Acoustic Aposematism (Clicking) by Caterpillars

Adult Lepidoptera are well known to perceive sound, such as the ultrasonic cries of insectivorous bats. Some even produce sounds that are used for social communication. Less well known is the sound production and reception of larvae.

Some caterpillars employ vibrational signals with ants (e.g., Lycaenidae and Riodinidae), communicate about space with conspecifics (e.g., Gracillariidae), or detect insect predators or parasitoids (e.g., Noctuoidea and Gracillariidae). However, “clicking” sounds are an audible sound produced by caterpillars of silk moth (Saturniidae) and hawk moth (Sphingidae).

This noise has also been described as “squeaking” or “crackling,” and originates with the mandibles. Defensive sounds are usually categorized as startle or warning behaviors; startle sounds warn a potential predator, causing momentary hesitation and escape of the potential prey, whereas warning sounds alert a potential predator that it is inadvisable to attack. Associated with the clicking sound is regurgitation behavior, and both actions follow disturbance of the larva.

Regurgitant usually is adverse to predators, and is a widespread defensive behavior among insects. Thus, clicking is thought to warn potential predators of an unpleasant experience if predation is attempted, but it is also possible that clicks function as startle sounds, allowing escape. This latter explanation seems unlikely, however, as caterpillars usually move very slowly, so escape is not very likely.