Amphicrossus japonicus is the first known facultatively aquatic nitidulid. The adult beetles breed in bamboo sap and subsequently enter water-filled bamboo culms. In water they breathe via a ventral air sheath held by hydrofuge pubescence. The beetles are facultative predators and hunt mosquito larvae, which they grab with their forelegs. The trend to facultative predation in Cucujoidea and the transition for beetles in general from semi-liquid decaying organic matter into water is discussed.