Caterpillar Spins a Web to Feast on Snails

Researchers in Hawaii have discovered a bizarre caterpillar that captures its prey — usually snails — in a web much like a spider's. Then the caterpillar feasts on escargot.

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Hawaii is home to some of the most unusual animals in the world, such as the endangered monk seal and the scarlet honeycreeper. That's a bird with a curved bill for sipping nectar from flowers. Well, now there is a new species to add to Hawaii's menagerie, one with a uniquely lethal technique for devouring its prey. NPR's Christopher Joyce reports.

CHRISTOPHER JOYCE reporting:

First off, you should know that the vast majority of caterpillars quietly graze on plants, not animals. But in Hawaii, odd things happen. Isolated from the rest of the world, many animals there evolved in bizarre ways. Biologists in Hawaii know that. Nonetheless they were puzzled by something they'd seen on the island of Maui, empty snail shells wrapped in what looked like silk webbing. And nearby, they found small caterpillars living inside their own silk casings.

A student brought some caterpillars to insect biologist Daniel Rubinoff at the University of Hawaii. He wondered whether this particular species had developed an escargot habit. Rubinoff was skeptical.

Mr. DANIEL RUBINOFF (Insect Biologist, University of Hawaii): Most of them will eat, say, carrots, algae, lichen or fish food. And I tried these guys on it, and they were not making it. They were dying. They were starving. They wouldn't eat those things at all. And so I thought, `Wow, maybe those guys weren't kidding on Maui. Maybe they do eat snails.' But it seemed ridiculous.

JOYCE: So Rubinoff said, `What the heck. Let's put snails in with the caterpillars and see what happens.' What ensued was an insect version of "Gulliver's Travels." That's where the shipwrecked and sleeping Gulliver is tied up by hundreds of tiny Lilliputians. Unlike the Lilliputians, however, the caterpillars were hungry.

Mr. RUBINOFF: They won't attack snails that are moving. They're waiting for them to sort of settle down and rest, and then they'll come up. And once they've, I think, decided there's a snail there, they start to vigorously spin silk, binding the snail shell to the leaves. And once it's, I guess, satisfied that the snail is really sucked in there, it crawls around to the lip of the shell, and then extends its body out of its case into the snail shell and consumes the snail, basically eating the snail alive in its own home.

JOYCE: Rubinoff says at first, he couldn't believe it. He took hundreds of photos to verify this. Not only did they eat snails, the caterpillars sometimes kept empty shells attached to their own casing, like trophies. Rubinoff prefers to think of this as camouflage, however. The snail-eating caterpillar, a new species in the genus hyposmocoma, moves with a rough crowd. There's an ambush caterpillar in Hawaii that grabs flies as they crawl by, and a spider that impales flying insects on its spearlike legs. But Rubinoff says this is the only caterpillar that spins a silken web to subdue live prey.

Writing in the journal Science, Rubinoff suggests that in Hawaii, there may have been few other animals that ate these snails as well as a mix of caterpillars that drove one species to try snails. No doubt, it was a brave caterpillar that first ate one. After that, it was clear snailing. Christopher Joyce, NPR News.

MELISSA BLOCK (Host): This is NPR, National Public Radio.

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