A rainforest katydid has ears that evolved to be remarkably like those of humans and other mammals. The insect's hearing organ, although tucked in the crook of its front legs, has components that echo the structures of our own middle and inner ear, researchers have discovered.
In humans the outer portion of the ear gathers sound waves and funnels them toward the thin membrane of the eardrum, also called the tympanic membrane. The eardrum's vibrations translate the pressure waves of sound to the three smallest bones in our body—the hammer, anvil and stirrup bones—which shake against a snail shell-shaped organ called the cochlea, whose curves are lined with sensory-cell hairs. The wiggling bones outside jiggle liquid inside the organ and trigger those cells, which send signals to the brain via the auditory nerve. Hair cells that respond to high frequencies are closest to the origin of wave propagation and cells that respond to low frequencies are located deeper within the cochlea.
Copiphora gorgonensis is a yellow-orange–faced katydid from Gorgona Island in Colombia. The insect's hearing organ comprises a tympanic membrane that connects to a thin cuticle plate. Sound waves rock the tympanum and thus the cuticle plate like a seesaw. But the seesaw is lopsided, with the fulcrum closer to one end. This setup translates larger movements from air pressure waves to smaller, more powerful motions in the cuticle plate. The plate creates ripples in a fluid-filled chamber akin to an unfurled cochlea. Inside this chamber sensory cells are arranged like a keyboard from high- to low-frequency sensitivity.
The cuticle plate plays the same role as the tiny hammer, anvil and stirrup of mammalian ears, but in miniature. The katydid's entire ear spans just 600 microns, a U.K.-based research team reports in the November 16 issue of Science. (A micron is one millionth of a meter.)
C. gorgonensis's exquisitely evolved ear may help it avoid predators, says Fernando Montealegre-Z, a sensory biologist now at the University of Lincoln in England and the study's lead author. These katydids communicate in ultrasound, a range too high for most ears in the animal world—and therefore most potential predators. The frequencies at which their sensory cells respond range from 10 to 50 kilohertz. A 40 micron-long section of cells is all that is necessary to hear fellow katydids, which sing at around 23 kilohertz. This finding suggests that the katydid can hear a range of high-frequency sounds, likely including the calls of echolocating bats on the hunt for a meal.
"[The finding] is yet another remarkable demonstration of convergent evolution," says Ron Hoy, a professor of neurobiology and behavior at Cornell University who was not involved in the work. He explains that sound travels well through the air, but when it meets a fluid interface most of the sound waves bounce off its surface. Because the sensory cells must be bathed in fluid to keep from drying out, however, hearing organs had to evolve structures to overcome this barrier. Exactly how this problem was circumvented in insect evolution remained a mystery until now. "It is really the first report of how insects get around this," he says. Although katydids are the only insects with this kind of ear, Hoy says he would be surprised if scientists didn't find other examples. He wrote a commentary to accompany the new report.
Montealegre-Z is also interested in a second katydid species that can detect sound waves between 10 and 160 kilohertz using only 14 sensory cells. He does not know if these cells are somehow responding to multiple frequencies or if the katydid instead hears a simpler translation of the full acoustical range. The efficiency of this tiny system could inspire engineers to create microsensors based on the katydid's ear design, he says.
Some microsensors are already used in directional hearing aids, Hoy explains. Katydid-inspired sensors could be less fragile, smaller and more sensitive, thus spurring applications we haven't thought of yet. "Who knows what they could be in the hands of an imaginative engineer?" he says.