In a small soundproof lab room at the University of Toronto’s Scarborough campus, surrounded by equalizers and amplifiers, there’s a tiny treadmill for insects that could help engineers design a better hearing aid for humans.
Ormia ochracea may only be the size of a fingernail, but the fly’s hearing ability is on par with that of any human trying to swat it. Native to the southern U.S., it’s drawn to crickets’ singing, but no one quite knows how it manages to pick out that sound amid the cacophony of the natural world and locate it so precisely — to within two degrees.
“It was a bit of a mystery how a small animal was able to do this,” says UTSC biologist Andrew Mason. But if he and his labmate Norman Lee can figure out how Ormia pinpoints individual sounds in a noisy setting, it could help solve the so-called cocktail party problem — the one that makes it tough for your grandmother to hear what you’re saying at family functions (and causes her to shout at you), because her hearing aid picks up too much background noise.
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As part of their research, Mason and Lee used wax to stick the fly (whose ancestors they’d procured from swampy northern Florida) to a pin suspended above a ping-pong ball, allowing the insect’s feet to touch down and creating a sort of treadmill. Then they played a simulated cricket sound and a distracting noise simultaneously, and watched as the grounded fly tried to locate and run toward the phantom cricket. The idea was to determine whether Ormia could block out unwanted noise when on the hunt.
When the biologists placed the noise source beside the one emanating the cricket sound — both stationed in front of the fly — Ormia ran just off centre, missing the direction of the cricket by a few degrees. When they moved the non-cricket noise farther from the cricket song, the fly became even more confused, veering way off course.
“There might be something in the brain that allows flies to separate these two items in nature that we’re not able to address yet,” Lee says. In May, he and Mason published their findings in the journal eLife.
Ormia’s super-hearing, facilitated by ears just half a millimetre long, has a macabre evolutionary function: the female deposits its spawn inside the crickets, who sing when looking for a mate. Black-striped larvae then hatch inside the doomed cricket and scrape at its innards for 10 days, Mason says, before “bursting out of the side like in Alien.”
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Unlike in humans, Ormia’s eardrums are connected — which Mason says could explain why the fly tries to interpret the different levels of sound it receives in both ears. Its eardrums work like a scale, and incoming noises tip the balance. In Mason and Lee’s experiment, the vibrations in the ear closest to the distracting noise overpowered the cricket song — but the other side of the eardrum could hear the cricket more clearly. This caused the fly to believe the cricket was off to the side, even though it was straight ahead.
Humans are able to locate and isolate the sounds they want to listen to, even in noisy environments, if the sources are far enough away from one another. (If two speeches are delivered on either end of a room, for example, you can tune into one or the other more easily than if the speakers were beside each other.) But for people who wear hearing aids, it’s impossible to focus on a single conversation in a noisy room: the whir of an air conditioner, lively chatter, and clinking glasses can all conspire to make socializing impossible, because hearing aids trick the ear into thinking all those sounds are the same distance away.
“There are a number of mechanisms that the auditory system uses to help focus selectively onto a single source,” Mason says. “These are defeated if you’re wearing a hearing aid, because the hearing aid amplifies everything indiscriminately and so the extraneous noise is harder to filter out.”
In their study, Mason and Lee found that, where physically separating sounds would be helpful to a human, it only made Ormia confused. But while the fly may not be able to isolate noises like we can, engineers have still made use of its sound-locating skills: Gunshot detectors in urban centres use Ormia-inspired microphones to locate shooters. Mics in some hearing-aid designs mimic the fly’s ear structure, even as biologists like Mason and Lee explore its limitations. And research groups around the world are working on hearing aids that would allow the wearer to home in on different frequencies — ones more useful to humans than that of a cricket’s song.
Meanwhile, back in the swamp, it seems crickets are conducting less tuneful mating rituals, as the females realize the best male singers are also the most likely to be parasitized by aurally gifted flies. “Some crickets remain silent,” Mason says, “because it doesn’t pay to sing anymore.”
Michelle Pucci has written for the Montreal Gazette, The Walrus, and CBC News.