Caterpillars’ chirp could be scary

Larvae of great peacock moths might signal that they’ll put up a fight

Should the need arise to say “back the bleep off” in caterpillar, it’s “chirp, chirp.”

DEFENSIVE ARTS | A caterpillar of the great peacock moth can say “leave me alone” in plenty of ways. Little clumps of spikes dotted along its body ooze a foul-smelling liquid, and buzzy chirps may serve as a warning that the creature is vexed enough to ooze. V. Bura
BAD DRIP A close-up of a great peacock moth’s clumps of spikes shows the strong-smelling droplets they release when under attack. The little bumps that support the spikes start yellow and change colors as the caterpillar grows. V. Bura

When harassed, the large, plump caterpillars of the great peacock moth make a run of sounds somewhat like the rasping of a fingernail over a comb, says Jayne E. Yack of Carleton University in Ottawa, Canada. These aren’t the clicking sounds she and her colleagues have studied before, but are more like a cricket’s chirps, she says.

Saturnia pyri chirp before or while they ooze foul-smelling droplets from their bristles. So the chirps might be a warning to attackers that there’s some serious resistance on the way, Yack and her colleagues propose online and in an upcoming Naturwissenschaften.

“Caterpillars are really important in an economic and ecological sense,” Yack says. They attack a lot of crops but also provide foods for many birds and gleaning bats. While most work has looked at adult moths, the study of caterpillar clicks and chattering has lagged, Yack says.

This new caterpillar paper is a “wonderful piece of natural history and behavior of the type that we need more of,” says moth-communication specialist William Conner of Wake Forest University in Winston-Salem, N.C. Now, he says, he’d like to see details of how predators react.

Unlike a lot of caterpillar communication, the chirps the team recorded in the lab travel by air and their frequencies overlap with bird, bat and human hearing ranges. Caterpillars give broadband chirps, with frequencies ranging from 3.7 kilohertz to the ultrasonic at 55.1 kilohertz, the researchers say. The team determined that caterpillars make the noises by scraping one of their mandibles against the inside of the other.

These long chirps may work a bit like the airborne clicking sounds made by a different caterpillar (Antheraea polyphemus) that Yack and her colleagues reported in 2007. That caterpillar clicks before or as it regurgitates foul-smelling glop on attackers.

The new paper describes another system in which caterpillars sound off as part of a suite of responses to attack. Rather than regurgitate, the great peacock moth caterpillar thrashes its body and oozes repellant droplets in addition to chirping.

“Strong and foul” is how Veronica Bura, a paper coauthor who works in Yack’s lab, describes the odor of the chemical droplets. She says it’s not an easy smell to describe. “It smells green, really green,” she says.

Droplets appear from the spiked nubbins that turn from yellow to blue to purple as the caterpillar grows. “They’re really pretty caterpillars,” she says.

From a bird’s point of view though, Conner says, “attempting to subdue such a caterpillar must be a memorable and unpleasant experience.”

Bura and Yack came across the chirp during a long-running project to survey caterpillars for sounds. The great peacock moth belongs in the same group as hawkmoths, which are emerging as a very chatty bunch of caterpillars, Bura says. She has more papers on the way.

More information is going to be necessary to see if these are true warning, or aposematic, signals, cautions Pedro Barbosa of the University of Maryland in College Park. Yet sound can have effects on its own as part of defense. For example, it might just be a “BOO” that startles a predator, without a warning of worse to come.

Yack argues that she thinks it’s unlikely that her caterpillars are startling predators: They don’t chirp and flee, she says. They chirp and keep on clinging to their perch.

When a researcher gives a quick squeeze to the rump of a great peacock moth caterpillar, the creature thrashes and makes a short buzzy chirp by rubbing its mouthparts together.

Credit: V. Bura

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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