Heleniella helvetica sp. n., a cold stenothermic species inhabiting the upper Rhône catchment in central Switzerland [Diptera, Chironomidae, Orthocladiinae]

A description of the male adult of Heleniella helvetica sp. n is provided based on material collected in pristine lotic habitats delimited by the crenal and rhithral of the cold stenothermic springfed stream, which is located in the upper Rhône catchment (Mutt stream, central Switzerland, altitude 1800-2600 m). H. extrema Albu, 1972 represents the nearest species to H. helvetica sp. n. from which it can be separated by the following combination of characters: chaetotaxy of the thorax; low value of the BR ratio; distribution pattern of setae on tergite IX and anal point; anal point broad basally, parallel-sided medially and bearing dorsal setae; inferior volsella long nose-like lobe projecting downwards; gonostylus twisted, macroseta unusually inserted medially on a distinct cylindrical and projecting prominence. H. extrema and H. helvetica sp. n. currently belong to the emended extrema-group, which includes two additional new undescribed species both confined to glacier habitats: H. sp.1, known from the Mutt stream located in central Switzerland (altitude 2200-2400 m); H. sp. A, known from the Eastern Pyrenees (France, altitude 2200-2300 m). Taxonomic remarks, key to male adults of known Heleniella species from Europe and comments on the ecology and geographical distribution of the new species are given.