Ormia ochracea, a parasitic fly, can determine the direction of a sound to within two degrees, a feat previously ascribed only to owls, cats, and humans. The discovery is surprising because flies ordinarily have no sense of hearing at all. Ormia, however, relies on its powerful ears to locate singing crickets, on which they deposit tiny larvae that eat the cricket from the inside out. Ormia's hearing mechanism is unique in that it involves a set of eardrums located behind the head. The eardrums act as tiny directional microphones, amplifying sound while indicating the direction from which the sound came. Unlike a regular microphone, whose hearing "membrane" is clamped down on all sides, Ormia's hearing mechanism consists of two membranes fastened with a hinge in the center so that it rocks like a see-saw. If sound waves come on both sides at exactly the same time and with the same amplitude, the see-saw doesn't move. But if sound comes to one side before the other, it moves because the two pressures are unequal.
"First the flies must locate and home in on the chirping cricket, Hoy notes. But for tactical reasons – 'probably to avoid being kicked off,' he says -- the flies usually land close by and walk the last few steps to their unsuspecting host…Ormia flies, the biologists discovered, can detect changes in sound-source position as small as 2 degrees. Even humans trying to detect who is speaking in a crowded room can't do better than that.
"'When you consider the size difference between humans and flies, Ormia flies are the real champions,’'Hoy says, describing a mechanical linkage between the fly's closely spaced eardrums as something like a playground teeter-totter, an earlier discovery made in collaboration with postdoctoral researcher Robert and Binghamton University engineer Ronald Miles. Sound waves cause each fly eardrum to beat out of phase with the other.
"'The near ear, the one closest to the sound source, responds more vigorously, compared to the far ear,' Hoy explains. Then the fly's nervous system (its fused neural ganglia and a tiny brain) instantaneously calculates the difference in pressure between the two ears and signals the fly's muscles to respond to the sound source. With human ears about 6 inches apart, we have about 10 microseconds to make the same calculation that the Ormia fly, with its half-millimeter head, makes in about 50 nanoseconds -- a thousand times faster.” (Cornell University 2001)