We all know how hard it can be to swat flies, but nobody knew how they pull off their incredible evasive manoeuvres. Now high-speed photography reveals that flies steer like helicopters, despite not having rotors.
Using cameras operating at 7500 frames a second, plus a winged robot,Florian Muijres of the University of Washington in Seattle and his colleagues studied how flies respond to approaching threats. First they presented fruit flies (Drosophila hydei) with an expanding dark circle on a screen, simulating a looming predator. The flies made rapid banked turns with just a few wingbeats.
“Astonishingly quickly, flies can compute where a threat is coming from and, like an aircraft, generate an evasive manoeuvre in the right direction,” says team member Michael Dickinson, also at the University of Washington. “They do this by very tiny changes in wing motion.”
Whereas aeroplanes can steer by swivelling about an axis that runs from nose to tail, almost like a car, flies do something different. They simultaneously roll their body and angle it either up or down while accelerating to make a speedy getaway.
It resembles how helicopters steer, says Dickinson. To turn, a helicopter tilts itself in the intended direction of travel. Aeroplanes also do this when they bank.
To confirm that they had understood what the fruit flies were doing, the team programmed a winged robot to mimic the flies. Its wing movements created the same turning forces as flies generate.
“Fruit flies turn to avoid fast-approaching objects in the same way as any air or space vehicle,” says Graham Taylor of the University of Oxford, who was not involved in the research. “What’s remarkable is the rapidity of the response, and the subtlety of the changes the flies make to their wingbeat when turning. The flies start turning away from approaching threats in half the time it takes you to start blinking at a camera flash, and finish throttling up in one-fiftieth of the time it takes you to complete that blink.”
So what is the best way to swat a fly? “It’s quite simple,” says Dickinson. “You just have to be very fast.”
Journal reference: Science, DOI: 10.1126/science.1248955