The Collaborative International Dictionary
Hydrophilus piceus
Water devil \Wa"ter dev"il\ (Zo["o]l.) The rapacious larva of a large water beetle ( Hydrophilus piceus), and of other similar species. See Illust. of Water beetle.
Wikipedia
Hydrophilus piceus
Hydrophilus piceus is a species of beetle in the family Hydrophilidae, the water scavenger beetles. It is known by the common name great silver water beetle.