Pionidae Thor, 1900

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 : 113-115

Family Pionidae Thor, 1900

Pionidae (in part): Cook, 1974a, p. 270.

Pionidae (in part): Smith, 1976, pp. 15–16, 88, 90–93.

Pioninae: Wainstein, 1980, pp. 172–173.

Pionidae (in part): K. O. Viets, 1987, p. 605.

Pionidae (in part): Smith & Cook, 1991, pp. 553, 554, 576–579, figs. 16.146, 16.147, 16.152–16.174, 16.176, 16.177, 16.179–16.181, 16.260, 16.268, 16.274, 16.275, 16.278–16.283.

Pionidae : Harvey, 1996, pp. 361–363.

Pionidae : Harvey, 1998, p. 89.

Pionidae (in part): Smith & Cook, 1999, pp. 117–118.

Pionidae : Smith et al., 2001, pp. 581, 612, 615, figs. 119–149, 327–344.

Pionidae : Walter et al., 2009, pp. 268–269, 284–285, 290, figs. 13.37A–13.37I, 13.54H, 13.55A–13.55L.

Pionidae : Smith et al., 2010, pp. 523, 524, 553, 556, figs. 15.119–15.149, 15.328–15.345.

Diagnosis. Larva (modified from Smith 1976): Character states of pioniform hygrobatoid mites sensu Smith (1976). Idiosoma elongate oval in shape, moderately flattened dorsoventrally, and bearing twenty pairs of setae. Dorsum nearly covered by entire dorsal plate bearing four pairs of setae anteriorly, namely vi, ve, si, and se. Dorsal plate surrounded by soft integument bearing two pairs of eyes anterolaterally and eight pairs of setae laterally including setae c1, c2, c3, d1, d2, e1, e2, and f1. Venter nearly covered by three pairs of expanded coxal plates. On each side, first coxal plate separate and second and third coxal plates fused with one another with suture line distinct laterally and obliterated medially where it merges with lateral coxal apodeme that is either nearly parallel to anterior edge of second coxal plate or nearly transverse. First coxal plate bearing setae 1a posteriorly and 1b at anterolateral angle; second coxal plate bearing setae 2b at posterolateral angle; third coxal plate bearing setae 3a anteriorly. Third coxal plate with medial coxal apodeme present or absent and with transverse muscle attachment scar present or absent. Posterior edge of third coxal plate convex or nearly straight, and lacking projections.

Excretory pore plate much larger than excretory pore and nearly circular, triangular, quadrangular, broadly heartshaped, or dumb-bell shaped, bearing setae ps1 medially and ps2 laterally; excretory pore sessile or borne on posterior projection of plate. Numbers of setae and solenidia on leg segments as follows: ITi 9 (+ ϕ1 and ϕ2); ITa 12 or 13 (+ ω); IITi 9, 11, or 12 (+ ϕ1 and ϕ2); IITa 12 or 13 (+ ω); IIITr 1 or 2; IIITi 9, 11, or 12 (+ ϕ); IIITa 11. Trochanter of first leg with seta ITr1 long and anteroventral in position.

Adults (modified from Smith 1976): Character states of pioniform hygrobatoid mites sensu Smith (1976). Dorsal integument mostly soft and bearing a pair of tiny dorsalia, partly covered by an array of small platelets, or nearly covered by an entire dorsal shield (may be sexually dimorphic). Venter with coxal groups surrounded except posteriorly by soft integument or fused with ventral shield in males, surrounded by soft integument or fused with ventral shield in females. First coxal plate with or without two to four thick setae anteriorly. Apodeme of first coxal group short, or long and extending posteriorly to beneath medial edge of third coxal plate. Suture line between third and fourth coxal plates oblique so that fourth coxal plate is triangular and forms no part of medial edge of posterior coxal group, or transverse so that fourth coxal plate is quadrangular and forms a substantial part of medial edge of posterior coxal group. Fourth coxal plate of males separate from or fused with one another and the acetabular plates posteriorly. Posterior edge of fourth coxal plate obliquely posterolaterally directed or nearly transverse, concave or sinuate, forming an angle or convexity with lateral edge of plate and usually with a conspicuous apodeme beneath this angle. Coxoglandularium I located between second and third coxal plates or fused with posterior edge of second coxal plate. Coxoglandularium II located between or at the line of fusion of fourth coxal plate and acetabular plate. Genital field bearing three to around one hundred acetabula per side, acetabula borne on acetabular plates in males, borne on acetabular plates, in soft integument, or with some combination of these conditions, in females. Acetabular plates of males entire and fused with one another to surround gonopore except, in some cases, anteriorly; compact, nearly semicircular in shape and confined to gonopore region, or elongate, tongue-shaped and extending laterally beyond posterolateral angles of fourth coxal plates; separate from or fused with fourth coxal plates anteriorly, and variously fused with ventral shield when present. Acetabular plates of females entire, variously fragmented, or absent; when present, compact and nearly triangular, circular, curvilinear or arcuate in shape and confined to gonopore region, or elongate, tongue-shaped and extending laterally beyond posterolateral angles of fourth coxal plates; surrounded by soft integument or variously fused with ventral shield when present. Genital field of males with gonopore situated in same plane as acetabular plates or in a genital pit, and with or without a complex sclerotized petiole posteriorly.

Gnathosomal base separate from first coxal plates, bearing or lacking an anchoral process. Pedipalp with femur lacking or bearing a ventral projection. Pedipalp tibia either shorter than three proximal segments combined and bearing fewer than six setae ventrally, or as long as three proximal segments combined and bearing more than ten setae ventrally; with or without one or two setiferous projections ventrally (may be sexually dimorphic); with most distomedial seta slender, peg-like, or spine-like, and either sessile or borne on a projection.

First leg with tibia bearing only straight setae distoventrally; tarsus straight, and either nearly cylindrical or strongly biconvex and expanded dorsoventrally. Posterior three pairs of legs with segments and their setae unmodified, or with various segments and their setae sexually dimorphic and characteristically modified in males.

Subfamilies included. Foreliinae Thor, 1923, Huitfeldtiinae Viets, 1924, Hydrochoreutinae Viets, 1942, Najadicolinae Viets, 1935, Pioninae Thor, 1900, Schminkeinae Schwoerbel, 1984, Tiphyinae Oudemans, 1941.

Distribution. World-wide.

Discussion. Cook (1974a) considered the family Pionidae to consist of five subfamilies, namely Huitfeldtiinae Viets, 1924 including only the genus Huitfeldtia, Pioninae Thor, 1900 including only the genus Piona, Nautarachninae Koenike, 1909 including only the genus Nautarachna (subgenera Nautarachna and Pionella ), Tiphyinae Oudemans, 1941 including the genera Wettina , Hydrochoreutes , Tiphys (subgenera Tiphys , Pionides , Acercopsis ), Acercella , Pionopsis (subgenera Pionopsis , Neotiphys ), Pionacercus (subgenera Pionacercus , Pionacercopsis ), and Foreliinae Thor, 1923 including the genera Forelia (subgenera Forelia and Madawaska ) and Pseudofeltria .

Smith (1976) subsequently revised the family based on a phylogenetic analysis of larval and adult characters and proposed a classification with three subfamilies, namely Wettininae Cook, 1956 including only the genus Wettina, Huitfeldtinae including the tribes Hydrochoreutini Viets, 1942 for the genus Hydrochoreutes, Foreliini for the genera Pionacercus , Pseudofeltria , and Forelia , and Huitfeldtiini for the genera Huitfeldtia , Acercella , Neotiphys , Pionides , Pionopsis , and Tiphys , and Pioninae including the genera Nautarachna and Piona . Smith concluded that the Holarctic genus Wettina represented an ancient lineage that was not demonstrably related to other genera placed in the family.

Simmons & Smith (1984) transferred the genus Najadicola Koenike, 1895 , and the subfamily Najadicolinae Viets, 1935 to the Pionidae from the family Unionicolidae based on synapomorphies of larvae and adults shared by members of Najadicolinae and Pioninae. Subsequently, Cook et al. (2000) removed Wettina from the Pionidae and placed it in a distinct family, the Wettinidae , along with a number of related austral genera previously classified in the aturid subfamilies Frontipodopsinae and Axonopsinae (discussed below).

Smith & Cook (1991, 1999) and Smith et al. (2001, 2010) adopted a version of pionid classification that essentially followed the cladistic arrangement proposed by Smith (1976), but progressively took into account the transfer of the Wettina -like mites from the Pionidae to the Wettinidae , the transfer of the Najadicolinae to the Pionidae , and the addition of the genera Schminkea Schwoerbel, 1984 , Australiotiphys Cook, 1986 , Twinforksella Cook, 1992 , and Larri Harvey, 1998 . Discovery of these austral pionid genera suggests that a number of pionid lineages are much older than previously thought. Accordingly, the tribes Hydrochoreutini, Foreliini, and Huitfeldtiini that had been placed by Smith (1976) in the subfamily Huitfeldtiinae were elevated to subfamily status and the Huitfeldtia -like mites and Tiphys -like mites were segregated into separate subfamilies. The resulting revised classification is summarized and discussed below.