Photo: Randi Silberman

Tiny Fly: Each prototype has a wingspan of 3 centimeters and weighs 60 milligrams, not including a battery and sensors. For now, the fly is connected to an external power source.

Using the results from Dickinson's models, I and others in Fearing's lab set out to replicate the insect's incredible wing motions. Part of the challenge is that many systems contribute to the flight of a fly, including eyes specially attuned to perceiving motion and powerful muscles that drive the wings to generate unsteady aerodynamic forces, on which the fly's maneuverability depends. Most insects control their wings by adjusting the amplitude of their wing strokes, the angle of attack, and the tilt of their strokes through tiny muscles in the thorax. Flies even have special sensory organs, called halteres, that sense body rotations during flight. These features are all key to flies' remarkable ability to hover, fly upside down, and land on walls and ceilings.

The main motivation for creating mobile robots is that they can go where humans cannot—to exposed points on a battlefield, for instance. Today, mainly the military can use such robots, because they cost on the order of US $100 000 each. To bring robots within the reach of law-enforcement and emergency-rescue services requires a totally new approach. We placed a great deal of importance on our choice of materials, which ultimately had to be cheap and fairly easy to work with. Durability was less important, because we envisioned a robot that could be replaced for less than $10.

We focused on the two-winged insects of the order Diptera, which includes houseflies, hoverflies, and fruit flies. Flies are the most able flyers on the planet, they're small, and they are naturally robust enough to survive collisions.

Flies achieve their astonishing maneuverability by moving their wings through complex, three-dimensional trajectories at frequencies that often exceed 100 hertz. The upstroke and downstroke patterns are almost symmetrical when flies hover but highly asymmetrical when they move forward or maneuver. Flies generate those large-amplitude, high-frequency wing strokes by using indirect flight muscles, so called because they deform a portion of the thorax rather than the wings themselves, inducing mechanical resonance in the fly's body. Smaller muscles connect directly to the wing hinge to fine‑tune the wing's movements.

Because of the small scale, the airflow around a fly is much more viscous than that around birds or fixed-wing aircraft. For insects, flight is somewhat like treading water. A fly's wing motions generate aerodynamic forces that can change magnitude drastically in a fraction of a second. Traditional aircraft wings, by contrast, are subject to fairly steady fluid flow. Because of this difference, the analytical tools that are used to predict the performance of an airplane are of little use in predicting the flight dynamics of an insect, making our job more difficult.

Over hundreds of iterations, our robotic fly has followed its own evolutionary path to more and more closely resemble the shape of a real fly. We borrowed two basic principles from biology—the ratio of the wing area to the body mass and the wingbeat frequency. Still, we need not copy nature slavishly by putting up with limitations on invertebrate biology that electromechanical devices do not share. Take, for instance, the elastic and structural properties of the insect thorax and wings. These body parts are made of chitin, a common polysaccharide, which though tough is nonetheless substantially weaker than carbon fiber.