A surprising reaction to sound
Most insects are able to “hear” sound, despite not having ears, through physical vibrations that travel through the surfaces they stand on, such as leaves or branches. However, biologists noticed something unusual about tobacco hornworm caterpillars. Despite having no eardrums, they would spring into the air as if surprised when people started chatting nearby.
Testing in a super-quiet room
To figure out how the bugs were hearing these conversations - whether it was through surface vibrations or other means - researchers placed them in an ultra-quiet room. Known as an anechoic chamber, this room was designed for high-precision acoustic testing. By playing different noises, the biologists discovered that the caterpillars were 10-100 times more sensitive to sounds travelling through the air than they were to vibrations on the ground.
The secret of the "microscopic haircut"
But how were they picking up airborne sounds without ears? The researchers suspected that the tiny bristles covering the caterpillars' bodies might be the key. To test this theory, they gave the insects a "microscopic haircut," carefully plucking the hairs from their bodies. Without their bristles, the bald caterpillars barely reacted to the sounds at all.
A built-in warning system
The biologists realised that these sensitive body hairs have evolved as an ingenious defence mechanism. The bristles allow the caterpillars to literally feel the specific buzzing frequencies of predators, such as wasps. When the sound triggers their hairs, the caterpillars can quickly jump or rear up to bite the wasp before it gets too close.
Inspiring future technology
Engineers and scientists are often inspired by the diversity of nature, and this fascinating new understanding of insects’ ability to hear is no different. At the moment, microphones are designed to mimic human ears, using pressure-sensing diaphragms to absorb the sound. By studying how these caterpillars use hairs to detect sound, engineers believe this could inspire the creation of a new class of microphones - much smaller and less expensive than today’s designs.
“While these caterpillars use tiny bristles on the outside of their bodies to feel sounds in the air, humans rely on thousands of microscopic hair cells deep inside the inner ear to send sound signals to the brain,” says Vincent Howard, Clinical Audiologist at Hearology®. “This incredible research is a wonderful reminder of how delicate and complex hearing is. It highlights exactly why we must do everything we can to protect our own highly specialised auditory systems from unnecessary damage.”
References
- Why does sound startle these caterpillars even though they don't have ears? - National Geographic article on the subject
- These caterpillars hear through hair, scientists discover - EarthSky, a well-established science and astronomy website, article on the subject
- These caterpillars can hear using tiny hairs on their bodies - Binghamton University article summarising the original research that made this discovery - which research was carried out at Binghamton University
