Hydrachna inermis Piersig, 1895

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 : 47

Hydrachna inermis Piersig, 1895

Species incerta

Location of type series: unclear, no type locality defined in original description, later published record: Leipzig, "Großzschocher, Teiche und Lachen in der näheren Umgebung" (Piersig 1897–1899).

Discussion: This species has been frequently recorded until recent times from various parts of Europe, but the diagnostic features have never been adequately defined. A character traditionally used to separate adult H. inermis from the similar H. processifera Koenike, 1903 , is the absence of a pair of minute sclerite dots in the frontal dorsum halfway between prae­ and postocularia. Apart from the highly questionable taxonomic value of this distinction, old descriptions of Hydrachna species did not reflect such details and it is probable that these minute sclerite dots were simply overlooked by Piersig, a question no longer resolvable since the type material has disappeared. A tentative distinction of larvae of the two species proposed by Sparing (1959) is based on variable characters and again questionable due to the lack of information on morphological details of adult H. inermis ( Davids 1973) . The description of larvae attributed to H. inermis by Wainstein (1980) does not include a discussion of these taxonomic problems. In this situation, H. inermis must be considered a species incerta; the question whether H. processifera is its junior synonym cannot be answered.