Acercopsis Viets, 1926

Smith, Ian M., Cook, David R. & Gerecke, Reinhard, 2015, Revision of the status of some genus-level water mite taxa in the families Pionidae Thor, 1900, Aturidae Thor, 1900, and Nudomideopsidae Smith, 1990 (Acari: Hydrachnidiae), Zootaxa 3919 (1), pp. 111-156 : 124-125

Genus Acercopsis Viets, 1926 stat. nov.

Tiphys (Acercopsis) : Cook, 1974a, p. 282, figs. 1200–1202. Tiphys (in part): Smith, 1976, pp. 52, 83, 92, figs. 45, 46. Tiphys (in part): Smith & Cook, 1991, pp. 554, 579.

Tiphys (in part): Smith et al., 2001, pp. 582, 615.

Tiphys (in part): Smith et al., 2010, pp. 524, 556.

Diagnosis. Larva: Character states of the subfamily Tiphyinae. Third coxal plate with transverse muscle attachment scar absent. Excretory pore plate nearly circular with setae ps1 well anterior to setae ps2; excretory pore sessile.

Adults (modified from Smith 1976): Character states of subfamily Tiphyinae. Fourth coxal plates with large, anteriorly concave projections covering insertions of fourth legs. Pedipalp with segments relatively long and slender and with tibia bearing a long, blade-like seta distomedially on a moderately prominent projection. Genital field bearing three pairs of acetabula. Males with genital acetabula relatively widely separated from one another and borne on slender acetabular plates that completely encircle gonopore and extend laterally beyond posteromedial angles of fourth coxal plates; second leg with tibia bearing two spatulate setae distoventrally and fourth leg with genu greatly expanded and bearing a row of blade-like setae and a few short swimming hairs on ventral edge. Females with genital acetabula relatively widely separated from one another and borne on acetabular plates that are nearly as wide as long, and with second and third acetabula on each side forming a row near posterior edge of plate that is nearly at right angles to long axis of idiosoma.

Type species. Acercus pistillifer Koenike.

Species included. Acercopsis pistillifer (Koenike) (Europe) , A. vernalis Habeeb (eastern North America). There is an undescribed species of this genus from eastern North America in the Canadian National Collection of Insects and Arachnids.

Distribution. Holarctic (Europe, North America).

Discussion. Cook (1974a) treated Acercopsis as a subgenus of Tiphys . Smith (1976) questioned the merit of considering Acercopsis as a subgenus but regarded this group of species, his pistillifer group, as the probable sister group of all other species of Tiphys . Here we propose elevation of Acercopsis to full generic status based on autapomorphic presence of spatulate setae on the tibia of the second legs in males and unique configuration of the genital field in both sexes. This placement is consistent with the apparent phylogenetic relationship between Acercopsis and Tiphys and helps to promote consistency in the classification of genus group taxa in the family Pionidae .