It is well documented that the lepidopterans (moths and butterflies) can produce acoustic signals. Moths and butterflies have tympanic ears which they use primarily to detect the ultrasonic emissions of echolocating bats; they also produce sounds to startle and confuse bats as they hunt for prey.
It has been known for over 100 years that caterpillars also produce acoustic signals. Several examples of this have been investigated. For example, larvae of the Australian common Imperial blue butterfly produce vibrational signals to communicate with ant symbionts, and some species of leaf miner and owlet moths can detect the sounds and vibrations made by predators and parasites, but, in general, the significance of acoustics in caterpillars remains unknown.
Now, a study led by Jayne Yack, a neuroethologist at Carlton University in Ottowa, shows that the silkmoth caterpillar (Antheraea polyfemus) produces clicking noises to deter predators. This clicking precedes the regurgitation of a repellent dark brown liquid, and acts to warn a predator of the imminent deployment of the caterpillar’s chemical defences. Of twelve other moth species examined, two more – the tobacco hornworm and luna moth larvae – were also found to do the same. The findings are reported in the current issue of the Journal of Experimental Biology.
Yack and her colleagues simulated predatory attacks on the caterpillars by pinching their head capsules with forceps, and filmed the defensive behaviours of the larvae. It was found that the larvae produced the clicking noises when their heads were pinched, and also when the researchers blew on them or rattled the enclosures in which they were kept. It was also found that the likelihood of the caterpillars producing the clicking sounds and regurgitating the repellent fluid was directly related to the severity of the attack.
The caterpillars produced trains of clicks lasting over 60 seconds. These are audible to humans; each click lasts approximately 25 milliseconds, and the entire train consists of 50-55 clicks. The clicks constitute a broadband signal which can be perceived by a diverse range of the caterpilllar’s natural predators, whose hearing ranges differ widely. Earlier studies suggested that caterpillars produce such sounds by opening and closing their mandibles. Using electron microscopy, Yack’s team confirmed this. They found that each of the silkmoth caterpillar’s mandibles has several tooth-like ridges; these serrated edges produce the clicking sound when thy come into contact with each other.
Electron micrograph of the serrated mandibles of the silkmoth caterpillar Antheraea polyfemus. (Scale bar = 250 µm; from Brown, et al, 2007.)
The regurgitated liquid was found to be repellent to both invertebrates (ants) and vertebrates (mice), but its composition is as yet unknown. Energy-wise, the fluid is costly to produce; the caterpillar recycles it sucking up any fluid that remains near its mouth after an attack. And, although the clicks are relatively loud (60-80 dB), they can only be heard in the caterpillar’s immediate vicinity. Thus, the caterpillar’s first line of defence is camouflage – it remains inconspicuous because of its cryptic colouration. But, if discovered by a predator, it clicks and regurgitates the liquid as a last resort. Yack would like to find out whether or not the clicks alone are sufficient to deter a predator, and to determine the composition of the regurgitant.
This film clip show a silkmoth larva emitting a train of clicks during a vicious attack from a chicken; a trail of regurgitant can be seen on the surface on which the encounter was staged:
Brown, S. G., et al. (2007). Clicking caterpillars: acoustic aposematism in Antheraea polyfemus and other Bombycoidea. J. Exp. Biol. 210: 993-1005. [Abstract]
Cocroft, & Rodriguez, (2005). The behavioral ecology of insect vibrational communication. BioScience 55: 323-334. [Full text]