Caterpillars with a taste for the flesh of snails
Biologist finds new species in Hawaii

Hawaiian snails, beware! A flesh-eating caterpillar is on the loose, and your shells betray you -- get off those leaves!

Daniel Rubinoff, a former UC Berkeley biologist and now director of the University of Hawaii's Insect Museum, has discovered a unique new species of caterpillar that traps the mollusks with silken strands from its own body and devours its helpless prey alive.

The caterpillars are the larvae of a widespread tribe of common Hawaiian moths and "represent a successful evolutionary experiment" that enables them to thrive very much as spiders do, Rubinoff says.

In this week's issue of the journal Science, and in a telephone interview, Rubinoff described the species he and his graduate student, William P. Haines, have discovered, and how it was found.

In a rain forest on the island of Maui, Rubinoff said, three local naturalists recently brought him some of the caterpillars. Festooned across the tops of the silken casings they spin to create their cylindrical homes were several small, empty snail shells.

The snails normally ooze along the leaves of trees in the forest, and the caterpillars snake long the same leaves -- but Rubinoff and Haines soon learned that the caterpillars refuse to eat the leaves in captivity and starve if that's all they're given to eat.

But when they come upon a slow-moving snail on a leaf in the laboratory, the caterpillars move in quickly, wrapping their silk threads around and around the shell until the mollusk is bound fast to its leaf and can't escape.

Then, at leisure, the half-inch-long caterpillar wedges its casing next to the snail's tightly bound shell, stretches out much of its body and pursues the snail as it tries to escape danger inside its shell, "from which there is no retreat," Rubinoff says.

The snail is eaten alive bit by bit, and the meal makes a leisurely full one for the tiny insect.

After dining on smaller snails, Rubinoff has observed, the insect will attach an empty shell to its casing. By decorating itself with several shells, the caterpillars have probably devised an effective camouflage against predatory beetles and ants that in turn would otherwise enjoy a caterpillar meal, he says.

Since identifying the Maui caterpillar species, Rubinoff says, he has found the same species on Molokai, on the Big Island of Hawaii among the rocks of fresh lava flows, and in forests ranging from sea level to 10,000 feet high on the slopes of Mauna Kea.

The caterpillars are members of the moth genus Hyposmocoma, which is unique to the Hawaiian islands and numbers about 350 species. Many feed on rotting wood, graze on lichens, or eat a wide variety of plants, but only Rubinoff's insect eats snails.

Rubinoff has named his new-found species Hyposmocoma molluscivora.

Caterpillars are the larvae of moths and butterflies, and Rubinoff's is the only one in the world known to eat mollusks of any kind or to use its silk to capture its prey, he says. The snail is a member of the more widespread genus called Tornatellides, which is found throughout the Pacific Basin, including the Galapagos Islands.

As he notes, the Hawaiian islands form the world's most remote land mass, and because of their isolation, many other animals on the islands have evolved unique strategies for feeding. There are other predatory caterpillars that ambush their prey, spiders that seize smaller insects in midflight, and damselflies that live on land rather than on water.

"Almost all insects are predators," Rubinoff says, "but to find a caterpillar going after a snail is a real shock. It's like finding a wolf diving for clams."