Although you may know them as hovering nuisances, fruit flies are formidable flying machines. They evade swatting humans with split-second aerial acrobatics , and they have more moves than your average jetfighter. Now, researchers have figured out how flies nail a relatively simple flying feat: setting their cruising altitude.
Researchers thought that flies chose their altitude based on optic flow, a phenomenon familiar to anyone who has ridden an airplane. When the plane is at low altitude, the ground seems to race by, but as the plane climbs, the ground seems to move along more slowly. According to the optic flow model, proposed in 2007, insects like to keep the ground moving at a certain speed (to avoid crashing), and they set their altitude accordingly. The model helped explain some weird bee behavior—bees often crash into mirror-smooth water, and they fly lower when a headwind slows them down—but it had never been tested in free-flying insects.
To test the idea, neuroscientist Andrew Straw of the California Institute of Technology in Pasadena and colleagues turned a wind tunnel into a closely monitored insect arena, a "many-camera beast" they nicknamed Flydra. Fruit flies released into Flydra met with a trippy turn of events. Once the insects reached the middle of the flyway, the team projected moving stripes on the floor that moved in pace with them, creating the illusion (for the flies at least) that the ground was still. In that situation, the optic flow model predicts that the flies would descend to get the ground "moving" again. But the flies didn't adjust their elevation.
Then Straw and colleagues tried something else. Using projectors, they cast part of the tunnel in shadow. The flies began to fly along the horizontal edge between the light and darkness, regardless of what was happening on the ground below.
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"This was really surprising for us," says Straw. "We could put that boundary at the top of the arena or the bottom of the arena—they always flew along the edge."
The team's observations, published  online today in Current Biology, suggest that flies base their cruising altitude on horizontal edges and landmarks—such as table surfaces or tree tops—and not on how fast the ground is moving beneath them. The edge-tracking strategy may enable flies to keep tabs on possible landing spots.
That may fly in the face of the optic flow model, but it's not necessarily shocking, says Cole Gilbert, a neuroethologist at Cornell University. Some data indicates moths also use edges to set their altitude, says Gilbert. "So this may be more widespread than what the little fly does. This may be the general principle" for all flying insects, he says.
The insight into fly flight will help robotics researchers as well, says Gilbert. Engineers have attempted to build flylike robots for military surveillance and other applications, but they haven't succeeded so far. "Maybe there are still some tricks we can learn from studying how these little bugs do it," he says.