How Caterpillars Hear: A Discovery with Microphone Implications

Few insects hear the way humans do. Some, like crickets and grasshoppers, have eardrum-like structures that detect sound as it travels through air, much like us. But for most insects, hearing means detecting mechanical vibrations traveling through surfaces like leaves, branches, or the ground.

Yet for the past year, every time Carol Miles, a biologist at Binghamton University, chatted with colleagues near her lab’s collection of tobacco hornworm caterpillars, the insects sprang into the air as if startled. “My [colleagues] would say, ‘Carol, you’ve got to shut up, you’re scaring the caterpillars’,” says Miles.

The curious effect of Miles’ gabbing led her to wonder how this was possible. These jade-colored caterpillars, found on farms and in backyard gardens across North and South America, eventually transform into Carolina sphinx moths, which possess eardrum-like structures near their mouths. As caterpillars, the same bugs possess no such structures, yet they respond to sound.

To find out why, Miles and her colleagues spent years completing a complex study that involved microscopic haircuts and access to one of the quietest rooms in the world. The results, announced at the joint meeting of the Acoustical Society of America and the Acoustical Society of Japan in December, suggest that tobacco hornworm caterpillars detect airborne sound using microscopic hairs on their bodies. Although this research has yet to be peer-reviewed, experts say the findings provide new insights into the evolution of hearing—and could even inspire the next generation of microphones.

To determine whether tobacco hornworm caterpillars could detect airborne sounds, not just the vibrations associated with them, Miles and her colleagues conducted a series of tests in Binghamton’s anechoic chamber, a specialized, ultra-quiet facility built for high-precision acoustic research. This room is so quiet that people inside can often hear their heart beating.

The researchers exposed the caterpillars to low and high-frequency sounds, both in the air and through surfaces via vibration, and recorded their responses. After crunching the numbers, the researchers found that the caterpillars were 10 to 100 times more responsive to airborne sound than to surface vibrations, a difference rarely seen in insects.

Having shown that caterpillars could indeed hear airborne sound, researchers wanted to understand how. They suspected the tiny hairs that cover the caterpillars’ bodies, but just to be sure, they plucked the hairs from the abdomen and thoraxes of several caterpillars, and repeated their experiment. As expected, the bald caterpillars barely responded to sound.

“We all knew that the body of caterpillars is covered with touch-sensitive bristles. What we did not know is that these bristles are sensitive to sound,” says Martin Göpfert, a professor of cellular neurobiology at the University of Göttingen in Germany, who was not involved with the study.

These caterpillars likely evolved this hearing system, Göpfert says, to evade their predators. According to the researchers, the caterpillars are not only capable of hearing the wing beat frequencies of wasps known to prey on them, but they also respond dramatically to them. When triggered, Miles says, the caterpillars will jump or rear up in defense, ready to bite anyone who gets too close.

Insects have evolved hearing independently at least 20 times, giving rise to a remarkable diversity of hearing systems vastly different than our own. This is why engineers often look to insect anatomy for inspiration when designing new devices to capture sound.

“Current microphones are made the way they are (i.e. with pressure-sensing diaphragms) because we are particularly arrogant animals and we have focused our attention at our own ears and paid little attention to the types of ears that other animals have,” said Ronald Miles, a professor of mechanical engineering at Binghamton University and co-author of the new study.

Miles, who has patented insect-inspired microphone designs before, says the tobacco hornworm caterpillars could help “inspire an entirely new class of microphones” that are smaller and less expensive than current designs.

Regardless of whether the caterpillars currently munching on your backyard tomato plant will inspire the next generation of microphone technology, the discovery of how they hear is undoubtedly beneficial to humans.

“Among animals, insects present the most diverse ears, and the richest pool of them,” says Göpfert. And these “insect ears” can often do things our human ears can't. “Waxmoths, for example, can hear very high sounds no other animal can hear, and some flies can pinpoint sound sources with a precision that seems unreachable physically,” says Göpfert. “Through insects, nature thus tells us how physical limitations of sound sensing can be overcome.”