The type-species of the genus Diorchis Clerc, 1903, D. acuminata (Clerc, 1902) Clerc, 1903, is re-described and illustrated on the basis of the type-material from Fulica atra L. from the Middle Ural, Russia, and new specimens from the same host species from Bulgaria. Since the type-series consists of specimens of two species, a syntype is designated as lectotype. The main differentiating characters of D. acuminata are the diorchoid rostellar hooks, 36-39 µm long, with a foliate epiphyseal thickening of the guard; cirrus-sac of variable length, usually reaching and often crossing the midline of proglottis, occasionally reaching antiporal osmoregulatory canals; evaginated cirrus with cylindrical basal part, bulbous middle part and pipette-like distal part; compact vitellarium situated dorsally to the ovary; copulatory part of vagina with muscular poral and middle portions and an antiporal sac-like reservoir; elongate eggs with polar filaments on their envelopes. The type-specimens of D. ransomi Johri, 1939 and D. longibursa Steelman, 1939 from Fulica americana Gmelin from USA are also re-examined and illustrated. Based on the present results, D. ransomi and D. longibursa are recognised as synonyms of D. acuminata. The previous records of the species are discussed. Diorchis acuminata is recognised as a specific parasite of Rallidae (mainly species of the genera Fulica and Gallinula) in the Holarctic.
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