HYGROBATIDAE KOCH, 1842

Gerecke, Reinhard, 2003, Water mites of the genus Atractides Koch, 1837 (Acari: Parasitengona: Hygrobatidae) in the western Palaearctic region: a revision, Zoological Journal of the Linnean Society 138, pp. 141-378 : 144

HYGROBATIDAE KOCH, 1842

Derived primarily from the fauna of temperate areas of the northern hemisphere, Hygrobatidae was for some time a distinct taxonomic unit, containing two large clades based on the presence (Megapusinae Thor, 1927 = Atractidinae Oudemans, 1941) or absence ( Hygrobatinae Koch, 1842 ) of a modified first leg (K. Viets, 1936). However, the detection of numerous additional genera, mainly in the southern hemisphere, has blurred both the external distinction from neighbouring families such as Aturidae Thor, 1900 , and the internal distinction between the two subfamilies synonymized by Cook (1974). At present, Hygrobatidae is defined by the following character combination (after Cook, 1974, modified): Lateral eyes beneath the integument; coxae 4 bearing a pair of glandularia; no movable genital flaps present, acetabula lying on sclerotized plates which in males normally fuse to a round genital shield completely surrounding the gonopore; a ventral seta always absent from second pedipalp segment.

Within this family there is a group of genera with characteristically modified first legs. The articulation furrow on the fifth segment bears a pair of variously modified setae arranged near its ventral margin; often the furrow is more or less extended; the medial seta arranged at the anterior margin of this segment is thickened basally and in most cases whip-shaped; the sixth segment bears numerous ventral setae arranged in a regular line or (mostly) in several distinct groups; in general, this segment is curved or variously modified. There is reason to assume that presence of this functionally not yet understood character combination documents a common, monophyletic origin of the genera in question ( Gerecke, 2000).

A further important character complex rarely found in other families of Hydrachnidia concerns the more or less complete fusion of the gnathosoma to the anterior margin of coxae 1. Various species of Australiobates Lundblad, 1941 show differing degrees of fusion in the first coxae, with each side medially separate and a completely free gnathosoma with long proximal anchoral process resulting in the formation of an entire coxognathosomal shield. In Tetrabates Thor, 1922 , the gnathosoma is fused to the first coxae in males, but free in females.

If we assume a common origin for all Atractides -like genera (with first leg modified as described above) from a Hygrobates -like ancestor (with coxognathosomal shield), the presence of an unfused gnathosoma should be interpreted as a secondary phenomenon. The two Palaearctic genera could represent a model for that process. In Mesobates Thor, 1901 , the proximal margin of the gnathosoma is completely fused with the first coxae, while in Atractides this fusion is clearly reversed. Only the presence of a small sclerotized bridge between the gnathosomal base and the anteromedial margin of the first coxae is a relictary character that testifies to Atractides being descended from more sclerotized ancestors. Also, from other points of view (fourth pedipalp segment lacking both an increased seta on the medial surface and a dense dorsodistal hair cover), Mesobates represents a plesiomorphic, Hygrobates -like morphological condition, while the first leg shows details characteristic for Atractides - like mites. An early radiation of the latter probaly took place in the southern hemisphere, and only two of the genera reached temperate and colder zones of the Palaearctic region. Here, as in most other parts of the world, Atractides split into a large variety of running water-, spring- and interstitial-dwelling species, while Mesobates is represented only by localized populations of a few species characterized by modest morphological divergence ( Gerecke, 2000).

Based on adult morphology (presence of a thickened medial seta on the fourth palp segment) Atractides is similar to two genera restricted to the southern hemisphere: Paraschizobates Lundblad, 1937 and Australiobates Lundblad, 1941 . For a better understanding of phylogenetic relations within Atractides - like genera, of the general direction of evolutionary trends, and of the sister genus of Atractides , a more profound morphological knowledge of pre-adult instars is required.

The following adult character combination is diagnostic for the genus (after Cook, 1974, modified). Coxae 1 (Cx-1) fused medially, gnathosoma separated from the anterior margin of Cx-1 by a nearly complete membranous furrow, but joined to it by a narrow median bridge; anchoral process of gnathosoma obviously included in the medially fused area of Cx-1; glandularia of Cx-4 located near anterior suture lines between Cx-3 and Cx-4; fifth segment of first leg with two variously modified ventrodistal setae and a basally thickened mediodistal seta; sixth segment of first leg varies in shape, but is in most cases bowed, with numerous ventral setae generally arranged in distinct groups; leg claws with ventral clawlet; fourth pedipalp segment with numerous dorsodistal fine hairs and a thickened medial seta, but without a ventral peg-like seta.