Photograph by Patrick Schmitz

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While underwater, a new species of amphibious caterpillar from Hawaii peeks out of its cocoon-like casing.

Photograph by Patrick Schmitz

First Amphibious Insects Found in Hawaii

But no one knows how the bugs breathe underwater.

Several new caterpillar species are equally at home on land or underwater, making them the first truly amphibious insects, scientists say.

The amphibious caterpillars—found only in Hawaii's fast-moving freshwater streams—belong to the moth genus Hyposmocoma, a group that includes more than 400 species.

The 14 newfound species are never seen far from water. But unlike purely aquatic caterpillars, these species can behave the same in water or on land for indefinite periods of time.

"When you put these guys in water, they run around and eat. You take them out, and they're perfectly fine too," said study co-author Daniel Rubinoff of the University of Hawaii.

"No other insect that we're aware of can do that. Actually, no other animal that I'm aware of can do that."

Some beetles can enter a dormant state to survive underwater for a while, and aquatic animals such as lungfish can live briefly on land by constructing mucus-lined cocoons. But these creatures are by no means "functioning normally" when out of their preferred element, Rubinoff said.

Amphibious Caterpillars Die in Still Waters

Like other Hyposmocoma species, the amphibious caterpillars spend their lives inside cocoon-like cases made of hardened silk, occassionally poking out their heads to feed or navigate.

Rubinoff and colleagues initially thought the cases functioned like scuba tanks, storing air for the caterpillars. (Related: "Water Spider Spins Its Own 'Scuba Tank.'")

But "when we dissected the cases underwater, there were no air bubbles," Rubinoff said.

It's still unclear how the caterpillars breathe underwater.

"It may be a specialized organ that we haven't found, or it may be that their skin is thinner than terrestrial [caterpillars'], which permits them to breathe directly through their skin," Rubinoff said.

The thin-skin theory could explain why the caterpillars are found only in fast-flowing streams, where the water is well oxygenated.

"If you put them in an aquarium, you need to put a bubbler in," Rubinoff said. "Take the bubbler out, and you're going to end up with stinking, rotting caterpillars in the water."

When it's time to turn into moths, the caterpillars seal themselves inside their silken homes while floating at or near the surface of the water. When the moths emerge, it's only a short trip to the surface and then to the sky.

"The moths, to our knowledge, are not swimmers," Rubinoff said.

Diverted Streams Harming Amphibious Caterpillars

Despite their flexibility in living conditions, the amphibious caterpillars are already endangered because of human activities, the researchers note.

For instance, many streams in Hawaii are being rerouted down cement causeways to irrigate sugarcane fields or for other uses, Rubinoff said.

When this happens, the amphibious caterpillars disappear.

"It's tragic, because these caterpillars are doing some pretty spectacular things that don't occur anywhere else in the world," Rubinoff said.

"They need some kind of conservation attention but have received none."

The research is detailed online this week in the journal Proceedings of the National Academy of Sciences.