Key to species of Onisimus

1. Antenna 1 peduncle article 1 longer than head, accessory flagellum 6-segmented, postero-ventral corner of 3 rd epimere almost straight ................................. O. platyceras

2. Antenna 1 peduncle article 1 shorter than head, accessory flagellum 4-segmented, postero-ventral corner of 3 rd epimere sharp ..................................................... O. caspius