Hydrachna geographica Müller, 1776

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 : 42-43

Hydrachna geographica Müller, 1776

Material examined: SMNH 6059, SV, Gästrikland, male; SMNH without number, Neuman coll., Gotland, female; SMNH 6060 SV Smaland, female.

Discussion: This species has been very often cited, recorded and also figured in the literature (for detailed accounts see K. Viets 1956, K.O.Viets 1987). However, there is still little information concerning measurements and general morphology of males ­ previous authors were content with defining the species by its large size and the particular, namegiving colour pattern of the dorsal idiosoma (e.g. Soar & Williamson 1925). Further characteristics in the re­examined specimens concern the presence of rod­shaped frontal sclerites flanking the postocularia (L 400 in male,> 500 in females) and two to three minute sclerite dots halfway between pre­ and postocularia; 6­8 setae on Cx­4; the gnathosoma with very long, weakly curved rostrum (L ratio gnathosomal rostrum/base> 1.5) in dorsal view distinctly set of from the base; a slender palp (L/H P­2> 1.5, P­3> 3.5) with high numbers of short setae on the dorsal surface of P­2 and 5­10 longer hair setae on the medial surface of P­3, but absence of setae from P­1. Important measurements are as follows. Male: Coxal field L 2000; frontal sclerite L 400, genital field L/W 880/840; gnathosomal base 660, rostrum 1000; chelicera L 2200, palp total L 1730, segments L/H P­1 300/460 (0.65), P­2 500/280 (1.79), P­3 620/140 (4.43), P­4 220/90 (2.44), P­5 L/H 90/40 (2.25); P­2 with 18 dorsal and six lateral, P­3 with three dorsal and eight lateral setae. Females: Coxal field L 2300–2600; frontal sclerite L 560–620, genital field L/W 630–700/840–1000; gnathosomal base 800, rostrum 1600; chelicera L 2800, palp total L 2280–2410, segments L/H P­1 400–420/590–640 (0.66–0.68), P­2 670–710/370–380 (1.81–1.87, P­3 850–930/190–195 (4.47–4.77), P­4 250/110 (2.27), P­5 L/H 100–110/ 50–60 (1.67–2.20); P­2 with 13–15 dorsal and two lateral, P­3 with three to eight dorsal and four to five lateral setae.

Together with the similar H. incisa and H. bulgarensis , H. geographica forms a morphologically distinct species group. All other European species are minor in dimensions, have muscle attachment sclerites associated with the preocularia, only one seta on Cx­4, a shorter gnathosomal rostrum, stouter palps, a less dense setation on P­2 (dorsally) and P­3 (medially), and often a dorsodistal seta on P­1.