Caterpillar Reactions in a Silent Chamber Point to Hearing Hairs

Tobacco hornworm caterpillars may detect approaching predators with the help of tiny hairs covering their bodies, according to research described by Phys.org. The findings offer a possible explanation for how the insects respond to sound despite having no obvious ears or tympanal membranes. The research team studied the caterpillars in an anechoic chamber, a heavily isolated room designed to block outside noise and vibrations. The chamber is supported on steel springs and separated from the ground, allowing researchers to examine the insects’ responses in an unusually controlled setting. For about a year, the researchers placed individual caterpillars on a platform and exposed them to vibrations at different intensities. An accelerometer recorded the movements passing through the platform. Depending on the strength of the stimulus, the caterpillars jumped, twitched or shuddered. The researchers used these observations to establish the minimum vibration level that produced a visible reaction. They then introduced airborne sound and measured the platform at the same time. The caterpillars continued reacting to sounds even when the vibrations reaching the platform were weaker than the level that had triggered reactions during the direct-vibration tests. The team said this suggested the insects were responding to airborne sound rather than merely sensing mechanical movement through the platform. The researchers next examined the caterpillars’ body hairs. In experiments conducted under a microscope, they removed either all of the hairs or selected groups with tweezers, then compared the insects’ responses before and after the procedure. Defensive reactions fell substantially across different sound frequencies, depending on which hairs had been removed. The work is still ongoing and has not yet been published in a scientific journal, according to the report. The researchers are investigating whether particular hairs are specialized for detecting particular frequencies. The possible biological mechanism could also have engineering applications. Conventional microphones detect sound pressure using membranes, while structures modeled on the caterpillar hairs might help measure the movement of air particles as well. Such systems could potentially provide information about both the strength and direction of a sound, including in future hearing-aid microphone designs.