Hydrachna extorris Koenike, 1897

Davids, Kees, Sabatino, Antonio Di, Gerecke, Reinhard, Gledhill, Terence & Smit, Harry, 2005, On the taxonomy of water mites (Acari: Hydrachnidia) described from the Palaearctic, part 1: Hydrachnidae, Limnocharidae and Eylaidae, Zootaxa 1061, pp. 36-64 : 42

Hydrachna extorris Koenike, 1897

Material examined: Holotype female NHUB 953 ­ without collecting site details, neither on the slide, nor in the original description.

Description: Frontal area in several pieces, with presence and arrangement of minute sclerites indeterminate. However, a pair of well developed crescent­shaped sclerites (L 300) associated with postocularia; coxal field L 1600; genital field L/W 550/780, with a minute indentation anteriorly and an extended furrow of porose sclerotization separating the two groups of acetabula; gnathosoma L 1500, rostrum L 1050 (rostrum/base 2.3), chelicera L 1960; palp measurements (L/H) P­1 220/430 (0.5), P­2 450/290 (1.6), P­3 540/ 138 (3.9), P­4 200/90 (2.2), P­5 75/50 (1.5), setation: P­1 0, P­2 dorsal 11, lateral 3, P­3 dorsal 3, lateral 1.

Discussion: This species provides a special problem: Notwithstanding the bad state of conservation of the holotype, several important characters are visible, and clear differences in comparison with all other W Palaearctic species can be found. Hydrachna extorris is similar to the members of the geographica group in the very long gnathosomal rostrum and a frontal area with rather large, crescent­shaped sclerites associated with the postocularia, but obviously without further sclerotizations. However, it differs in the presence of only one seta on Cx­4 and having a rather stout palp (L/H P­2 1.6, P­3 3.9, P­4 2.2) with lower setae numbers. In the shape of P­3 it is similar to H. processifera , a species with shorter postocular sclerites and a much shorter P­2 (L 240­270). The fact that no further records have been published since the first description suggests that this species of completely unclear geographical origin does not belong to the rather intensively studied E uropean fauna.