Hydrachna incisa Halbert, 1903,

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: 45-46

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http://doi.org/ 10.5281/zenodo.170186

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Hydrachna incisa Halbert, 1903


Hydrachna incisa Halbert, 1903 

Hydrachna halberti Soar, 1908  , syn. nov.

Hydrachna levis Williamson, 1913  , syn. nov.

Hydrachna levis acuminata K.O. Viets, 1954  , syn. nov.

Material examined: Hydrachna incisa  , Holotype male, National Museum of Ireland, Dublin; Carrigaline County Cork, April 1900; Hydrachna halberti  NHML, holotype? "Chas. D. Soar, Osborne Dyke, Nor­Broads, 1906 "; " 1929 ­ 11­20 ­ 264. C. D. Soar coll."; Hydrachna levis Williamson, 1913  , Holotype female, NHML, " Hydrarachna levis Williamson 1929  ­ 11­20 ­ 265. Nor.­Bds. 1900 Type " " Type species 32.H. C.D. Soar coll. Balsam, 1902 "; Hydrachna levis acuminata  Holotype male SMF K.O.Viets 1087; Paratype male SMF K.O.Viets 1088, Schmiedsee (Rüthsee) 21.5.1928; one male, Greece, Thessalia, Pinios delta, small lake surrounded by carr near Stómio, 8.5. 1992, Smit coll.

Discussion: So far, Hydrachna incisa Halbert, 1903  in the adult stage was only known from the male. Numerous characters indicate that it is closely related to H. geographica  . Shared features are: (1) large general dimensions; (2) frontal area with only one pair of longish sclerites near postocularia and a few sclerite dots halfway between lateral eyes and postocularia; (3) more than one seta on Cx­ 4; (4) gnathosomal rostrum extremely elongated, and (5) palps slender and with high numbers of setae on P­ 2 and P­ 3, but lacking a dorsal seta on P­ 1. The species differs from its sister taxon in the morphology of the frontal platelets (not straight, but curved and crescent shaped), distinctly shorter (maximum L <400), a lower number of setae on Cx­ 4 (2–5), the more slender segment P­ 2 (L/H 1.9–2.7, in H. geographica  <1.9) and, of particular taxonomic importance, the shape of the anterior margin of the male gonopore (straight or with a minute indentation only). The genital field (shown too elongate in Halbert's Fig. 2View FIGURE 2. A – F,) has a L/W ratio of 700 / 660. The longish sclerite located halfway between the lateral eyes and the postocularia described for H. incisa  by Halbert could not be found in the type specimen ­ most probably, the author confused pieces of muscles located in this area for them. If muscle attachments are recognized as areas with interrupted papillosity, two or three such dots can be identified in this area, arranged in a similar way as in H. geographica  .

With regard to all diagnostic character states listed for H. incisa  , the holotype of H. halberti  is in good agreement. The specimen obviously represents the female of that species and must be considered its junior synonym. As a particular feature it has asymmetrical palps (right P­ 2 distally narrowed to a H of 160, minimum H of left P­ 2 200). See below for a general characterisation of H. incisa  females.

From a re­examination of the holotype of H. levis  it is clear that Williamson was wrong in stating that the integument of this species was without papillosity. The papillate upper integument layer of this specimen has been detached from large parts of the body surface, but remnants are still visible on the membranous integument near the insertion of right IV­L and around the base of the detached gnathosoma. Thus, the key character traditionally used for defining this species (e.g. Soar & Williamson 1925) is based on an error of observation. As no significant morphological differences could be detected, Hydrachna levis  is considered a junior synonym of H. incisa  .

In his description of H. l. acuminata, K.O. Viets  regarded the presence of integumental papillae (believed to be absent in H. levis  ) as a diagnostic feature. The presence of high numbers of setae on P­ 2 and P­ 3 (visible with difficulty only in the thick type preparations) was obviously overlooked by Viets, as indicated by his text and Fig. 3View FIGURE 3. A. In fact, the two male specimens of the type series are in perfect agreement with H. incisa  and this taxon is obviously a further junior synonym of that species.

The synonymization of the two species H. halberti  and H. levis  , both described from females, provides the opportunity for the first description of the female of H. incisa  . Some important measurements are: coxal field, total L 1400–2000, genital field L/W 460–600 / 640–830, frontal sclerite L 280–320, gnathosoma base 600 (damaged in H. levis  ), rostrum L 1100–1300; chelicera L 2150–2680; palp total L 1785–2050, segments L/H P­ 1 220–270 / 370–470 (0.57–0.59), P­ 2 580–660 / 250–310 (2.13–2.32), P­ 3 680–800 / 140–150 (4.86–5.33), P­ 4 240–250 / 90–100 (2.50–2.67), P­ 5 65–70 / 35–45 (1.44 –2.0); L ratio P­ 2 / P­ 3 0.83–0.85; P­ 2 with 17–21 dorsal and 4 lateral setae, P­ 3 with 5–6 dorsal and 6–7 lateral setae.


Natural History Museum, Tripoli


Forschungsinstitut und Natur-Museum Senckenberg














Hydrachna incisa Halbert, 1903

Davids, Kees, Sabatino, Antonio Di, Gerecke, Reinhard, Gledhill, Terence & Smit, Harry 2005

Hydrachna levis acuminata

K.O. Viets 1954

Hydrachna levis

Williamson 1913

Hydrachna halberti

Soar 1908