Well, it may not be smoking, but it’s whistling. According to MSNBC science news, researchers have found that the walnut sphinx moth caterpillar (Amorpha juglandis), can make whistling noises by forcing air through its external breathing holes (“spiracles”). The paper, by Bura et al., is in the December issue of the Journal of Experimental Biology.
To confirm their idea, researcher Veronica Bura at Carleton University gently applied latex over all eight pairs of the caterpillars’ abdominal spiracles and then uncovered each pair systematically while pinching the larva. The whistles definitely came from the eighth pair, generating trains of whistles lasting up to four seconds each, and spanning frequencies that ranged from those audible to birds and humans up to ultrasound.
Why do they do this? The video below gives one clue, showing that they whistle when they’re attacked (they also thrash about):
To test this theory, Bura et al. exposed caterpillars to a bird predator, a trio of yellow warblers (Dendroica petechia). When attacked, the caterpillars produced sounds (and thrashed), and the birds were scared off, even when they attacked for a second time. None of the test caterpillars were injured. The authors conclude that the sound is a key part of the caterpillar’s anti-predator defenses. I find this intriguing but unproven, since the authors apparently didn’t do the required controls in which birds attacked caterpillars whose whistle had been silenced by occluding their eighth spiracles (as another control, they could just occlude the seventh pair of spiracles, which also help breathe but can’t whistle). The sound, then, may play some role or no role (it could, after all, be the thrashing that scares off birds). More work is needed here!
Bura, V. L., V. G. Rohwer, P. R. Martin, and J. E. Yack. 2010. Whistling in caterpillars (Amorpha juglandis, Bombycoidea): sound-producing mechanism and function. J. Exp. Biol. 214:30-37.