Humans and other mammals have the ability to pinpoint sound sources because of the finite speed of sound combined with the separation between our ears. The spacing of several centimeters or more creates a slight difference in the time it takes sound waves to hit our ears, which the brain processes perceptually so that we can always experience our settings in surround sound.
Insects generally lack this ability because their bodies are so small that sound waves essentially hit both sides simultaneously. Many insects do detect sound vibrations, but they may rely instead on visual or chemical sensing to find their way through the fights, flights and forages of daily life.
O. ochracea is a notable exception. It can locate the direction of a cricket's chirp even though its ears are less than 2 mm apart -- a separation so slight that the time of arrival difference between its ears is only about four millionths of a second (0.000004 sec).
But the fly has evolved an unusual physiological mechanism to make the most of that tiny difference in time. What happens is in the four millionths of a second between when the sound goes in one ear and when it goes in the other, the sound phase shifts slightly. The fly's ear has a structure that resembles a tiny teeter-totter seesaw about 1.5 mm long.
Teeter-totters, by their very nature, vibrate such that opposing ends have 180-degree phase difference, so even very small phase differences in incident pressure waves force a mechanical motion that is 180 degrees out of phase with the other end. This effectively amplifies the four-millionths of a second time delay and allows the fly to locate its cricket prey with remarkable accuracy.
Such an ability is almost the equivalent of a human feeling an earthquake and being able to discern the direction of the epicenter by virtue of the difference in time between when the right and left foot first felt the tremor -- except the fly's h
|Contact: Jason Socrates Bardi|
American Institute of Physics