Hydrachna globosa (De Geer, 1778)

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 : 43-44

Hydrachna globosa (De Geer, 1778)

Hydrachna globosa neumani Lundblad, 1962 , syn. nov. Hydrachna globosa rotundata Láska, 1964 , syn. nov.

Material examined: Hydrachna globosa neumani Lundblad, 1962 , holotype female SMNH 5988, Gotland, Kinner 1874, leg. Neuman. Hydrachna gallica 10.06.1998, Belarus, Orla, oxbow­lake on the right bank of river Neman, sandy­clayey unvegetated bottom, 20 cm deep, leg. det. M. Cichocka one male, one female.

Discussion: The variable shape of the dorsal plates has led to the description of a high number of taxa most of which are now regarded as junior synonyms of H. globosa (see K.O. Viets 1987). In particular Thor (1916) introduced a series of subspecies names for specimens which, arranged together, represent a morphological continuum rather than distinct taxonomic units (Lundblad 1962). One of these, Hydrachna gallica Thor, 1916 , is often regarded as a separate species and regularly reported from Eastern Europe. Biesiadka & Cichocka (1994) observed larvae of H. globosa s. str.­like adults parasitizing Nepa cinerea L., but larvae of adults attributed to H. gallica were restricted to Ranatra linearis . In contrast, Lundblad (1962) could not detect any stable diagnostic morphological difference between the two taxa. Also our comparative investigation comparing material attributed to H. gallica by Cichocka, and specimens classified as H. globosa by Lundblad does not reveal clear morphological differences at the adult stage, apart from slightly major measurement values in H. gallica . Some examples are ( H. globosa in parentheses): frontal shield L male 800 (590–670), female 800 (700); genital field L/W male 500/540 (450–470/370–480), female 350/520 (290/490); palp total L male 885 (850) female 980 (885); chelicera L male 1150 (970), female 1250 (1050). Also with regard to the shape and setation of sclerites and palp segments, there are no remarkable differences between the specimens examined, with the exception of the position of the median eye at the posterior margin of the lateral eyes (behind the postocularia) in H. globosa , at the centre or anterior margin of the lateral eyes (between postocularia) in H. gallica (Cichocka, pers. comm.). The significance and stability of this rather minute difference should be investigated with more material from a wider geographical range.

Lundblad (1962) introduced the name Hydrachna globosa forma neumani for a specimen with a rather slender palp. The type specimen is obviously a juvenile. The longitudinal faults on the ventral side of palp segments, namely of P­3 demonstrate that they were originally stouter (as typical for H. globosa ), and the slender shape is caused by shrinking of the segments. Hydrachna globosa neumani is obviously a synonym of H. globosa .

A short time later, and probably without knowledge of Lundblad's revision, H. globosa rotundata was published as a further subspecies, from a rice field in Slovakia ( Láska 1964). The name­giving rounded shape of lateral (and medial) frontal sclerite margins agrees perfectly with Thor's description of H. gallica , a species synonymized by Lundblad (1962) with H. globosa . Also the slightly protruding ventral margin of P­3 is a character state lying within the individual variability of H. globosa . Hydrachna globosa rotundata is yet another junior synonym of H. globosa .