Assessment of lateral compression of the idiosoma in adult water mites as a taxonomic character and reclassification of Frontipodopsis Walter, Wettina Piersig and some other basal Hygrobatoidea (Acari : Hydrachnida)

Systematists have frequently interpreted lateral compression of the idiosom a in post-larval instars of water mites as a synapomorphy indicating common ancestry. This paper re-examines the evidence to support this assumption a nd concludes that lateral flattening has occurred independently several tim es during water mite evolution, and especially often in various basal clade s of the superfamily Hygrobatoidea. Reassessment of the phylogenetic relati onships of the genera attributed to these clades results in improved unders tanding of the early evolution and biogeography of hygrobatoid water mites and necessitates redefinition of several clades and significant revision of the family classification. The South African genus Karlvietsia K.O. Viets, 1962 is transferred from the subfamily Frontipodopsinae of the family Atur idae to the Hygrobatidae. The Australian genera Tasmanaxona Cook, 1986, Whe enyella Cook, 1986 and Wheenyoides Harvey, 1990 are also removed from Front ipodopsinae and placed with the genus Wettina Piersig, 1892, here removed f rom the family Pionidae, in the family Wettinidae Cook, stat. nov. Based on apparent relationship with members of Wettinidae, the South African genus Stormaxonella K.O. Viets, 1962 is transferred from the Aturidae and provisi onally placed in the Wettinidae. The clade including the predominately Hola rctic genus Lethaxona K.H. Viets, 1932 and the Western Hemisphere genus Let haxonella Cook, 1963 is recognised as the probable sister group of Wettinid ae, and these genera are consequently removed from the Aturidae and placed in Lethaxonidae, fam. nov. The genus Frontipodopsis Walter is reassigned fr om Aturidae to the now monobasic family Frontipodopsidae K.H. Viets, stat. nov. Finally, five new species are described, including Frontipodopsis (Fro ntipodopsella) sudafricanus, sp. nov., Wettina occidentalis, sp. nov., Karl vietsia simplex, sp. nov., Hygrobates (Hygrobates) gereckei, sp. nov. and H ygrobates (Hygrobatides) frontipodoides, sp. nov., and both Karlvietsia bre vipalpis K.O. Viets and K. angustipalpis K.O. Viets are redescribed.