Meat-Eating Caterpillar: It hunts snails and ties them down

A newly named species of Hawaiian caterpillar sneaks up on a resting snail and quickly spins silk strands around it, lashing it to the spot. The caterpillar then reaches into the snail shell’s opening and has lunch.

SLOW FOOD. Hawaiian caterpillar (arrow) pokes out of its mottled gray case to wrap silk around a snail. Science

These larvae of a small moth, newly named Hyposmocoma molluscivora, are the first mollusk-eating caterpillars that scientists have officially described, says Daniel Rubinoff of the University of Hawaii at Manoa. A few other snail eaters have been spotted in Hawaii but not yet studied.

Only 1 percent of the world’s moth and butterfly caterpillars must consume meat, and most of them hunt soft-bodied insects called scales, say Rubinoff and William P. Haines, also of Manoa, in the July 22 Science.

“The snail eating is pretty extraordinary,” says Rosemary Gillespie of the University of California, Berkeley, who studies the evolution of Hawaiian arthropods.

A few longtime observers of island moths told Rubinoff that they’d seen caterpillars “messing around snail shells,” he says. They suspected that the caterpillars were hunting snails. But Rubinoff admits, “With all due respect to the folks who turned out to be right, I thought, ‘Oh sure.'”

He had been studying the Hyposmocoma genus, which is so diverse that it includes species that dive underwater. Caterpillars in this Hawaiian genus drag around with most of their bodies inside cases that Rubinoff compares to miniature toilet paper rolls. The caterpillars attach bits of orange and green lichen, and even small snail shells, to the outside of their cases.

Rubinoff first encountered snail hunting not in the Hawaiian forests but in his lab. He was trying to feed a batch of unfamiliar case bearers collected in Maui. “For most [caterpillars], you throw in some fish food and carrots, and they’re happy,” he says. But these strangers shunned his whole smorgasbord.

“Finally, we threw in a couple of snails, and, bam, they were eating,” he says.

Rubinoff says that he’s never seen attacks on a moving snail. Instead, a caterpillar sidles up to a sitting snail, pokes around gently as if confirming its target, and then starts an elaborate tie-down that takes about an hour.

At least four other Hyposmocoma species of snail-capturing caterpillars live in the Hawaiian Islands, Rubinoff says. A possible fifth species was collected high in rainforests on the island of Kauai by Steven Montgomery, a consulting conservation biologist in Waipahu on Oahu.

James Costa of Western Carolina University in Cullowhee, N.C., says that a few other insects, such as certain ground beetles, specialize in attacking snails. “It’s incredibly unusual for a caterpillar,” he says.

Such oddball lifestyles tend to develop in isolated ecosystems where there’s a limited variety of creatures, as in Hawaii. “There’s more room for evolutionary experimentation,” says Gillespie. For example, Hawaiian insects evolved without ants as predators or competitors for food.

Ants and other aliens are moving in now, though, and Montgomery warns that native caterpillars and snails may not survive the ecosystem upset.

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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