Redescription of Monorcholepis dujardini (Krabbe, 1869) and M. passerellae (Webster, 1952) (Cestoda: Cyclophyllidea: Aploparaksidae) in passerine birds from the Holarctic Region.

Two species of avian tapeworms, Monorcholepis dujardini (Krabbe, 1869) and M. passerellae (Webster, 1952), of the cyclophyllidean family Aploparaksidae Mayhew, 1925 (earlier included in the Hymenolepididae) are redescribed. Relative to congeners, the morphology of the strobila of both species shows strong similarities including a unique form of the cirrus. Separation of these species, however, can be based on the number (40-53 and 25-31, respectively) and size (18-25 microm and 14-18 microm) of the rostellar hooks, although their shape in specimens of both species varies considerably. We examined specimens of Monorcholepis dujardini in Turdus iliacus L., T. philomnelos Brehm and T. nerula L. from the Curonian Spit (Baltic Sea, Russia), T. naumnanni Temminck from Chukotka, Motacilla alba L. from the Kuril Islands (Russia) and Passerella iliaca iliaca (Merrem) from Alaska (U.S.A.). Specimens in T. iliacus from the collection of O. Fuhrmann (Natural History Museum, Geneva, Switzerland) were re-studied. The type material (holotype and paratypes) designated as M. dujardini neoarctica (Webster, 1955) in Ixoreus naevius naevius (Gmelin) from Douglas Island, U.S.A. was re-examined and validity of this subspecies was rejected. A redescription of M. passerellae (Webster, 1952) was based on material in P. iliaca iliaca from Wisconsin (type specimen), and Point Barrow, Alaska, and on the one specimen recorded for the first time, in Turdus iliacus from the Palaearctic (Curonian Spit). Species of the genus Monorcholepis Oshmarin, 1963 and subgenus Aploparaksis (Tanureria) Spassky et Yurpalova, 1968 are characterized by similar topography of the gonads. These generic taxa and the interrelationships of constituent species are discussed.