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Amphibious Caterpillars Discovered in Hawaii

Scientists at the University of Hawaii have discovered the first-ever species of insect that are able to survive an entire life stage spent both above and below the water’s surface. In mountain streams across the islands of Hawaii, the researchers observed the larvae (caterpillars) of the moth genus Hyposmocoma feeding and breathing both underwater and away from streams on dry rocks. The scientists said that the caterpillars can breathe and feed indefinitely and equally well both above and below the water’s surface, and can mature either submerged or completely dry. The amphibious caterpillars possess no gills or plastron, common structures for underwater respiration in other insects. When submerged, the caterpillars likely rely on the direct diffusion of oxygen through the hydrophilic skin along their abdomens, the researchers said. Perhaps as a result of their need for direct diffusion, the caterpillars occur only in fast-flowing, well-oxygenated streams, the authors wrote, and quickly die in stagnant water. Genetic analysis of DNA from 89 species of Hyposmocoma indicated that the amphibious lifestyle is an example of parallel evolution; the analysis showed evidence of at least three independent invasions of the water by strictly terrestrial clades (evolutionary groups including a single ancestor and all its descendants), beginning more than six million years ago, before the current “high islands” existed (note: high islands are of volcanic origin and are distinguished from “low islands,” which are formed by sedimentation or uplifting of coral reefs). The authors noted that why and how Hyposmocoma, an overwhelmingly terrestrial group, repeatedly evolved unprecedented aquatic species is unclear, although there are many other evolutionary anomalies across the Hawaiian archipelago. How and why certain species of Hyposmocoma have overcome the physiological limitations of breathing directly from both water and air will be the focus of future research, the researchers said. Interestingly, all caterpillars in the genus Hyposmocoma spin silk cases embedded with minute objects from their environment (pebbles, diatoms, algae, and lichens) to protect and camouflage their bodies, serving as essential shelter both in and out of the water. The caterpillars quickly perish when removed from their cases. The article on amphibious caterpillars was published online on March 22, 2010 in PNAS. [PNAS abstract]