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Amazing Little Ears

Humans detect the direction a sound comes from by comparing the difference in time of arrival, or loudness of the sound, so why is there a difference?

The Human Ear

Photo: Phil Dragash (Flickr)

An photo of the human ear on a person right side looking from behind the person.

Humans detect the direction a sound comes from by comparing the difference in time of arrival, or loudness of the sound reaching our two ears.

We are able to this because our ears are separated by the width of our head. A sound wave traveling from the left reaches the left ear sooner, and is louder than at the right ear.

How would this be different if our ears were right next to each other?

Scientists puzzled about this very question when studying very small insects, such as the tiny parasitic fly Ormia ochracea.

Ormia females deposit their larvae inside a live cricket. The larvae develop inside the cricket before they emerge, eventually killing the host.

The females locate crickets by ear, using only the sound of cricket chirps to guide them. However, with ears separated by less than half of a millimeter, the time and intensity difference between a fly’s ears should be much too small to detect. How do Ormia do it?

It turns out that the Ormia ear is quite unusual. The fly’s two ear drums are connected in the middle by a small bridge. The connection between the two ears is a form of mechanical coupling. It basically acts as an amplifier of the minute differences between the two ears. The unique anatomy allows Ormia flies to accurately detect the direction of sounds that would be impossible for other insects their size.

The discovery also has exciting potential applications for humans.

Development is already underway for a small hearing aid containing a microsensor patterned after the Ormia fly’s ear. The new hearing aid would allow people with hearing in only one ear to localize sound, something that current hearing aids can’t do.

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