New research explores small fly’s amazing sense of directional hearing

May 15 2017

Ormia ochracea's sense of directional hearing is second to none in the animal kingdom.

But, according to new University of Toronto Scarborough research, what makes its hearing so incredible may also complicate efforts in using it as a model for new technology, including hearing aids.

"These flies have highly specialized ears that provide the most acute directional hearing of any animal," says Andrew Mason, a professor of biology at U of T Scarborough. "The mechanism that makes their hearing so exceptional has even led to a range of bio-inspired technology, like the mini directional microphones used in hearing aids."

Ormia is a small, yellow, nocturnal fly native to the southern United States and Mexico. The female uses its exceptional hearing to locate the songs of male crickets, where it deposits its larvae. The larvae then burrow inside the cricket, eating it alive in the process.

While that is pretty extraordinary in itself, what makes the fly truly remarkable is its mechanically-coupled ears. Unlike most animals that have two separate ears, both of Ormia's eardrums are connected together, kind of like a seesaw with a rigid joint in the middle that can bend. When one of eardrums vibrates from a sound wave it pushes the other, and the tiny time difference it takes to activate one ear drum allows the fly to figure out which direction the sound is coming from.

"It's interesting that something so small can be sensitive to the direction of sound," says Mason. "They're tiny relative to the wavelength of sound they're able to localize, so they shouldn't be able to do what they do but they can because of the mechanical coupling."

Engineers are interested in using the same principle found in Ormia's coupled eardrums to develop artificial sensors. These sensors could better locate signals for a range of uses where the size of the object relative to the signal might be a limiting factor - from hearing aids, to gunshot detectors, to different types of radar.

How these are affected by distracting noise is a major obstacle. When it comes to hearing, especially with hearing aids, engineers need to solve something called the "cocktail-party-problem," that is, how to improve signal detection in noisy environments, like tracking one conversation at a crowded party. Animals usually deal with this problem through something called spatial release from masking (SRM) that allows for better signal detection when there's a separation between what they're interested in hearing and any distracting noises.

"If the distracting noise is farther away from the interesting signal, then it causes less interference," explains Mason.

But new research by Mason and his former PhD student Norman Lee, lead author and now an assistant professor of Biology at St. Olaf College in Minnesota, has found that Ormia's mechanically-coupled hearing system prevents it from using SRM.

In a series of lab tests they found that when a distracting noise was introduced to one side of the fly, it diverted it away from the cricket sound it should be interested in. They found that since the fly's hearing is extremely directionally sensitive, and because of its mechanically coupled hearing system, a noise placed at one side obscures the signal in one ear.

"A distracting noise that is more to one side will cause an auditory illusion by obscuring the signal in that ear," says Mason. "It essentially ends up fooling the fly into perceiving that the signal is coming from one place, so it ends up pushing it away from the actual cricket sound."

Mason's research, which was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) and will be published in the biology journal eLife, may be of interest to evolutionary biologists since it shows that sources of noise can interfere with the pattern of cricket songs. This in turn affects the fly's ability to find a host to lay its parasite.

Mason says engineers may also be interested because it highlights the possible limitations of mechanically-coupled hearing systems. While Ormia offers a great directional system that offers a fast and accurate response, it's not clear how the flies navigate more complex situations, like multiple signals and noisy environments.

"These flies are very accurate for one thing, which is detecting cricket sounds, but that comes at a cost since they've evolved to focus on this very restrictive set of information," he says.

Somehow, he adds, the flies are able to overcome these apparent limitations in nature. How they're able to do this will be an important area of Mason's future research.