Why You Can't Swat a Fly

The reason you can’t swat a fly is that, for a creature with a brain hardly deserving of the name, the fly is a marvel of calculating ability. But before I explain what scientists led by Michael Dickinson of the California Institute of Technology (that would be the Dickinson whose e-mail is "flyman") have learned about how the fly brain calculates the location of the looming swatter, formulates an escape plan and plants its legs in an optimal position to hop out of the way (all within about 100 milliseconds of spotting the swatter), let’s cut to the chase: the best way to swat a fly, Dickinson says, is “not to swat at the fly’s starting position, but rather to aim a bit forward of that to anticipate where the fly is going to jump when it first sees your swatter.”

Where will it jump? Uif the descending swatter (they used a 6-inch-diameter black disk, dropping at a 50-degree angle toward a fly) comes from in front of the fly, the fly moves its middle legs forward and leans back, then raises and extends its legs to push off backward, away from the swatter. Are you approaching your quarry from behind? The fly has a nearly 360-degree field of view and can see behind itself, so when it spies the swatter behind it it moves its middle legs a tiny bit backward and flies forward. With a swatter from the side, the fly keeps its middle legs still and leans in the opposite direction before jumping. The idea is to position its center of mass so that when the legs push off the fly will evade the swatter.

“When the fly makes planning movements prior to takeoff, it takes into account its body position at the time it first sees the threat,” Dickinson says. “The fly somehow ‘knows’ whether it needs to make large or small postural changes to reach the correct preflight posture.” It does all this “long” (in fly time) before it takes off. “These movements are made very rapidly, within about 200 milliseconds” of seeing the swatter, says Dickinson, “but within that time the animal determines where the threat is coming from and activates an appropriate set of movements to position its legs and wings.”

Don’t believe the folk wisdom that if you approach the fly really, really slowly so your swatter doesn’t stir the air then the fly won’t notice. The Caltech scientists found that flies can tell you’re coming by sight alone—and remember that 360-degrees of vision thing.

It is Dickinson’s hope that discoveries about the fly’s neuronal processing will shed light on more complicated brains, not that his work will help humans kill flies better. His admiration for the little guys’ abilities, in fact, have made him hope that people will “think before they swat.”

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