A Hyposmocoma caterpillar can breathe oxygen from the water directly through the skin of its abdomen. (Patrick Schmitz, Rubinoff Lab)

Researchers in Hawaii have found several species of moth whose caterpillars can live both under water and on land, making them unique among insects.

Biologists Daniel Rubinoff and Patrick Schmitz of the University of Hawaii observed the caterpillars of 12 species of moth living equally well under water and on dry rocks away from streams. All the species are members of the genus Hyposmocoma, a type of moth unique to Hawaii.

These moths are the only known insects that can survive one of its life stages — in this case, the larval stage — above or below the water's surface.

The moths live around rainforest streams in the mountains of the Hawaiian Islands.

"The adult moths don't swim, but fly between the emergent rocks in the stream, courting and mating. They lay their eggs above the waterline on these same rocks," said Rubinoff, in an email.

When the larvae emerge from the eggs, they can survive either in the stream or on dry rocks, living on dried algae and lichens.

"The caterpillars … can cruise in and out of the water at will for unlimited periods of time," said Rubinoff.

The caterpillars can even pupate under water, spinning a silk casing around themselves. In the 12 amphibious species, the researchers found three different case shapes, representing different lineages: burrito, bugle and cone.

"Dissection of the cases while under water reveals no air bubble to mediate gas exchange, and the larvae possess no gills or plastron, common structures for underwater respiration in other insects," wrote the authors of the study, published this week in the Proceedings of the National Academy of Sciences.

The researchers surmised that the caterpillars must breathe through direct exchange of oxygen with the water through the skin of their abdomens.

The caterpillars can live only in fast-moving stream water that's well-oxygenated and die quickly in stagnant water, which supports the direct exchange hypothesis, the researchers said.

A DNA analysis of the moth species found that the amphibious nature of the caterpillars might have evolved three separate times during the history of the Hyposmocoma genus.

"The three aquatic groups we present are not very closely related to each other either, which is part of what is so remarkable about their evolution," said Rubinoff.

Hyposmocoma has over 400 different species, and the amphibious species aren't the only unusual ones.

In 2005, Rubinoff's group found a Hyposmocoma moth whose caterpillars use their silk threads to capture and eat snails, the only known caterpillars to eat mollusks.