Eardrums Provide Directional Hearing

Biological Strategy

AskNature Team

Sense Sound and Other Vibrations From the Environment

For living systems, sensing sound and other vibrations is important for communicating and detecting conditions within their environment. Living systems must locate a signal’s source so that they can move toward it (such as when it is food or a potential mate) or away from it (such as when it is a predator). To prompt an appropriate response, living systems must sense these signals, recognize their amplitude or volume (which is sometimes very low), and determine their direction. Living systems must be attuned to signals relevant to them and able to distinguish these from irrelevant sounds to avoid expending unnecessary energy. For example, owls’ ears are asymmetrically placed. This enables them to detect sounds more accurately, which helps them locate small prey at night and avoid wasting energy chasing down irrelevant sounds.

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Insects

Class Insecta (“an insect”): Flies, ants, beetles, cockroaches, fleas, dragonflies

Insects are the most abundant arthropods—they make up 90% of the animals in the phylum. They’re found everywhere on earth except the deep ocean, and scientists estimate there are millions of insects not yet described. Most live on land, but many live in freshwater or saltwater marshes for part of their life cycles. Insects have three distinct body sections: a head, which has specialized mouthparts, a thorax, which has jointed legs, and an abdomen. They have well-developed nervous and sensory systems, and are the only invertebrate that can fly, thanks to their lightweight exoskeletons and small size.

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The eardrums of a parasitic fly can help locate its host through sound thanks to the see-saw shape of the membranes.

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.

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