Helen's twins in the Balkans: discovery of two new Tonnoir, 1919 species closely related to Peus, 1958, with systematic revision of the "lacustris" group (Diptera, Ptychopteridae).

Keresztes & Kappert, and Keresztes & Török, both belong to the subgenus Ptychoptera (Paraptychoptera)Tonnoir (1919) and are described from boggy headwaters in the south Balkan area. These new species are closely related to the range-restricted Peus, 1958, which is known only from Oiti village, Mount Oeta, Phthioitis region, Greece and, together with , forms a morphologically well-defined unit in the subgenus Paraptychoptera. Based on cladistic analyses of 53 different morphological characters using the male antenna, wing, and genital structures, a general revision of the "lacustris" group is proposed with a dichotomous key of species.