Ditching its evolutionary cousins' customary fare of leaves for an Atkins diet, a newly discovered species of caterpillar in Hawai'i traps snails with its silk — like a spider — and then eats them alive.

They're the first known snail-hunting caterpillars, and part of a poorly understood group of carnivorous critters that smashes the caterpillar's pacifist image.

The researchers, who report the species in today's issue of the journal Science, said all of these odd varieties come from the Hawaiian Islands — an isolated evolutionary laboratory known to yield novelties found nowhere else on Earth.

Feasting on snail flesh brings some distinct advantages, experts said. Rather than munch on leaves all day, the caterpillars get ready-made meals by snaring a resting snail.

"If I had a choice between escargot or leaves, I would pick escargot," said Steven L. Montgomery, a conservation biologist who discovered other insect-eating caterpillars while a student at the University of Hawai'i in the 1970s.

Like soldiers bearing grisly tokens of their conquest, the snail-stalking caterpillars sometimes attach hollowed-out snail shells to their tough outside casings and drag them from meal to meal.

Study leader Daniel Rubinoff of UH's College of Tropical Agriculture and Human Resources said the shells seem to be a form of camouflage — not a "trophy."

The snails they studied were on Maui, but Rubinoff said others have since been found on Moloka'i, Kaua'i and the Big Island.

The caterpillars appeared at first to be scavengers. But in Rubinoff's laboratory the caterpillars refused to eat leaves or insects and began to starve. Acting on a hunch, the researchers threw in a snail — and the test caterpillar went right after it.

"First time that happened, I thought it was a fluke; we got a wacko caterpillar," said Rubinoff, a professor in the school's Plant and Environmental Protection Sciences. "But then we found several caterpillars who did the same thing."

The rainfall in Hawai'i promotes the growth of mold and fungi, providing a buffet for snails — and an opportunity for caterpillars. At some point in their evolutionary path, these caterpillars had discovered that snail protein is a richer source of nutrition than leaves.

Measuring less than half an inch and as slow as — well, a snail — the caterpillar must wait for a snail to come to rest before it strikes.

Shrouded in a silk casing decorated with lichen and snail shells, the caterpillar inches toward its resting prey, never wasting time on dead snails. It must be sure not to disturb the snail, which will clamp its vulnerable opening to the leaf or drop to the ground at any sign of danger.

Slowly and gently, the caterpillar spins a sticky sheet of silk that binds the snail to the leaf — a spiderlike tactic unique among caterpillars. After 30 to 60 minutes of silk-weaving, there is no escape for the snail.

The caterpillar extends its head and legs from its case and chases the snail — very slowly — into the inner recesses of the shell until there's nowhere to run, and the cornered snail is eaten alive.

After its larval stage, the caterpillar becomes a moth. Rubinoff and his team named the creature Hyposmocoma molluscivora in honor of its status as the only known caterpillar that eats mollusks.

Of the more than 150,000 species of moths and butterflies worldwide, only about 200 species are believed to be predators while they're caterpillars; butterflies and moths eat little or no food during their brief adult lives. Other caterpillars also make silk, but they use it to make cocoons or to move from one branch to another.

Biologists may have come close to discovering the new snail slayer more than 20 years ago. In 1984, Montgomery said, he came across a snail tied to a fern by silk. Today he realizes it was likely the calling card of the snail-hunting molluscivora. But at the time he dismissed the sighting.

"I thought it was a schizophrenic caterpillar that wanted to get rid of some silk," Montgomery said.

The Associated Press contributed to this report.