BioEd Online

Researchers have found a tiny caterpillar in the rainforests of Maui, Hawaii, that weaves silk to tie up sleeping snails. Most caterpillars feed on plants, but Hyposmocoma molluscivora feasts on the flesh of living molluscs.

The caterpillars, which grow up to be moths, perform this trick by camouflaging themselves in a purpose-built silk casing stippled with foliage and lichen. When it encounters a resting snail (Tornatellides), the caterpillar immobilizes it by binding it to leaves with silk threads comparable to those in a stiff spider web.

"The larvae stretches from its case into the shell and pursues the snail until it is trapped," says Daniel Rubinoff, whose paper is published this week in Science¹. "It then eats the snail alive in its own house."

They don't do this just to add spice to their diet, adds Rubinoff, who is an entomologist at the University of Hawaii at Manoa in Honolulu. They have an inflexible digestive system that is unable to process other food types, a common trait in caterpillars.

The caterpillar spends around 25 minutes weaving the silk trap, and takes all day to devour its meal.

Silken bonds

Out of more than 150,000 types of moths and butterflies of the order Lepidoptera in the world, about 200 are predatory, and all of these are silk spinners.

"The real zinger here is that this is the only one that uses silk to capture prey," says Rubinoff. The others use their silk for more benign purposes, such as spinning coccoons.

So far H. molluscivora have only been found in Hawaii. But on that isolated island archipelago they are quite widespread: researchers have found predatory caterpillars capturing and consuming snails on four of the five islands.

The discovery provides evidence of the importance of isolation in the evolution of novel traits. "A system like Hawaii, which doesn't have any social insects such as ants or wasps, is less diverse. But it is also more tolerant of evolutionary experiments or pathways," says Rubinoff.

It also emphasizes the need to preserve unique ecosystems such as the Hawaiian rainforests, say the researchers.