Many insects have larvae that are aquatic but an adult that is terrestrial. Until early in the 20th Century, it was widely assumed that no adult insect could live under water for sustained periods. The French Biologist, F. Brocher, changed these beliefs with his observations of Elmis aenea that were reported in his 1911 paper, Recherches sur la respiration des insectes aquatiques adultes. Brocher observed beetles living underwater for extended periods and noted a layer of air held close to its body by hydrofuge hairs on it cuticle or plastron. Brocher suggested that the plastron was involved in respiration. His observation stimulated further research into plastrons as breathing mechanisms in adult insects.
Elmis aenea is a riffle beetle in the family Elmidae. These beetles are common in fast running water that is well aerated and near lake shores where wave action aerates the water.