In the silence of a summer garden, tobacco hornworm caterpillars cling to branches and quietly feed on leaves. Then, suddenly, one freezes. It has detected danger just in time—a wasp is approaching from behind and assessing its potential prey.
Although tobacco hornworm caterpillars appear to have no ears, they can detect natural enemies such as wasps. So how does a caterpillar know when a wasp is nearby?
Scientists do not yet fully understand how caterpillars sense approaching predators. However, our team of biologists and engineers is working to solve this mystery. Ongoing research suggests that tobacco hornworms can hear with highly sensitive tiny hairs located on their bodies.
Learning how these remarkable organisms detect and respond to their surroundings could help scientists better understand the natural world. It may also inspire the development of smaller, less expensive microphone technologies.
Because hornworm hairs are extremely sensitive, researchers must study them in an environment free from outside noise and vibrations. One of the best places to conduct hearing research is an anechoic chamber, a specially designed room that absorbs sound and isolates experiments from external disturbances.
What Happens in an Anechoic Chamber?
An anechoic chamber is one of the quietest places in the world. Its specialized walls block outside sounds, while a sturdy steel spring system supports the “floating” chamber and prevents it from touching the ground. This separation helps isolate the room from external vibrations, allowing researchers to study even the smallest movements.
Inside the chamber, we studied how caterpillars respond to vibrations. Over the course of a year, researchers placed individual caterpillars on a platform and transmitted vibrations through it at different intensities.
To measure the platform’s movement and the precise vibrations reaching each caterpillar, the team used an accelerometer. Some caterpillars jumped in response to strong vibrations, while others convulsed or trembled when exposed to intense physical movement.
These observations revealed a consistent response threshold. When vibrations were weaker than that level, the caterpillars showed no visible reaction.
After identifying this pattern, we tested additional caterpillars in the anechoic chamber using airborne sound rather than direct platform vibrations. The goal was to determine whether caterpillars could detect sound through the air independently of vibrations transmitted through the platform.
Sound is broadly defined as a form of vibration or energy that can become audible when it reaches a listener’s sensory system. However, airborne sound can also cause nearby objects to vibrate. To make sure the caterpillars were responding to sound itself—not vibrations traveling through the platform—we used accelerometers to measure platform movement during each experiment. We compared vibrations produced by airborne sound with those caused by direct physical stimulation.
The caterpillars continued responding to airborne sound even when the resulting platform vibrations were below their direct-vibration response threshold. This result suggests that tobacco hornworms can detect airborne sounds through a hearing mechanism separate from their ability to sense physical vibrations.
Finding the Caterpillar’s “Ears”
So where are a caterpillar’s “ears”? More precisely, which body structures allow it to detect sound?
Animals can detect sound in two primary ways: by sensing sound-wave pressure or by detecting the movement and speed of the particles that make up a sound wave.
Scientists have traditionally associated hearing with a tympanic membrane organ. In most mammals, the tympanic membrane vibrates in response to sound pressure, and those vibrations move nearby bones and other structures that transmit signals to the nervous system.
Source: www.wired.com