Hydrachnidia Krantz, 1978 - water mites
   Superfamily Hygrobatoidea Koch, 1842
     Family Pionidae Thor, 1900
       1 Subfamily Foreliinae Thor, 1923
       2 Subfamily Huitfeldtiinae Viets, 1924
       3 Subfamily Hydrochoreutinae Viets, 1942 [monotypic]
       4 Subfamily Najadicolinae Viets, 1935 [monotypic]
       5 Subfamily Pioninae Thor, 1900
       6 Subfamily Schminkeinae Schwoerbel, 1984 [monotypic]
       7 Subfamily Tiphyinae Oudemans, 1941