Now that summer is here, my son spends a lot of time outdoors, poking around the forest surrounding our house. He bothers a lot of insects in the name of play – and although he has no intention of eating any of them, an 8-year-old boy is still a pretty fair approximation of a predator. So he gets to see, first hand, a lot of the adaptations insects have evolved to discourage the large and hungry from turning them into new and exciting menu items. Stings are popular, if obvious. Bad smells are a surprise (and delight), as is the most devious camouflage. I’ve had to tell him bright colors mean “I taste awful,” not “Your big sister really wants to see me.” But I can’t wait to see what will happen when he stumbles across a North American walnut sphinx moth caterpillar. When they’re pinched, they whistle. Loudly. That’ll surprise him.
Surprise, of course, is the whole point. Hand a predator something unexpected when they come poking around – a loud noise, some snakelike eyespots, a lump of vomit -- and they might just go looking somewhere else for their meal. And that’s exactly what happened when Veronica Bura, a graduate student in Jayne Yack’s lab at Carleton University, put walnut sphinx moth caterpillars in a cage with a caterpillar-eating yellow warbler. In each case, the bird pecked at the caterpillar, the caterpillar squeaked like a wheezy dog toy (for an example of the noise, see this movie), and the bird decided not to risk eating that noisy thing. Although they're soft and tasty, all of the caterpillars survived being experimentally exposed to a predator without injury.
Where does their whistle come from? Unlike Lauren Bacall, these inch-long grubs have no lips. They don’t even have lungs – like all insects, they breathe through a distributed network of tubes that stretches throughout their bodies. Air enters the tubes through spiracles, a set of holes on the insect’s surface, and moves through the tubes to the animal’s tissues via a combination of passive diffusion and – in some species – a bit of active pumping by muscles in the thorax and abdomen. Each spiracle is attached to a system of muscles and levers that let the insect open and close the hole at will: an important bit of anatomy that lets them control how much moisture they lose as they breathe. But they can’t exactly pucker up.
They can, however, blow. High-speed video of whistling caterpillars showed that the insects contract their thorax and the front part of their abdomen about 100 milliseconds before they start making sound. Bura and her colleagues figured that the caterpillars were whistling by forcing air through their spiracles.
Still, all spiracles are not created equal. Like all caterpillars, walnut sphinx moth larvae have eight pairs of them along the sides of their abdomen, and the last pair is longer and wider than the others. The whistling comes from this large pair of spiracles: when the researchers blocked the last pair of spiracles with dots of makeup latex and pinched the caterpillars, there was no noise. Removing the latex plugs from that last pair of spiracles restored the insect’s whistle. And a pinch after blocking any other pair of spiracles with latex was rewarded with sound. It’s not yet clear how the caterpillar manipulates that last pair of spiracles to make air vibrate when it’s pushed through the opening, but further study could show whether they’re contracting muscles to narrow the opening or partly blocking it like the valve in a dog’s squeaky toy. However they do it, it’s certainly effective at alarming birds and 8 year olds alike.
References:
Bura VL, Rohwer VG, Martin PR, & Yack JE (2011). Whistling in caterpillars (Amorpha juglandis, Bombycoidea): sound-producing mechanism and function. The Journal of experimental biology, 214 (Pt 1), 30-7 PMID: 21147966
(with help from)
Schneiderman, H. A. 1960. Discontinuous respiration in insects: Role of the spiracles. Biol. Bull. 119 (3): 494-528.
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