A piece in ScienceNews reported some new research (not yet published) that updated an old but good paper on the famous “whistling caterpillar.” I refer to the walnut sphinx caterpillar (Amorpha juglandis), which, when disturbed—as in a predator attack—emits a high-pitched whistle from its spiracles (the body openings used for respiration).
An earlier paper (from 2010) by Veronica Bura et al. in the Journal of Experimental Biology (reference and free link below) studied the whistling behavior, showed how the sound was made, and found that it apparently startles bird predators, who flee and thus allow the whistler to live another day. Newer work, however, shows that the whistle is even more amazing than thought, for it appears to mimic the sounds that other birds make when they see a predator: an “alarm call.”
But first, let’s look at the magnificent caterpillar. Nice one, eh?
The video below (you can see similar behaviors in the supplementary material to the paper) shows how the caterpillar whistles when it’s disturbed by experimenters, a disturbance designed to mimic a bird attack. (This caterpillar may also be attacked by other vertebrates like bats, snakes, and frogs.) You may notice that the “squeak” sounds like a bird, and that’s important.
In 2010, Bura and her colleagues did a number of studies on this species to locate the source and examine the effect of this sound. The salient results:
- They measured the characteristics of the sound with sonograms. That needn’t detain us except to note that the median length of the squeak produced by these artificially disturbed larvae was about two seconds.
- The sound was always accompanied by a contraction of the front of the body that lasted about half a second. I suspect this contraction is what forces air through the spiracles. Here’s what it looks like (figure from the paper), along with a recording of when the sound was produced
- The most obvious source of the sound would be the spiracles, the holes in the body wall through which larvae (and adult insects) breathe, taking in oxygen through diffusion (or active body movement) and expelling carbon dioxide. The association of the sound with body contraction makes this hypothesis more credible. The authors then covered up the spiracles with removable make-up latex to see if the spiracles were responsible, and, if so, which ones. They found that when all spiracles were covered, no sounds were emitted in mock attacks (pinching the beast). When the spiracles were uncovered, one by one, the authors found that only the posterior pair of spiracles (there are eight pairs) were involved, for only this pair, when covered, eliminated no sound. The posterior (“A8”) spiracles are also larger than all the others.
- The authors also did “laser Doppler vibrometry” to see whether the air and sound emitted during the squeak came out of a specific pair of spiracles. This involves bouncing a laser beam off of a small piece of tissue paper suspended near a spiracle. Vibrations of the tissue produced by emitted air will be detected by the laser. They found that the laser showed a big signal only when the tissue was by the A8 spiracle rather than a control (A5) spiracle, and that the vibrations disappeared when the spiracle was occluded by latex. Here’s a figure showing the results of the vibrometry. You can see a bit of vibration from A5 (which didn’t disappear when the spiracle was covered), but a a big pulse from A8 (pictures of the spiracles are also shown), which did disappear when the spiracle was covered with latex. Thus, the squeak is produced by air coming out of spiracle A8:
- Finally, the big question: does the squeak really deter predators? The authors tested this by exposing caterpillars to three captive (but wild-caught) yellow warblers (Dendroica petechia), who attacked the larvae regularly. In every case, when the bird attacked the caterpillar, the caterpillar produced a squeak (squeaks weren’t made when the larva wasn’t disturbed). And in every case, the bird was deterred by that squeak and fled: every bird hopped or flew away when the sound was made, and one bird even dove down into the vegetation as if it were attacked by a predator”. (That’s important; see below.) No caterpillar was harmed in the least by these attacks, so the sounds seem pretty effective in deterring predation.
Here’s a pretty lousy figure from the paper showing a bird being deterred by the sound and diving for cover:
But crucial experment wasn’t done. If it is the caterpllar’s squeak that deters the predator, the predator wouldn’t be deterred if the sound were eliminated. In other words, they should have done the same tests using warblers with the A8 spiracles of the caterpillar occluded with latex. If the authors are right, those caterpillars should have been nommed. That would have been an easy experiment to do, but for some reason it wasn’t. Thus the crucial information—that the sound itself deters predation—is missing.
The update by other investigators was, perhaps, motivated by one sentence in Bura et al.’s paper, which I’ve already alluded to:
In one case, the bird dived away from the caterpillar into thicker vegetation in a manner similar to how it would react to a predator.
That suggests, to the curious and atttentive mind, that the caterpillar’s squeak might imitate the “alarm” calls that birds give when predators nearby. And indeed, a new meeting abstract by Jessica Lindsay and Erik Greene (link and title below), which expands the previous results, suggesting that that the caterpillar noises aren’t just disturbing, but are disturbing because they mimic the “alarm calls” that birds give when predators like hawks are around. The birds react as if they hear the alarm call, leaving the insect alone not because they’re simply startled, but because they need to find cover fast. Here’s part of that abstract; the paper has yet to be published:
This will undoubtedly be published soon as a full paper, but in the meantime it’s a cool suggestion. I’ve put below what I think is a “seet” call of a chickadee, which resembles the call of the caterpillar above. I may be wrong about the “seet”, so birder/readers who have better examples are welcome to post them below:
J. Lindsay and E. Greene. Whistling caterpillars mimic the alarm calls of birds. Presented June 12, 2015, at the 52nd annual conference of the Animal Behavior Society, Anchorage, Alaska.
V. Bura et al. Whistling in caterpillars (Amorpha juglandis, Bombycoidea): Sound-producing mechanism and function. Journal of Experimental Biology. Vol. 214, January 1, 2011, p. 30. doi:10.1242/jeb.046805.