Taxonomic revision of the Holarctic diving beetle genus<i>Acilius</i>Leach (Coleoptera: Dytiscidae)

<jats:p><jats:bold>Abstract. </jats:bold>A full taxonomic revision is presented of the Holarctic diving beetle genus<jats:italic>Acilius</jats:italic>Leach (Coleoptera: Dytiscidae) belonging to the subfamily Dytiscinae and tribe Aciliini.<jats:italic>Acilius confusus</jats:italic><jats:bold>sp.n.</jats:bold>, previously confused with<jats:italic>Acilius fraternus</jats:italic>(Harris), is described from north‐eastern U.S.A.<jats:italic>Acilius latiusculus</jats:italic>LeConte and<jats:italic>Acilius simplex</jats:italic>LeConte are<jats:bold>new synonyms</jats:bold>of<jats:italic>Acilius abbreviatus</jats:italic>Aubé. Lectotypes are designated for the following names:<jats:italic>A. abbreviatus</jats:italic>,<jats:italic>A. semisulcatus</jats:italic>Aubé,<jats:italic>A. fraternus</jats:italic>,<jats:italic>A. guerryi</jats:italic>d'Olsoufieff,<jats:italic>A. sinensis</jats:italic>Peschet,<jats:italic>A. duvergeri</jats:italic>Gobert,<jats:italic>A. brevis</jats:italic>Aubé,<jats:italic>A. fasciatus</jats:italic>(De Geer),<jats:italic>A. maccullochii</jats:italic>(Kirby),<jats:italic>A. tomentosus</jats:italic>Motschulsky,<jats:italic>A. sulcatus blancki</jats:italic>Peyerimhoff,<jats:italic>A. subimpressus</jats:italic>Motschulsky and<jats:italic>A. laevisulcatus</jats:italic>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<jats:italic>Acilius</jats:italic>species is included and each is fully illustrated. The general biology and a taxonomic history of the genus are provided.<jats:italic>Acilius abbreviatus</jats:italic>and<jats:italic>A. semisulcatus</jats:italic>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.</jats:p>