Parasite to prosthetic: The Ormia fly showed a scientist and an engineer how they might make a better hearing aid. (Image copyright Dennis Kunkel Microscopy, Inc.)
Big Ideas From Small Places
A small yellow fly lands on the back of a common field cricket. To entomologists, this fly is Ormia ochracea. It has no common name, so we’ll simply call it the ormia fly. In order for this fly to reproduce, a female finds a cricket, deposits her larvae, which burrow into the cricket’s body, grow, mature and then dig their way out so the maggots can pupate—killing the host in the act of egress. Beyond revealing another of nature’s secrets, this gruesome scene launched a completely unexpected trip into biotechnology.
This journey began in the 1970s, when Bill Cade—then a graduate student at the University of Texas at Austin and now president of the University of Lethbridge in Alberta, Canada—discovered the parasitic action of ormia on crickets. Moreover Cade showed that these flies find their cricket host by hearing and homing in on the male cricket’s calling song—that familiar summer chirp. Nonetheless Cade never got around to finding out exactly how the ormia hears a cricket.
Twenty years later Daniel Robert arrived as a new postdoctoral student in my lab at Cornell University in Ithaca, N.Y. Robert had worked on hearing in locusts for his doctoral thesis, in Switzerland. We chatted about potential projects and remembering Cade’s work, I suggested that he head to the southeast United States to bring ormia back to my lab—where I’ve spent most of my career studying the acoustic communication of insects. That very cold day in February, Robert liked the idea of warming up in Florida, so he agreed to take on the project of determining the kind of ear that permits this fly to hear crickets and home in on them.
Kudos For Collaborating
When Robert returned with enough flies to start a colony, he started looking for the ears. For Robert, this turned out to be relatively easy. His trained eyes homed in on a pair of thin, transparent membranes just behind the fly’s head that look a lot like the "eardrums" of crickets and grasshoppers—some of the many insects that I’d studied over the years. Sure enough, they turned out to be this fly’s ears.
The next step was to measure this ear’s response to sound—in ormia’s case, to cricket-like sounds. For measuring vibrations, the gold standard is Doppler laser vibrometry (DLV), which engineers use to study mechanical structures—for example, spinning hard drives. With a price tag of $150,000 or more, though, DLV instruments exceed the budget of most biologists. Added to that operating one takes technical know-how—like a mechanical engineer’s knowledge. My lab lacked the instrument and the engineer.
Around this time a department party brought together faculty, family and close friends. Carol Miles, another postdoc in my lab, brought her husband, Ron to the party. As it turns out, Ron is a mechanical engineering professor from Binghamton University, about an hour away from Cornell. We soon learned that Ron Miles is an acoustics expert—one who has a DLV. Moreover Miles once worked at Boeing, where he used DLV to study the vibrations of various parts of the 747. At that party, Miles started thinking about moving his attention from problems large to small—from a jumbo jet to a fly’s ear. We developed an ongoing collaboration.
Working together, we quickly revealed that an ormia’s ears—each about half a millimeter across—provide great sensitivity and directional information with which this fly can locate a singing cricket with unerring accuracy. Even more interesting, the design of this fly’s ears is unique—at least among any known in the animal kingdom. This fly’s paired eardrums are anatomically and mechanically coupled to each other, such that they form a resonating system in which the two eardrums vibrate in a push–pull manner.
Bioacoustics To Biotechnology
This was a very nice discovery, delighting us biologists because we could claim to have uncovered a new mechanism for directional hearing. Plus we had solved the ormia-ear problem. Moreover Miles realized immediately that this fly-ear design was remarkable as a piece of acoustical engineering. The ears are really tiny, yet they are very sensitive to sound volume and direction. Miles wondered whether these features could spawn a new kind of directional microphone—one on the order of 1 millimeter across, built of a thin silicon membrane and designed to respond to directional sounds as can an ormia’s eardrums. Modern nanofabrication facilities, like the one in Cornell’s engineering college, easily make silicon devices on this scale, and cheaply, too.
Miles’s insight was to apply a strategy called “biomimicry.” This means taking a biological system for solving some problem—like sound localization in an insect’s auditory system—and applying similar features to a device for human applications.
One obvious application for an ormia-inspired directional microphone is in the field of hearing-aid technology. Currently, directional hearing aids are very expensive, but a hearing aid in which several directional microphones could be mounted and that fits into the ear canal—the most popular and desirable type—would constitute a design breakthrough. The key to commercialization is cost, which could be one advantage of an ormia-microphone design. Since the microphone membrane would be made of a very small chip of silicon, fabricated en masse by nanofabrication techniques, the promise of a cheap directionally sensitive microphone might be possible. Although it is still early days in this project, its progress is promising.
It is very satisfying as a biologist to realize that a discovery made in an animal—especially one completely neglected except by entomologists—turned out to be interesting for its unique biological features and might form the basis for a biomedical-engineering project that could enhance the quality of life for the hearing impaired. As this story shows the pursuit of knowledge for its own sake, no matter how seemingly trivial or irrelevant it might sound at first, may yield solutions for problems, which scaled up, just might be put to good use by people.
