Genus Pionacercus Piersig, 1894 Pionacercus (Pionacercus): Cook, 1974a, pp. 284–285, figs. 1203, 1206, 1207. Pionacercus (in part): Smith, 1976, pp. 44–46, 79–81. Pionacercus (in part): Smith & Cook, 1991, pp. 554, 576, 578. Pionacercus (in part): Smith et al., 2001, pp. 612, 615, fig. 336. Pionacercus (in part): Smith et al., 2010, pp. 553, 556, fig. 15.337. Pionacercopsis: Gerecke, 2014, pp. 59–60, fig. 2. Diagnosis. Larva: Partially described by Lundblad (1927), but insufficiently known to permit listing of diagnostic character states. Adults (modified from Smith 1976): Character states of subfamily Foreliinae. Idiosoma with dorsal integument mostly soft and bearing only tiny platelets associated with glandularia and idiosomal setae, or mostly covered by a dorsal shield that extends anteriorly to and including level of postocular setae in males; mostly soft and bearing only tiny platelets associated with glandularia and idiosomal setae in females. Venter with coxal plates and genital field incorporated into a ventral shield in males, but surrounded by soft integument bearing only tiny sclerites associated with glandularia and idiosomal setae in females. Fourth coxal plate with medial edge relatively long in males, short but not reduced to angles in females. Genital field with three pairs of relatively small acetabula borne on ventral shield flanking gonopore in males and on triangular acetabular plates flanking gonopore in females, extending laterally to but not beyond posterolateral angles of fourth coxal plates. Pedipalp segments moderately long and slender and with tibia bearing two slender setae on small projections ventrally and a sessile, peg-like seta distomedially. Male third leg with tarsus bearing slightly reduced claw sockets that are terminal in position and small but only slightly modified claws; fourth leg with genu short, moderately expanded, and bearing a row of moderately long, slightly thickened setae ventrally, a patch of slender swimming setae anteriorly, and patches of slender swimming setae and thick, blade-like setae distoventrally, lacking a prominent distoventral projection, tibia relatively long and slender and bearing a row of slender swimming setae ventrally and several swimming setae distally, tarsus curved dorsally, bearing a row of thick, peg-like setae that extends both proximal and distal to deepest part of curved portion, and bearing thick, highly modified claws. Type species. Pionacercus leuckarti Piersig. Species included. Pionacercus leuckarti Piersig (Europe), P. uncinatus (Koenike) (Europe), P. norvegicus Thor (Europe), P. japonica Imamura (Japan). There are five undescribed species of Pionacercus from North America in the Canadian National Collection of Insects and Arachnids and larvae have been associated with adults by rearing for one of them. One of these species is also represented in the Habeeb Collection and was incorrectly reported as P. leuckarti Piersig (Habeeb 1958). Distribution. Holarctic (Eurasia, North America). Discussion. Cook (1974a) treated Pionacercus (including the subgenus Pionacercopsis) in the subfamily Tiphyinae. Smith (1976) placed Pionacercus (including Pionacercopsis) in the tribe Foreliini along with the genera Forelia and Pseudofeltria. Smith & Cook (1991, 1999) and Smith et al. (2001, 2010) treated this clade as the subfamily Foreliinae. Gerecke (2014) proposed placing Pionacercus and Pionacercopsis in the Foreliinae, apparently unaware that Smith (1976) had already assigned them to Foreliini. Here we follow Gerecke (2014) in considering Pionacercus and Pionacercopsis as separate genera.

Published as part of 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), pp. 111-156 in Zootaxa 3919 (1) on pages 118-119, DOI: 10.11646/zootaxa.3919.1.6, http://zenodo.org/record/244587