Hanging by a thread: Post-attack defense of caterpillars

Abstract

Caterpillars use a diverse range of anti-predator defenses, including camouflage, making and hiding in shelters, mimicry, regurgitating, rolling, and biting. Some caterpillars also drop to the ground and hang themselves by a silk thread. This hanging behavior has been described for a long time but has surprisingly been overlooked in entomological research. In this study, we aimed to identify (1) the taxonomic distribution of the species showing the hanging behavior and (2) the type of sensory stimulus that induces the hanging behavior. We first located caterpillars in the forest and stimulated each caterpillar with three different types of sensory stimuli sequentially: visual approaching, wind-blowing, and poking. For those who responded to none of the stimuli, we further delivered harder tactile stimuli (pinching and detaching from the plants by grabbing) and observed whether they performed the hanging behavior. Among the surveyed 88 species, 46 species (comprising 11 different families) were confirmed to perform the hanging behavior. Most species responded to neither visual nor wind stimulus, but about half of the tested individuals responded to one of the tactile stimuli. Our results suggest that hanging by a silk thread is widespread across a range of lepidopteran groups, and they use this behavior as a post-attack defense.