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Flesh-Eating Caterpillars Discovered in Hawaii

John Roach
National Geographic News
July 21, 2005
 
In Hawaiian rain forests, scientists have discovered caterpillars with a taste for escargot: They trap snails on leaves using silk webbing and then eat them alive.

These are the first caterpillars known to eat snails or mollusks of any kind, an evolutionary adaptation likely enabled by the island chain's isolation. The insects are also the first caterpillars known to use silk to ensnare prey in a spiderlike fashion.

"It's really remarkable—even within a diverse genus it's remarkable," said Daniel Rubinoff, an entomologist at the University of Hawaii at Manoa, who led the discovery team.

The caterpillars belong to the genus Hyposmocoma, a large and diverse group of moths and caterpillars that have adapted to nearly every ecological niche on the Hawaiian Islands, Rubinoff said.

Of the more than 350 known species that make up Hyposmocoma, most have plant-based diets. But researchers have discovered four species of caterpillars that eat snails to date.

To attack their shelled prey, the flesh-eating caterpillars wait for snails to come to rest. After poking them to make sure they're still alive, the caterpillars then use silk to bind the snails down.

Next, the caterpillars, which are covered in their own silk casings, emerge from their casings to reach into the snails' shells and begin eating the trapped snails alive.

Some satiated caterpillars then attach the empty snail shells onto their speckled casings. This likely serves as camouflage.

Rubinoff and graduate student William Haines describe the discovery of one of the four snail-eating caterpillar species in tomorrow's issue of the journal Science.

Why Hawaii?

Rosemary Gillespie, an insect biologist at the University of California at Berkeley, said the discovery fits the image of Hawaii as a place full of unusual evolutionary surprises.

"It's so isolated. Prior to human contact very few organisms got there, and the ones that did get there found an open palate upon which to diversify," she said.

Most Hawaiian organisms exploit niches like those that similar organisms use on the mainland, Gillespie said. "But you also get oddballs showing up that evolve to do something you don't expect them to do at all."

In addition to the newly discovered flesh-eating caterpillars, the Hawaiian Islands are home to caterpillars and spiders that snatch flies out of midair and devour them.

Rubinoff was skeptical when colleagues first reported the possibility of snail-eating caterpillars on Maui. Of the approximately 150,000 described species of moths and butterflies, only about 200 species are predators, and all of those eat other insects.

He thought his colleagues had discovered a quirky individual but decided to take a look.

Rubinoff collected some caterpillars from the site and tried to feed them lichens, carrots, and other common Hyposmocoma caterpillar foods. They ate nothing. Then he fed them a snail, and a caterpillar quickly went to work, binding it to the glass of the laboratory jar.

Since then, Rubinoff and his lab have confirmed the existence of flesh-eating caterpillars on the islands of Molokai, Kauai, and the Big Island (Hawai'i). The caterpillars are all found in rain forest habitat and dine exclusively on snails.

"Not only is it not a fluke, but it is a radiation of snail-eating caterpillars," he said.

Habitat Conservation

The question Rubinoff now ponders is why this feeding strategy evolved in Hawaii and apparently nowhere else in the world.

"Maybe because of Hawaii's isolation, it spawns evolutionary experiments that don't seem to occur anywhere else on the planet," he said.

And perhaps the "incredibly ecologically diverse genus" of Hyposmocoma is the pre-requisite that allowed for the evolutionary experimentation, he added.

The flesh-eating caterpillars are found in Hawaii's rain forests, which are becoming increasingly rare. The discovery of the insects serves as "another reason to save what's left of the native Hawaii biota," Rubinoff said.

Saving and studying these unique insects and their habitat may eventually shed additional light on how evolution operates, he noted.

Gillespie, the Berkeley researcher, said researchers first need to collect more information on where these flesh-eaters fit in the huge and varied genus of Hyposmocoma.

"Then we can get insight into how evolution works," she said.

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