Acilius semisulcatus male. Keyed using Taxonomic revision of the Holarctic diving beetle genus Acilius Leach (Coleoptera: Dytiscidae) (Bergsten & Miller, 2006; Systematic Entomology 31 (1), 145-197). A number of holarctic species have a black ventral body surface (the metacoxa and metasternum at the very least). Of these species, A. semisulcatus and abbre*viatus are distinct in having "abdominal sternum II laterally and a large distal part of metafemur normally yellow" (clearly seen above). A. abbre*viatus is a western species, and can be differentiated from semisulcatus by the preapical band of the elytra and the shape of the mesotarsal claws.