This scratchy hiss is the closest thing yet to caterpillar vocalization

Tap — gently — the plump rear of a young Nessus sphinx hawk moth, and you may hear the closest sound yet discovered to a caterpillar voice.

Caterpillars don’t breathe through their mouths. Yet a Nessus sphinx hawk moth, if disturbed, will emit from its open mouth a sustained hiss followed by a string of scratchy burplike sounds. “Hard to describe,” says animal behaviorist Jayne Yack of Carleton University in Ottawa, who urges people just to listen to it for themselves.

This newfound noise from young Amphion floridensis may startle birds or other would-be predators not expecting something as generally quiet as most caterpillars to erupt in sound.

The discovery marks the fourth sound-producing mechanism in caterpillars that Yack and colleagues have found. Some caterpillars use their spiracles, respiratory pores along the flanks, to toot sounds. Caterpillars take in oxygen and release waste carbon dioxide through these pores. These gases, which don’t depend on the caterpillar version of blood to travel throughout the body, move through a branching air duct system of increasingly tiny pipes. Two other kinds of caterpillar noises involve mouthparts rubbing against each other. But none of those noisemakers are involved here, researchers report online February 26 in Journal of Experimental Biology.

Instead, the new anatomical studies and computer modeling suggest that these caterpillars speak by pulling air in through their mouths and into their guts and then releasing it. The rush of air inward could create the first hissing part, and outrushes could make the string of scratchy burps. There’s no sign of a special sound-making flap in the gut, but air whooshing through a constriction could make noisy turbulence. That could give a caterpillar voice its own version of teakettle squeals. In miniature, of course.