A cinnabar moth (Tyria jacobaeae) caterpillar, July 8, 2011.
| Photo Credit: Quartl (CC BY-SA 3.0)

Caterpillars have a sixth sense that most land-based animals do not. They can sense electric fields around them with small bristles called setae on its body — a feat called electroreception. British researchers have discovered this in laboratory experiments and their findings were published recently in the Proceedings of the National Academy of Sciences.

They studied four species of caterpillars: cinnabar moth, scarce vapourer moth, European peacock butterfly, and common wasp.

Researchers have long known that aquatic and amphibious use electroreception to detect both predators and prey. Since 2013, scientists have also found electroreception in arthropods like bumblebees, hoverflies, and spiders. However, none of these land-based creatures use the ability to defend against predators.

Caterpillars, which are also arthropods, may break this mould, using electroreception to sense predatory insects nearby.

“It works exactly the same way as [brushing a] rubber balloon on your hair,” said first author Sam England. He is a postdoctoral researcher at the Museum für Naturkunde in Berlin and was at the University of Bristol before.

Static charges accumulate on both the balloon and the hair, leading to a mutual electrostatic attraction. The balloon can then move the hair without needing contact.

As an insect’s wings flap through the air, static charges build up on them. When it nears the caterpillar, the setae senses these charges by building up charges of its own. This mutual interaction gives rise to an oscillating electric field.

In the study, researchers stimulated caterpillars with a live electrode carrying a voltage oscillating at 180 Hz, to mimic an approaching wasp. In response, the caterpillars flailed and coiled, a sign that they were stressed. Caterpillars around an electrode without the voltage didn’t behave this way.

The caterpillars’ setae responded to frequencies of 50-350 Hz, possibly to help distinguish between threats and non-threats, Dr. England said. The setae vibrated the most when the field frequency was 220.3 Hz — close to the rate at which many predator insects flap their wings.

He suspected the caterpillars could have evolved to tune to their predator’s wingbeats. “These animals have had a lot of evolutionary pressure upon them to evolve defences because so many animals like to eat them.”

That said, scientists also “already know” that caterpillars don’t rely on electroreception alone to sense predators, Dr. England added. It supplements the other five senses.

He also said that “sensory pollution” could damp the caterpillars’ electroreception. The setae are sensitive to voltage frequencies also present in overhead power cables, around 50-60 Hz. This ‘exposure’ could desensitise the setae and diminish the caterpillars’ ability to spot predators with them.

Dr. England said he is keen on studying this issue next.

Karthik Vinod is an intern with The Hindu.