Urticating hairs are possessed by some arachnids (specifically tarantulas) and insects (most notably larvae of some butterflies and moths). The hairs have barbs which cause the hair to work its way into the skin of a vertebrate. They are therefore an effective defence against predation by mammals.
One of the more well known moth larvae that possess urticating hairs are those of the Pine Processionary moth (Thaumetopoea pityocampa). The Pine Processionary moth are a major pest in pine forests/plantations and the caterpillars form silken nests within the tree. The caterpillars form characteristic processions with each caterpillar following the previous one (head to tail) from a tree to an area where they burrow underground and pupate. Although they use pheromone trails the caterpillars rely on touching the hairs on the rear of the preceding caterpillar to form the procession.
Like most tarantulas, the Mexican Red Kneed Tarantula (Brachypelma smithii), possesses urticating hairs.
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