Rainforest Insects Hear Like Humans

Bugging out. The cucurbit beetle (Diabrotica speciosa) stops producing mating pheromones when it senses decreasing air pressure.

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Deep in the South American rainforest, katydids perk up their tiny ears to listen for the clicking of bats. Apart from being on the insects' forelegs, those ears are remarkably similar to our own, new research shows. And by imitating the tiny structures, researchers speculate, engineers might create microscopic acoustic sensors.

Mammalian ears have three distinctive features: a tympanic membrane, or eardrum, that vibrates with incoming sound waves; three delicate ear bones that transmit the vibrations into the inner ear; and the cochlea, a fluid-filled coil of sensor cells that respond to vibrations at different frequencies, arranged from high to low like the keys on a piano.

The fluid is necessary for the sensor cells to survive. But it also deflects much of the sound waves' energy—the same reason noises sound muted when you stick your head underwater, says Ronald Hoy, a neurobiologist at Cornell University who focuses on acoustic communication in insects. For sensor cells to receive signals, he says, there must be a mechanism for transforming the large, weak airborne sound waves into smaller but more powerful fluid-borne sound waves that the sensor cells can detect—a process called impedance matching. That's where the three-bone structure of the mammalian middle ear comes in—and, Hoy says, with its intricate structure and mechanical precision in converting these signals, the middle ear bones have long been considered "a whiz-bang miracle."

Scientists have known for decades that, like mammals, katydids and related insects have a miniature tympanic membrane; they also have a flat strip of sensor cells in their legs. But how these insects convert airborne sound waves to fluid-borne sound waves was less clear—particularly as researchers had not yet identified a fluid-filled organ that the waves could travel through.

Now, researchers at the University of Bristol and the University of Lincoln, both in the United Kingdom, think they have discovered a mechanism in the ears of Copiphora gorgonensis, a species of katydid from the forests of Colombia, that rivals the middle ear bones in mammals. The structure is so small—no bigger than a few hundred microns—that it couldn't have been discovered by dissection, Hoy says. "Even if you had golden hands, it would be easy to miss."

Instead, biophysicist Fernando Montealegre-Z detected the structure while examining three-dimensional images of the insects' inner ears from high-powered CT scans—a noninvasive technique used to examine fossilized bones and other artifacts without causing damage. As Montealegre-Z examined the images, he noticed a small, cone-shaped cavity behind the katydids' tympanic membrane. Two slivers of cuticle hinged on either side of the cavity with the short ends sticking out of the cavity. Further examination showed that the cavity was filled with oily liquid. When vibrations hit the tympanic membrane and moved the short ends of the slivers outside the cavity, the long ends inside the cavity wobbled, sending smaller but more powerful waves rippling through the fluid to the row of sensor cells.

This sound-converting mechanism is like that in humans but much simpler, says bionanoscientist Daniel Robert, one of the study's authors. That's exciting, he says, because it's "probably less delicate and more robust," and should inspire engineers to make extremely small, sensitive microphones that can hear in ultrasonic frequencies, as well as improve medical devices like hearing aids.

Catherine Carr, a neuroscientist at the University of Maryland, College Park, agrees that the study is "very exciting"—particularly because the katydids have overcome the problem of impedance matching "on such a tiny scale."

Despite their huge differences, katydids and humans have "the same job description," Hoy says: They have to hear through the air even though their sensory cells have to be kept wet. Before this study, he says, scientists were "clueless" about how insects like katydids accomplished that task. "What blows me away," he says, "is the evolutionary convergence between insects and mammals." -Emily Underwood

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