Revision of the Oriental and Australasian diving beetle genus Sandracottus Sharp, 1882 (Coleoptera, Dytiscidae, Dytiscinae)

A comprehensive revision is presented of the Oriental and Australasian diving beetle genus Sandracottus Sharp, 1882 (Coleoptera: Dytiscidae: Dytiscinae: Aciliini) and seven junior subjective synonyms are proposed. Sandracottus guerini Balfour-Browne, 1939, syn. nov. is a junior subjective synonym of S. femoralis Heller, 1934; S. manipurensis Vazirani, 1969, syn. nov. of S. hunteri (Crotch, 1872); S. mixtus Blanchard, 1843, syn. nov. of S. chevrolati (Aubé, 1838); and S. angulifer Heller, 1934, syn. nov., S. nauticus Sharp, 1882, syn. nov., and S. palawanensis Satô, 1978, syn. nov. of S. maculatus (Wehncke, 1876). Finally, S. vijayakumari Anand et al., 2021, syn. nov. is a new synonym of S. dejeanii (Aubé, 1838). New status is assigned to S. bakewellii guttatus (Sharp, 1882), stat. nov. as well as S. hunteri (Crotch, 1872), stat. rev. Lectotypes are designated for the following taxa: Dytiscus flavocinctus Guérin-Méneville, 1830, Hydaticus chevrolati Aubé, 1838, Hydaticus insignis Wehncke, 1876, Sandracottus baeri Régimbart, 1899, Sandracottus bizonatus Régimbart, 1899, and Sandracottus ornatus Sharp, 1882. A neotype is designated for Hydaticus maculatus Wehncke, 1876. In total, three Oriental species, two of which also occur in the East Palaearctic, six Southeast Asian species, one species from New Guinea and the Moluccas, and one from Australia with an endemic subspecies in Central Australia (S. bakewellii guttatus) are recognised. Each taxon is presented with a diagnosis, habitat preferences, conservation assessments, distribution data, and a comprehensive bibliography. Important characters (habitus, dorsal colouration, median lobes and parameres) are illustrated. All currently valid taxa are redescribed. Sandracottus jaechi Wewalka & Vazirani, 1975 from Sri Lanka, S. bizonatus from Borneo, S. insignis from the Philippines and S. rotundus Sharp, 1882 from Sulawesi are recommended to be listed in the next IUCN red data book. A key to all species is provided.