Hydrodromidae K. Viets, 1936

Gerecke, Reinhard, 2020, The early derivative water mites (Acari: Hydrachnidia, superfamilies Eylaoidea, Hydrachnoidea and Hydryphantoidea) of Madagascar, Zootaxa 4727 (1), pp. 1-77 : 15

Family Hydrodromidae K. Viets, 1936

This is a monotypic, morphologically very distinct family with uncertain relationships to other hydryphantoids. All investigated species of the genus Hydrodroma are, at the larval stage, parasitic on dipterans of the families Chironomidae and Chaoboridae ( Meyer 1985, Smukalla & Meyer 1988). In deutonymphs and adults, a strong morphological homogeneity is observed with regard to numerous character states, and the type species H. despiciens (Müller, 1776) was for a long time considered the rare example of a water mite with a cosmopolitan distribution, being also reported from Madagascar. However, stable, species-specific patterns are found in leg setation as well as other character states such as integument structure, size and arrangement of lateral eye lenses, size of leg claws and proportions of leg segments (Gerecke 2017 and bibliography cited there). In the material available for this study, seven species were identified. They all have in common a similar size (body length in general between 700 and 1500 µm) and a remarkable intraspecific variability in proportions of palp segments. As, in certain limits, structure and arrangement of integument papillae may differ between various body parts, integument structure is described here for the posterior dorsum. Swimming setation is rather homogenous in the anterior pair of legs (absent from I-L, on II-L 0-2 on segment 5), but shows species-specific patterns on III-/IV-L.