Weighing just a milligram and protected only by a thin exoskeleton, the fruit fly lives in constant danger of being squashed. But what it lacks in toughness, Drosophila melanogaster makes up for in speed. And a new study reveals that the fly's escape maneuvers are more than simple reflexes. Researchers have discovered that the fly anticipates the direction of a looming threat and makes split-second movements that better prepare it to take off in the opposite direction. The findings reveal a level of movement planning rarely seen in such a simple organism.
Everyone knows flies can get off the ground quickly, but researchers generally assumed there wasn't much more to their takeoff than a jump and a few flaps of the wings. Biologists Gwyneth Card and Michael Dickinson of the California Institute of Technology in Pasadena have shown that there's a lot more involved, thanks to high-speed video cameras that shoot 5400 frames per second. As flies jumped to avoid a small Plexiglas disk sliding toward them on a string, the team noticed something astounding: A full 100 milliseconds before the insects beat their wings to escape, they shuffled their feet and shifted their weight so that their center of mass was just above their two middle legs, which they use to jump. "They do this elegant little ballet to position themselves for the jump," says Dickinson. The routine differs depending on the direction of the threat, always preparing the fly to lunge in the opposite direction, the researchers report online today in Current Biology.
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"The idea that the flies anticipate and plan what they're going to do is very cool," says Ralph Greenspan, a neurogeneticist at the Neurosciences Institute in San Diego, California. "It shows an awful lot of sophistication for a brain with only about 100,000 neurons."
Dickinson says his next goal is to figure out how the fly's relatively simple nervous system transforms a visual input--swatter at 3 o'clock!--to a plan for evasive action. Meanwhile, he admits the findings could have implications for people who'd rather swat flies than study them. "Most people swat exactly where the fly is," Dickinson says. But by the time the swatter makes contact, the fly has usually flown. "You should overshoot the fly," he says.