Can a Caterpillar Hear Without Ears?

Can a caterpillar hear without ears? It may initially seem an odd question, yet a team of researchers chose to investigate it.

A chance observation in the laboratory developed into research revealing how a small garden insect detects danger through sound. The finding could also help to improve sound-detection technology in the future.

Caterpillars react to sound

The research story began several years ago at Binghamton University, where Carol Miles, an associate professor of Biological Sciences, spotted an unusual behaviour.

When she spoke in a laboratory containing caterpillars, they would jump. Their behaviour resembled a startle reaction, much like the human response to an unexpectedly loud sound.

“Every time I went ‘boo’ at them, they would jump,” said Professor Miles. “And so I just sort of filed it away in the back of my head for many years. Finally, I said, ‘Let’s find out if they can hear and what they can hear and why.’”

The observation remained in her mind, and the question resurfaced much later: could caterpillars truly hear sound despite having no ears?

Studying caterpillar sound

To investigate the question, Professor Miles joined forces with Ronald Miles, a professor of mechanical engineering at Binghamton University.

Ronald Miles had previously examined how creatures such as flies and spiders react to sound. His earlier research had even resulted in a patent for a new microphone.

“There’s an enormous amount of effort and expense on technologies for detecting sound, and there are all kinds of microphones made in this world. We need to learn better ways to create them,” he said.

“And the way it’s always been done is to look at what animals do and learn how animals detect sound.”

The combination of biology and engineering made the pair well suited to investigate how caterpillars sense sound.

Testing sound in silence

The researchers required a highly controlled environment to test sound responses accurately. They conducted the experiments in Binghamton University’s anechoic chamber, among the quietest rooms in the world.

The chamber excludes external noise and stops echoes, giving researchers complete control over sound.

“It allows us really extremely accurate control over the sound field. And that’s important if you’re trying to figure out what it is that the animal is responding to,” said Ronald Miles.

“So with this, we can give the animal just sound and no vibration, or just vibration and no sound, and we can accurately measure just how much of each the animal is responding to.”

This arrangement allowed the team to distinguish airborne sound from vibrations travelling through surfaces, including plant stems.

Testing sound versus vibration

The researchers played low-frequency sound at 150 hertz and high-frequency sound at 2000 hertz to examine the reactions of tobacco hornworm caterpillars.

They compared responses to sound moving through the air with responses to vibrations passing through the surface beneath each caterpillar.

“We wanted to find out whether they are responding to this airborne sound or just the sound induced vibration of the base, through their feet,” said study co-author Sara Aghazadeh.

“They are always on the stem of the plant, so we thought maybe the vibration of the plant is the reason for them to detect sound.”

The findings were unambiguous: caterpillars responded 10 to 100 times more strongly to airborne sound than to vibrations transmitted through a surface.

A small insect with a big ability

“I know this sounds really silly, but there are videos of another species of caterpillar responding to sound on social media,” co-author Aishwarya Sriram said.

“So we knew they responded, but we just didn’t know if it was sound or vibration. We were extremely happy when we found out they detect airborne sound as well as base vibration.”

The results demonstrated that caterpillars depend on more than touch or movement. Sound carried in the air has an important role.

Listening through hair

The next issue was how this sound detection operates. In the absence of ears, another bodily structure needed to be responsible.

The team explored this by gently removing minute hairs from the caterpillars’ bodies, concentrating on the abdomen and thorax.

“We found that, yes, the caterpillar can detect sound rather than the sound induced vibration of the substrate,” Aghazadeh said. “So at that time, we wanted to figure out if they are hearing, where are the ‘ears’ of this caterpillar?”

Once the hairs had been removed, the ability to detect sound declined markedly. “A lot of other insects respond to sound, because sound causes motion of the air, and they have little hairs that can respond,” Ronald Miles said.

The hairs move in response to air vibrations and function as sound sensors.

Why certain sounds matter more

The team also investigated why some sound frequencies prompted more powerful reactions. Predators may provide part of the explanation.

“The wing beat frequencies of these predatory wasps are around 150 or 100 to 200 Hz, so I think the caterpillars think that there is a predatory wasp hovering near or above the caterpillar,” Sriram said.

“And that’s why it primes itself when it hears the sound, and it reacts with a jump startle, or a freeze, or a twitch response.”

This capability could aid caterpillar survival by enabling an early reaction to threats.

An apparently ordinary garden insect has therefore revealed a sophisticated method of sensing sound.

By examining how tiny hairs register air movement, scientists could create improved microphones requiring less power while offering greater sensitivity. Major ideas can sometimes arise from the smallest creatures.