Hydrachnidia Krantz, 1978 - water mites
   Superfamily Arrenuroidea Thor, 1900
     01 Family Acalyptonotidae Walter, 1911 [monotypic]
     02 Family Amoenacaridae Smith & Cook, 1997 [monotypic]
     03 Family Arenohydracaridae Cook, 1974 [monotypic]
     04 Family Arrenuridae Thor, 1900
     05 Family Athienemanniidae Viets, 1922
     06 Family Bogatiidae Motaş & Tanasachi, 1948
     07 Family Chappuisididae Motaş & Tanasachi, 1946
     08 Family Harpagopalpidae Viets, 1924 [monotypic]
     09 Family Hungarohydracaridae Motaş & Tanasachi, 1959
     10 Family Kantacaridae Imamura, 1959 [monotypic]
     11 Family Krendowskiidae Viets, 1926
     12 Family Laversiidae Cook, 1955 [monotypic]
     13 Family Mideidae Thor, 1911
     14 Family Mideopsidae Koenike, 1910
     15 Family Momoniidae Viets, 1926
     16 Family Neoacaridae Motaş & Tanasachi, 1947
     17 Family Nipponacaridae Imamura, 1959 [monotypic]
     18 Family Nudomideopsidae Smith, 1990
     19 Family Cladomomoniidae Orghidan & Gruia, 1980 [monotypic]