Taxonomic revision of the Holarctic diving beetle genus Acilius Leach (Coleoptera: Dytiscidae)

A full taxonomic revision is presented of the Holarctic diving beetle genus Acilius Leach (Coleoptera: Dytiscidae) belonging to the subfamily Dytiscinae and tribe Aciliini. Acilius confusussp.n., previously confused with Acilius fraternus (Harris), is described from north-eastern U.S.A. Acilius latiusculus LeConte and Acilius simplex LeConte are new synonyms of Acilius abbreviatus Aube. Lectotypes are designated for the following names: A. abbreviatus, A. semisulcatus Aube, A. fraternus, A. guerryi d'Olsoufieff, A. sinensis Peschet, A. duvergeri Gobert, A. brevis Aube, A. fasciatus (De Geer), A. maccullochii (Kirby), A. tomentosus Motschulsky, A. sulcatus blancki Peyerimhoff, A. subimpressus Motschulsky and A. laevisulcatus Motschulsky. In total, six Palaearctic and seven Nearctic species are recognized, none of which is Holarctic in distribution. Each species is presented with a diagnosis, full description, habitat preferences, conservation assessments, distribution data and a comprehensive bibliography. Distribution data are presented with a map and a list of countries, or for Nearctic and eastern Palaearctic species, states or larger provinces. A key to all Acilius species is included and each is fully illustrated. The general biology and a taxonomic history of the genus are provided. Acilius abbreviatus and A. semisulcatus are found to hybridize in a narrow zone through middle British Columbia and south-western Alberta, geographically comparable with a previously recognized hybrid zone of several other insect species pairs. More than 5000 specimens were examined.