Taxonomic characters of the male endosomal structure in the genus Rheumatogonus Kirkaldy (Hemiptera: Gerridae), with descriptions of four new species from Borneo and Sri Lanka

The species of Rheumatogonus (Gerridae: Ptilomerinae) from Thailand, Borneo, Mindanao and Sri Lanka are revised. Four new species, collected from northern Borneo and Sri Lanka, are described: R. esakii spec. nov. from Brunei, R. vantoli spec. nov. and R. inusitatus spec. nov., both from Sabah, and R. cheliforus spec. nov. from Sri Lanka. Redescriptions of the genus and the four known species assigned to this paper are provided. A key to species with diagnostic illustrations is presented. The male genital structures are illustrated in detail, as they provide the most reliable specific characters. The pilosity on the connexival segment VI is used for identifying the females.