On the systematic of the water mite Piona annulata ( Thor , 1900 ) ( Acari , Hydrachnidia : Pionidae )

The water mite Piona annulata (Thor, 1900) was described and known only from the female. Male of this species have been previously unknown. The descriptions of the female of this species (Lundblad 1962, etc) are incomplete, complicating the identification of species. The aim of the paper is to description of male and redescription of female Piona annulata. The material was collected by the author in the Yaroslavl Province of Russia. Idiosomal setae are named according to Tuzovskij (1987). Furthermore, the following abbreviations are used: P–1–5, pedipalp segments (trochanter, femur, genu, tibia and tarsus); I–Leg–1–6, first leg, segments 1–6 (trochanter, basifemur, telofemur, genu, tibia and tarsus) i.e. III– Leg–1 = trochanter of third leg; L – length; W – width; n = number of specimens measured; all measurements are given in micrometers (μm).

Pedipalp (Fig. 5) stout: P-1 short, with single short dorsodistal seta; P-2 with five short subequal dorsal setae; P-3 with three short unequal setae, lateral seta slightly longer than both other setae and situated a little proximally to middle of segment; P-4 with two distinct setal tubercles, lying behind each other; ventrodistal peg-like seta short and located on small tubercle; P-5 short, with proximal solenidion, four thin setae and four very short, thick distal spines.
I/II-Leg-5/6 strong thickened distally (Fig. 6); III-Leg-5 long, expanded distally, III-Leg-6 comparatively short and club-shaped (Fig. 7); IV-Leg-4 thick, with a deep concavity bearing numerous unequal spine-like setae, IV-Leg-5 narrowed in anterior two third and expanded distally, IV-Leg-6 thin straight, with three to four to thick setae (Fig. 8).Number of swimming setae as follows: one to two on I-Leg-4, two on I-Leg-5 and II-Leg-4, three on II-Leg-5 and IV-Leg-4, five to six on III-Leg-5, seven to nine on IV-Leg-5; III-Leg-4 with two to three rudimentary swimming setae.Claws of tarsi I-II relatively large, with two subequal clawlets (Fig. 9).Claws of legs III asymmetrical (Fig. 10); large claw with thick, long straight dorsal clawlet and a relatively short, thin slightly curved ventral clawlet; small claw with two thick pointed subequal clawlets.
Female.Dorsum similar as in of the male.All coxal groups separated and covering about half of the ventral surface in mature specimens (Fig. 11).Anterior coxal plates with short apodemes.Sclerites bearing setae Hv free.Medial margin of coxal plate IV 1.5-2.0times longer than medial margin of coxal plate III.Posterior margins of coxal plates IV forming obtuse angles, apodemes slightly developed.Genital opening and acetabular plates approximately equal in length.Acetabular plates bowed, narrow, with 9-15 pairs subequal acetabula in a single row; in additional, medially to acetabular plates one or two acetabula lying free in soft integument.Each acetabular plate with single anterior and two to three posterior genital setae; in additional, two (occasionally three) genital setae situated on small platelet between acetabular plate and anterior genital sclerite on each side.Pedipalp compact (Fig. 12): P-3 with three short unequal setae, base of lateral seta located proximally to middle of segment; P-4 slender than in male, both setal tubercles small and distinctly separated.
Legs thin and slender.I-Leg-4 with single short swimming seta, I-Leg-5 with two (occasionally with three) short swimming setae (Fig. 13).Legs II-IV with long swimming setae, with the following numbers: one to three on II-Leg-4, three on II-Leg-5, four on III-Leg-4, five to six on III-Leg-5, three to four on IV-Leg-4 and seven to nine on IV-Leg-5.
Remaks.The adults of P. annulata is similar to Piona nodata (Müller, 1776) and differs from them by the following characters (characters states of P. nodata are indicated in parenthesis, comparison is given with specimens collected in the Yaroslavl Province): both sexes: I-Leg-5 with two, occasionally with three, swimming setae, Figs. 6, 13 (with five to nine swimming setae); male: I/II-Leg-5/6 strong expanded distally, Fig. 6 (I/II-Leg-5/6 along entire length nearly an identical thickness); proximal projection of the ejaculatory complex with single coil, Fig. 4 (with three coils); female: P-4 both setal tubercles small and distinctly separated, Fig. 12 (both setal tubercles comparatively large and lying close to each other); two, occasionally three, genital setae situated on small platelet between the acetabular plate and anterior genital sclerite on each side, Fig. 3 (all genital setae situated on the acetabular plates).The groups of genital setae anterior to genital field in the females P. annulata separated from acetabular plates (Thor, 1901); however, as have noted Smith et al. (2015), these groups of genital setae occasionally is separately on one side and fused on the other.The groups of genital setae anterior to genital field separated from the acetabular plates in all females P. annulata collected in the Yaroslavl Province.