The morphology of adults, deutonymphs and larvae of water mites of the superfamily Hydrovolzioidea is studied, taking as exemple the species Hydrovolzia cancellata Walter, 1906 and Acherontacarus rutilans E. Angelier, 1951. All active stages, especially larvae, show strong differences from each other in many important character states. As a consequence, a new diagnosis of the superfamily Hydrovolzioidea is given, and the former subfamily Acherontacarinae elevated to family level. A diagnosis for all active instars of the families Hydrovolzioidae and Acherontacaridae nov. stat. is proposed. New data on larval behaviour, host range, coevolution and zoogeography are provided.