A miniature robotic helicopter has revealed a simple yet effective visual trick that lets insects fly so adeptly without sophisticated avionics.

Besides explaining how insects zoom around and land without crashing into the ground, the technique could potentially be used to help control aircraft.

As insects fly forwards the ground beneath them sweeps backwards through their field of view. This "optical flow" is thought to provide crucial cues about speed and height. For example, the higher an insect's altitude, the slower the optical flow; the faster it flies, the faster the optical flow.

Previous experiments involving bees suggest that optical flow is crucial to landing. Maintaining a constant optical flow while descending should provide a constant height-to-groundspeed ratio, which makes a bee slowdown as it approaches the ground. Distorting this optical flow can cause them to crash land instead.

General flight

Now Nicolas Franceschini at the University of the Mediterranean in Marseilles, France, and colleagues have shown the same technique may explain more general flying behaviours.

They fitted a miniature helicopter with a simple software feedback loop to ensure that optical flow remains constant as it flies along. This allowed the tethered micro-copter to take off gracefully, maintain altitude over varying terrain and land, all without any means of directly measuring its speed or height. A video produced by the researchers shows the micro-copter in action (43.7 MB .avi format, requires DivX).

The fact that insects are such effective fliers could all be thanks to a similar feedback mechanism hardwired into their brains, Franceschini says.

Bee brains

Maintaining a constant optical flow should be relatively easy for an insect, says Rob Harris, a specialist in insect vision at the Centre for Computational Neuroscience and Robotics and the University of Sussex, in Brighton, UK, who was not involved with the project.

Maintaining a constant optical flow precludes the need to calculate height and groundspeed manually. "You have to assume they are not doing complicated trigonometry in their little brains," he adds.

"It explains about 70 years of experiments," Franceschini says. For example, it explains why bees sometimes drown when flying over still water. Without any features on the surface of water, a bee detects no optical flow and instinctively descends, eventually landing in the water.

Franceschini is currently talking to helicopter manufacturers about developing optical flow regulators for their aircraft. Such feedback mechanisms would be lightweight and trivial to develop and could help prevent crashes, he claims.

Journal reference: Current Biology (vol 17, issue 4, manuscript 5340)