Bugged

Frog-biting flies tap into mating calls to target prey

BY ELIZABETH GARDNER

Midges, tiny mosquito-like flies that prey on túngara frogs, home in on the frogs’ mating calls to locate them. But how they eavesdrop on the calls — perhaps through an unusual ability of their antennae or yet-to-be-discovered tiny ears — is still a mystery.

Like World War II spies, the midges tap into and exploit a code that should be undetectable to them. The calls are made at a frequency the midges shouldn’t be able to receive through their antennae, especially at the long distances from which they are hearing the frogs calling. They also put the amorous frogs in the position of being unable to attract a mate without also inviting bloodthirsty and disease-carrying pests to dinner.

Ximena Bernal, assistant professor of biological sciences at Purdue, is the first to rigorously study how frog-biting midges hear.

“If we could model technology after this amazing feat of nature, we could greatly improve the quality and comfort of hearing aids.”

Ximena Bernal, Assistant Professor of Biological Sciences


Nature’s superior technology

Many insects have tympanic ears, which can be located almost anywhere on their bodies. But within this family of flies and their close relatives, none are known to have them. The midges’ entire bodies are only a few millimeters long, and the discovery of such a tiny ear could inform the creation of smaller and better microphones or speakers that could provide a variety of useful applications, including improving current hearing aids, Bernal says.

“We don’t yet know how the midges are hearing the frogs, but another, larger fly that listens in on cricket calls was found to have an ear with incredibly accurate hearing,” Bernal says. “It would be like a person hearing a cellphone conversation three football fields away who could pinpoint the yard line, the side of the field and the exact location of the phone. This shows that small ears can be very accurate. If we could model technology after this amazing feat of nature, we could greatly improve the quality and comfort of hearing aids.”



Missing ‘chucks’

Bernal’s focus is animal communication and the ecological and evolutionary implications of the frog-fly interactions.

Studies had shown that female túngara frogs prefer more frequent and complex calls, with what are called “chucks” added to the end of the call, she says.

The simple call sound is like the blast from a video game laser gun, while the “chucks” are a short stuttering sound, more like the noise of a video game rebuke when a player attempts the wrong move.

“You wonder, if females prefer calls with many chucks, why aren’t the males always adding chucks to their calls?” she says. “We discovered the calls with chucks are also more attractive to midges and frog-eating bats. This dynamic creates some really interesting scenarios for the frogs. If the frogs don’t call, they won’t get females. They have to balance the risk of being attacked with the need to find a mate.”

In response, the frogs modulated the complexity of their calls and rate of chucking, adding the attractive acoustic ornaments when the risk from predators is low and/or they know females are listening, she says.



An outsider’s insight

Bernal now works with a collaborator at Cornell University whose focus is insect hearing. At first, entomologists and insect researchers were skeptical of her findings. “Because I am an outsider to the field, I wasn’t aware of how crazy the idea was,” she says. “I didn’t rule out things entomologists might have ruled out as too far-fetched. I was curious, and I pursued it. It may be that they have a tympanic ear we haven’t found, or they use their antennae in a way we can’t explain. Either would be surprising.”

Bernal has published papers on the phenomenon in journals that include Ethology, Animal Behaviour, and Ecology. She is currently investigating hundreds of species of frog-biting midges to understand how flies started hearing and paying attention to the calls of frogs.