Hydrachna distincta 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 : 41

Hydrachna distincta Koenike, 1897

Material examined: Holotype female SMNH 555, "Holstein, Plön, Drecksee Dr. Zacharias leg. 8.7.86 ".

Discussion: The preparation is completely desiccated, with most legs detached and broken into parts, gnathosoma detached, laterally, with base destroyed, chelicerae broken into pieces, one palp entire, the other lacking P­3­5; frontalia and lateral eyes separate. The ventral idiosoma is mounted upside­down (genital field and coxae visible from the internal side). In this position it is seen that acetabula also extend to the internally­bent lateral margin of the genital plate, here obviously facing the surrounding folded in membranous integument. The palp (obviously not distorted) and gnathosomal rostrum agree with H. conjecta . In the original publication Koenike described the genital field as similar to H. globosa . Only later ( Koenike 1909) did he describe the longish shape of this organ; all later text books ( Soar & Williamson 1925, Sokolow 1940) obviously relied on this publication or on the figure given by K. Viets (1936) of the holotype. However, as indicated by the key of Soar & Williamson and the examination of a slide in NHML erroneously ascribed to this species (see under H. conjecta ), generally most weight was given to the shape of the frontal plates. Most records of H. distincta probably refer to specimens of H. conjecta with an equally concave medial margin of the frontalia (see there): This is the case in a specimen in the collection of Koenike attributed to H. distincta with a question mark, and surely also in the specimens recorded from Ireland by Halbert (1911), who was the only author to describe the male of H. distincta . As this author explained, his identification relied exclusively on the shape of the frontal platelets. In view of the male genital field agreeing perfectly with that of H. conjecta, Halbert was the first to suggest a possible synonymy of the two species. Nevertheless, the most unusual character of H. distincta is the elongated female genital field projecting anteriorly between the Cx­3, a feature not found in any other European species. As no further female specimens with such a morphology have been described in the past 100 years, it is postulated that this species is based on a misshaped individual of H. conjecta (see Lundblad 1962a for descriptions of similar morphological deviations in other species of the genus). However, as Wainstein (1976, 1980) described diagnostic characters in the shape of tibiotarsi in larvae which he attributed to H. distincta , the question concerning the existence of two possible H. conjecta ­like sister species remains open and needs further investigation. An earlier publication of the Russian author ( Wainstein 1966) suggests that in separating the two species, he also gave most weight to the frontal plates.