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How A Tiny Fly's Ears Could Help You Hear Better

Ormia ochracea is not a very likeable creature, even by fly standards.

This parasitic fly likes to leave its larvae on the backs of crickets. The larvae burrow inside the cricket and then proceed to eat the cricket alive.

But humans who have struggled with hearing loss might soon be thankful for one at least small part of this fly – its ears.

Ormia ochracea has developed very specialized ears that let it locate crickets by following the sound of their chirp. Scientists are using these ears as inspiration in developing microphones for the next generation of directional hearing aids.

"The thing that makes it very special is that the fly ear is so small," says Neal Hall, an assistant professor of electrical and computer engineering at the University of Texas. Hall and his team have developed a prototype of a microphone inspired by the Ormia ochracea ear, which was published Tuesday in Applied Physics Letters.

Humans have large noggins, says Hall, and these big heads of ours to help us figure out which direction a sound is coming from.

"We have a significant separation between our ears," he explains, "so sound arrives at one ear just a split second before the other. Our brain ... looks at those very minute differences in time of arrival to locate the object."

But the fly head is tiny – its ears are separated by only a millimeter, which is about the thickness of the average fingernail – so sound arrives at both ears at almost the exact same time.

To overcome the limits of its itsy-bitsy head, the fly has evolved a special way of hearing: Its two eardrums are connected by a small rigid structure that behaves like a teeter-totter, and this teeter-totter amplifies very small differences in the arrival time of sound.

"It's like having two microphones in one that are linked together by this teeter-totter," says Hall.

The teeter-totter mechanism in the fly ear was first explained by mechanical engineer Ronald Miles and neurobiologist Ronald Hoy in 1995 – NPR's Morning Edition even featured a segment on the discovery way back in 1999.

Since then, a number of scientists have strived to create tiny, man-made microphones that mimic the teeter-totter mechanism in the fly ear. Within the past year, teams lead by Ronald Miles at Binghampton University and Miao Yu at the University of Maryland have also published directional microphone prototypes inspired by the Ormia ochracea.

Hall says that in his microphone, the motion of the teeter-totter is detected using a special type of material that emits an electrical signal when it changes shape. This approach is not as sensitive to direction as some of the other approaches, but it may be simpler and more energy efficient.

"We've made a big leap forward in terms of reducing power consumption and the readiness of the technology to make an impact," he says.

Of course, most people aren't interested in chasing down crickets to feed to their kids for dinner. But these fly-inspired microphones could be applied to a number of more human endeavors – smartphones, defense tracking – or directional hearing aids.

"The number one complaint of hearing aid users is that they cannot hear in noise," says Ruth Bentler, who studies the effectiveness of directional hearing aids at the University of Iowa. "As soon as you have any degree of hearing loss and you walk into a crowded restaurant, it becomes difficult to hear speech."

Bentler says the solution for many is directional hearing aids, which use one or more microphones to cancel out noise coming from the side or from the back of the head. These hearing aids are "designed to be more sensitive to noises coming from the 'look' direction," she says.

"The teetering mechanism has some significant design advantages over how one would normally try to implement a directional hearing system," says Hall. Using a single teeter-totter mechanism could reduce power consumption – which is always an issue in battery-powered hearing aids – and help them maintain calibration over time.

But even though scientists are getting close to replicating the capabilities of Ormia ochracea's ears, Hall says he's still impressed at the capability of this little fly.

"It is the equivalent of if you were just standing on the ground and all of a sudden the ground starts shaking because there was an earthquake, and I told you I can tell just by my feet that the epicenter of the earthquake was in Costa Rica," Hall says. "The fly does something equally remarkable in locating sound given the proximity of its ears."

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