensor data, along with inputs from the pilot's controls, to continually calculate where the plane is and should be and then adjust the plane's control surfaces—such as the flaps, ailerons, and rudder—accordingly.