Hygrobatidae Koch, 1842

Gerecke, Reinhard, 2021, The water mites of the family Hygrobatidae (Acari, Hydrachnidia) in Italy, Zootaxa 5009 (1), pp. 1-85 : 6-7

Family Hygrobatidae Koch, 1842

Diagnosis: Lateral eyes beneath the integument. Coxae in most European taxa in three groups, rarely fused by secondary sclerotization to a single coxal shield or a ventral shield that can include the genital field. Cx-IV typically with a pair of glandularia (in extra-European species occasionally shifted to Cx-III). Legs with or without swimming setae. Genital sclerites not movable; three to many pairs of acetabula lying on plates flanking the gonopore. Male genital sclerites in all European taxa anteriorly and posteriorly fused to form a round plate, completely surrounding the gonopore. Gnathosoma in all European species either posteromedially fixed by a fine, hardly visible bridge to, or broadly fused with, Cx-I. A ventral seta always absent from P-2.

Discussion: In a global perspective, water mites of the family Hygrobatidae represent not only a wide variety of morphological adaptations, but they offer also a lot of taxonomic difficulties, not only with regard to the distinction of the numerous genera, but also concerning the separation from related families ( Cook 1974, Smit 2020). However, the genera known from Europe are easily distinguished by a clear-cut combination of character states, with the medial margin of the anterior coxal plate formed exclusively by Cx-I, Cx-II not extending to the midline of the body ( Gerecke et al. 2016). Representatives of three genera are recorded from Italy. Species of the only further genus known from Europe, Mesobates Thor, 1901 , combines characters of European Hygrobates (broad fusion of the gnathosomal base with the anterior coxal plate) and Atractides (I-L-5 with a pair of strong ventral sword setae, I-L- 6 bowed). It consists of species mostly restricted to circumpolar regions. The detection in a stream in Greece of an isolated population of Mesobates forcipatus Thor, 1901 , a species otherwise known from Scandinavia only, suggests that this species might be present also elsewhere in the Mediterranean area ( Gerecke 2000).

Genus Atractides Koch, 1837

Diagnosis: Dorsal and posteroventral idiosoma often completely membranous, but in some species covered by variously developed plates or shields; if the dorsal integument is strongly lineated, no muscle insertion sclerites are developed. Cx-I fused medially. Gnathosoma without a posterior “anchoral process”, not broadly fused with Cx-I, but joined by a very narrow median bridge. Glandularia on Cx-IV near suture line Cx-III/IV. Legs only occasionally (in one Italian species) with swimming setae. I-L-5 with a short ventral seta and two characteristic distal setae variously modified in shape and arrangement (“S-1/-2” - similar in shape, or S-1 longer and slenderer than S-2). I- L-6 variously modified - often curved, distally or proximally thickened, stout or very slender. Genital field usually with three, rarely with more pairs of acetabula. Gnathosoma and palp various in shape, palp often showing sexual dimorphism. P-4 bearing two fine ventral setae and a strong, often peg-like seta in medioventral position.

Discussion: The function of the “ Atractides hands”, the characteristically modified I-L, is unclear - the anterior legs appear not to be involved in prey uptake, but are in general hold extended in anterodorsal direction. In addition to the subgenera represented in Italy, the only further European subgenus is Maderomegapus Lundblad, 1941, introduced, originally at genus rank, for Atractides (Maderomegapus) hystricipes ( Lundblad, 1941) , endemic to Madeira.

Subgenus Atractides s. str.

Diagnosis: Without pronounced sexual dimorphism in setation of first and second legs; ventrodistal tip of I-L-5 not extending beyond dorsodistal margin of the segment, S-2 not extremely inflated; I-L-6 not extremely shortened and basally thickened; P-1 subquadratic or only slightly elongated (length/height ratio <1.5); chelicera robust (length/height ratio <8.0).