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Psst. This fly's ears can rival a cat's

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4:11pm, June 18, 2004
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Owls and cats turn out to have a not-so-charismatic rival in their much-admired ability to pinpoint a sound's origin: a sound-tracking parasitic fly.

The flies are too tiny for the usual direction-fixing physiology to work, says Andrew C. Mason of the University of Toronto at Scarborough. Yet an unusual eardrum structure allows Ormia ochracea to pinpoint a sound's source to within 2 compass degrees, Mason reported this week in New Orleans at the meeting of the Society for Neuroscience.

Neuroscientists discovered the fly's unusual ear structure several years ago. "What we're talking about now is how well it works," Mason says. "In terms of directional hearing, the flies are as good as any system ever studied."

Before the discovery of the structure, neuroscientists had thought that the flies' tiny size ruled out the directional-tracking method that bigger animals use: comparing a sound's loudness and arrival time at the two ears.

It turns out, however, that the fly's eardrums are connected. Sound hitting the drum nearer the source kicks up a response in the other one almost instantaneously. The transferred vibration in the far eardrum interferes with its direct reaction to the sound. The stronger vibration, therefore, marks the ear closer to the sound.

The flies manage such keen source tracking with a remarkably stripped-down nervous system. A person pools responses from several thousand sensory cells in each ear, yet O. ochracea has only about 100 such cells per ear.

Most flies don't have any hearing systems, Mason notes. Yet the female O. ochracea follows cricket chirps to find hosts for the parasitic larvae that she bears.

Mason measured auditory talent by tethering flies to a modified computer trackball that served as a treadmill. As Mason broadcast cricket chirps from different locations, a computer analyzed the fly's motions. In some experiments, Mason also measured nerve impulses triggered by the chirps.

The fly's novel ear will open many new directions for hearing research, predicts another Ormia researcher, Daniel Robert of the Universität Zurich in Switzerland. Researchers at the State University of New York in Binghamton, for instance, are trying to model a small directiontracking microphone after the fly's ears. That could prove useful for designing hearing aids.

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