Wormholes, sensory nets and hypertrophied tactile setae: the extraordinary defence strategies of Brenthia caterpillars

Metalmark moth larvae in the genus Brenthia (Lepidoptera: Choreutidae) live beneath gauzy web shelters spun over a leaf surface and chew an escape hole into the floor of their nests through which they dart when alarmed. We provide a video and data showing that these larvae, when their webs are disturbed, shoot through their wormhole to the opposite side of the leaf at extremely high speeds: the fastest caterpillar passed through to the opposite leaf surface in only 100 ms. Experimental data show that Brenthia larvae use their silken web shelters as an extension of their sensory system. Although such usage of webbing is not surprising given what is known about spider webs, this is the first time such a behaviour has been shown to occur in Lepidoptera. Two grossly hypertrophied abdominal setae, directed behind the body, are not pulled through the wormhole, but instead remain intercalated into the silken lattice of the web shelter. By leaving the two setae engaged, the caterpillar is able to monitor both sides of its host leaf simultaneously. We report analogous escape hole behaviours in four other microlepidopteran families: Elachistidae, Gelechiidae, Oecophoridae and Pyralidae. In at least one genus of the latter family (Monoloxis), two caudally directed abdominal setae are hypertrophied in the same fashion as in Brenthia. We suspect that both escape holes and hypertrophied abdominal setae are more common and taxonomically widespread than presently recognized and that both ‘strategies’ will be shown to occur among other microlepidopterans.