Scientists Put Caterpillars in an Ultraquiet Chamber to Learn How They Hear Without Ears
Researchers have found that tobacco hornworm caterpillars can hear with tiny sensitive hairs on their bodies rather than ears. The discovery could lead to new microphone technology.
In a quiet summer garden, a caterpillar rests on a branch, calmly feeding. Moments later it abruptly freezes, sensing a wasp approaching from behind. Although tobacco hornworm caterpillars show no obvious ears, they can perceive such dangers. A team of biologists and engineers has been investigating this ability and believes the caterpillars hear through tiny, extremely sensitive hairs on their bodies.
The hairs are so responsive that experiments must take place in absolute silence. Researchers used an anechoic chamber, one of the quietest places on Earth. The room rests on heavy steel springs, keeping it detached from the ground and blocking outside vibrations and noise.
For a full year, scientists set up caterpillars on a platform each day and sent vibrations through it at many different strengths. An accelerometer recorded the motion and precise vibrations passing through the platform. Depending on the force, the caterpillars would jump, twitch, or shudder. Eventually the team determined the exact threshold below which the insects no longer visibly reacted.
In the next phase, the researchers switched to airborne sound. The caterpillars continued to respond even when the airborne stimulus was weaker than their earlier vibration threshold. To rule out that they were merely detecting platform vibrations caused by the sound, the accelerometer was used to compare platform motion under sound stimulation and under direct vibration. The continued reactions suggested the caterpillars were genuinely hearing sound through the air.
This raised the question: where, or what, are their ears? Insects typically detect sound pressure through tympanal organs, but caterpillars lack obvious membranes. After reviewing earlier research and examining the insects under a microscope, the scientists focused on distinctive body hairs. In follow-up experiments, they removed either all or selected hairs with tweezers under a microscope and observed the insects' responses to sound. The defensive reactions dropped dramatically, and the effect varied with the removed hairs and tested frequencies. The findings have not yet been published in a journal.
The work could inspire new acoustic technology. Standard microphones use membranes to detect sound pressure. If engineers imitate caterpillar hairs, future microphones might also measure air particle velocity and determine the direction from which a sound wave comes. Such directional microphones could be especially useful in hearing aids, giving users information about both loudness and origin of a sound.
