Hydrodromidae, K.Viets, 1936

Gerecke, Reinhard, 2017, The water mites of the genus Hydrodroma (Acari, Hydrachnidia, Hydrodromidae) in Europe and Africa, Ecologica Montenegrina 13, pp. 1-24 : 2-4

Family Hydrodromidae

The family Hydrodromidae is an isolated clade in the Hydryphantoidea. In addition to the doubtful genus Oxopsis Nordenskiöld, 1905 , known from a single specimen collected in Sudan (see Cook 1974), representatives, recorded from all continents except for Antarctica, are very similar in general idiosoma shape and attributed to one single genus, Hydrodroma .

Concerning many features of idiosoma and gnathosoma, hydrodromids are characterized by a high degree of homogeneity. In the following, important character complexes are briefly surveyed - for a diagnosis of the family, see Di Sabatino et al. (2010):

1. Idiosoma integument completely soft. Except for appendages, gnathosoma, sclerotized coxal and genital plates and the unpaired excretory pore, no external sclerotization (e.g., muscle insertions, dorsal or ventral plates, secondary sclerite borders) is developed ( Figs 1 A-B). The lateral eye lenses lie free under the integument and are not enclosed in capsules ( Fig. 1 A ) - in contrast to most other water mites families.

2. The uppermost layer of the integument is characterized by dense papillosity. In a deeper stratum, a fine net of meshes is formed ( Schmidt 1935): in top view, six finely sclerotized bars extend from the base of each papilla to form a network of equilateral triangles. Each meeting point of these lines is positioned below the tip of one of six surrounding papillae ( Fig. 1 D ). Formation of the integument provides important information for species distinction: Papillae may differ in shape species-specifically, and they can be homoiomorphic (of one type) or heteromorphic (forming a pattern of two different types). In species with heteromorphic papillae, every large papilla is surrounded by six small papilla-like elevations, resulting in a honeycomb-like structure in top view ( Fig. 2 B ).

3. The genital field is rather uniform and without distinct sexual differences. Paired, movable halfmoonshaped plates bear a rather high number of undifferentiated, small and round acetabula and medially several rows of fine setae ( Fig. 1 B ). In general, females tend to have larger genital plates and higher numbers of acetabula, but with a broad overlap in variability range ( Wiles 1985). Fine sexual differences in setation were found by Meyer (1983) in H. pilosa (sub nom. H. despiciens : male with> 45 pairs, seta L generally> 100 µm; females with <45 pairs, setae generally <100 µm) and by Wiles (1985) in H. despiciens (male: all setae hollow; females: posterior as in males, anterior setae solid, flat, best visible under phase contrast).

4. Legs are rather uniform and, except for differences in segment size, without sexual characteristics. All leg claws are rather simple, sickle shaped, with a fine dorsal clawlet, but without denticulation or a claw blade. Differences between species may be found in the absolute and relative size of claws (larger in stream dwelling species), proportions of legs segments (stouter in stream dwelling species) and, in surprisingly stable species-specific patterns, number and arrangement of long, fine swimming setae. In addition, arrangement and shape of other leg setae, in particular those at distal margins of leg segments 3-5, may provide character states useful for species recognition.

5. Morphology of mouthparts, in particular setation of palps, have often been highlighted in earlier species descriptions, but such characters are often difficult to verify. A very distinct character state of all Hydrodroma species is the very long dorsodistal extension of P-4 ( Fig. 1 C ). The fact that palp segments are weakly sclerotized, with P-2-4 often strongly movable against each other, results in a considerably wide range of measurement values. In the case of the European species, the shape of gnathosoma and its appendages is of limited value for species discrimination ( Wiles 1985).

Figures

Figure 1. Hydrodroma, general morphology. A. Hydrodroma pilosa, living specimen in oblique dorsal view; note the pale pigment dots of the separated, not encapsulated lateral eyes; B. H. pilosa, male in ventral view (legs detached); note dense coxal setation, paired genital plates with numerous acetabula and a circular anal pore sclerite. C. H. pilosa male, palp, medial view; note the extremely elongated dorsodistal extension of P-4. D. Hydrodroma sp. from Madagascar, at right papillose integument, at left underlying triangular net structure (A., D.: photograph Gerecke; B.-C. from Di Sabatino et al. 2010).

Figure 2. A-D: Hydrodroma despiciens, neotypes, Denmark. A. male, integument papillae near lateral eye in tangential view; B. male, integument in top view; C. male, anterior coxal plate with medial setal tubercles on Cx-I; D. female, genital field; E. female, integument in tangential view; F. male, chelicera.