Adolescence is a tough time for anyone, but what if, on top of your growing pains, you had to learn how to breathe underwater? Hawaiian caterpillars take it all in stride. Researchers have discovered 12 species of Hawaiian moths whose caterpillars are equally comfortable submerged in a stream or beached on a bone-dry strip of land. The feat makes them unique among insects, and maybe even among animals, the researchers say.
Many animals lead double lives, spending time in water and on land. But these versatile creatures have limits. For instance, many toads live mainly on land but have to return to the water to lay eggs. And although some aquatic insects can hibernate to withstand periods of drought, they're not really amphibious—they have to put their lives on hold to get through a dramatic change in circumstance.
So when Daniel Rubinoff, an entomologist at the University of Hawaii, Honolulu, first saw a caterpillar that seemed to have no preference at all for water or land, he couldn't believe it. Rubinoff studies Hyposmocoma, a moth genus found exclusively in Hawaii. The 400 species of Hyposmocoma share a common trait—the larva, or caterpillar, spins a case for itself out of silk and whatever materials, such as algae or tiny pebbles, are handy. Entomologists group the species by the shape of this case. The species with burrito-, bugle-, and cone-shaped cases, favor volcanic rocks near streams. But on several specimen-gathering expeditions, Rubinoff saw species he had thought were terrestrial living in the middle of streams.
This alone would have been remarkable—only 0.5% of caterpillars can breathe underwater—but since these creatures had been originally seen on land, they couldn't be simply another aqueous species. So Rubinoff brought his catch back to the lab. He submerged the caterpillars in tanks for weeks at a time; when the insects flourished, he stranded them in petri dishes with only a bit of carrot and no water. The caterpillars seemed equally at ease in both situations. Whether they're under water or without a drop of moisture for the duration of their adolescence, "these guys don't care," says Rubinoff.
Other aquatic caterpillars have gills, which don't function out of water, but these creatures seem to breathe through their skin. The streams' swift flow brings in plenty of oxygen, and the caterpillars anchor themselves to submerged rocks with silk lines.
These amphibious caterpillars are unique among insects, Rubinoff and colleague Patrick Schmitz report online today in the Proceedings of the National Academy of Sciences. But Rubinoff thinks the underwater mountaineers might be unique among all animals, even amphibians. "No other animal that breathes air can handle being submerged for a month," he says.
But it might simply be the case that such versatility hasn’t been documented, says Christopher Raxworthy, a herpetologist at the American Museum of Natural History in New York City. "It's unusual, but I could imagine a [similar] situation" with amphibians.