The Secret Life of Numbers: 50 Easy Pieces on How Mathematicians Work and Think (2006)
Chapter: 43 Buzzing Around Corners (Biology)
43
Buzzing Around Corners (Biology)
Can anyone claim not to have been driven crazy by the sound of buzzing flies? Fly swatters are of little help, since just about every time one tries to swat, the insect manages to avoid the threatening object by making split-second changes to its flight path. This is not surprising since flies require only 10 wing beats to make an acrobatic turn, lasting all of a 20th of a second. But how do flies perform these so-called saccades, these sudden turns in midair?
Two factors could conceivably affect the aerodynamics of a fly. One is the friction of the fly’s skin in the air. The other factor that could play a role is the body’s inertia, which keeps the flying fly on its course. For 30 years the assumption was that the aerodynamics of large animals, such as birds and bats, is determined by inertia. Flies, it was commonly thought, were too small for their inertia to have any significant impact. Scientists maintained instead that sudden changes in the flight direction of small animals were determined by the friction of their skin with the air. Flies swim in the air, as it were.
But Steven Fry from the Institute of Neuroinformatics, a joint institute of the Swiss Federal Institute of Technology and the University of Zurich, together with his colleagues Rosalyn Sayaman and Michael Dickinson from the California Institute of Technology, put an end to this—as it turns out—erroneous belief. In a paper published in the journal Science, they investigated the aerodynamic mechanisms underlying the free-flight maneuvers of fruit flies (Drosophila melanogaster).
The researchers set up three high-speed digital cameras in a specially equipped lab. At a speed of 5,000 images per second, each camera filmed the maneuvers of the flies as they approached, and avoided, an obstacle.
The recorded data were then loaded onto a computer-controlled mechanical robot. It consisted of artificial insect wings, constructed to scale and submersed in a tank filled with mineral oil. With the help of their robot fly, the three scientists were able to measure the aerodynamic forces generated by the wing motions of flying insects.
Their experiment yielded some remarkable observations. To initiate a saccade, the fruit fly creates torque through slight differences in the motions of its two wings. But the main point of interest was the fly’s behavior after the onset of a turn. If indeed friction in the air were the determining factor when the fruit fly performs a saccade, a few beats of the wing would suffice to surmount the resistance. Then the fruit fly could quickly adjust its wings to their normal position and continue on with the flight ahead. But the researchers noticed something different. A split second after the onset of a turn, the fly produces a reverse torque with its wings for the duration of no more than a few wing beats.
Why does the fly do this? After initiating a turn—but already having stopped producing additional torque with its wings—inertia causes the fly to continue to rotate. Similar to a figure skater performing a pirouette, the fly continues spinning around its own axis. To counteract this continuing rotation of its body, the fly “puts on the brakes.” Since this countersteering technique is only necessary to counteract inertia, the three researchers proved that it is inertia and not friction that is the determining factor when the fruit fly takes to the air.
