The quest for the identity of Orthoceratiumlacustre (Scopoli, 1763) reveals centuries of misidentifications (Diptera, Dolichopodidae).

Recently, a species of Orthoceratium was collected in Greece that differs morphologically from the European species commonly presumed to be Orthoceratiumlacustre (Scopoli, 1763). Verification of the identity of the Greek species through comparison with 460 specimens of Orthoceratium from 17 West Palaearctic and one Afrotropical country, and examination of existing type material, revealed that the species recognized as O.lacustre in northwestern Europe for over 250 years is actually O.sabulosum (Becker, 1907), the other known species in the genus, which was originally described from Tunisia. Although the types of O.lacustre have been lost, a comparison of the distribution ranges of both species in Europe provided evidence that the species collected in Greece is conspecific with O.lacustre. Both species have distinct distributions in the West Palaearctic, with O.lacustre largely restricted to the northern border of the Mediterranean basin, and O.sabulosum more widespread, occurring in northwestern Europe, the western, southern, and eastern Mediterranean, the Middle East, and the Afrotropical Region (Tanzania). Both species are redescribed and fully illustrated, a neotype is designated for O.lacustre and a lectotype for O.sabulosum, and a key to males and females is provided. The misidentifications that lasted for over two centuries are explained by the omission by previous authors to study the type specimens, and inaccuracies in species descriptions and keys.