Plagiorhynchus formosus Van Cleave, 1918, a Synonym of Plagiorhynchus cylindraceus (Goeze, 1782) Schmidt and Kuntz, 1966

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When he first described Plagiorhynchus formosus, Van Cleave (1918, Trans. Am. Microsc. Soc. 37: 19-48) did not differentiate his new species from European species, other than to state that the eggs lack polar elongations. He did not consider it to belong in the genus Prosthorhynchus, even though he included, but did not mention, the description of that genus by Kostylev (1915, Ann. Mus. Zool. Acad. Imp. Sci. 20: 389-394) in his Literature Cited. It does, however, fit into the concept of Prosthorhynchus, and was transferred there by Travassos (1926, Mem. Inst. Oswaldo Cruz 19: 31-125). While subsequent authors debated the validity of these two genera, Schmidt and Kuntz (1966, J. Parasitol. 52: 520-527) reduced them to subgenera in the genus Plagiorhynchus Liihe, 1911. In his early years of publication, Van Cleave emphasized the distinctness of New World from Old World acanthocephalan faunas. Later authors seem to have taken this for granted, for there is no study which compares Plagiorhynchus formosus with its European counterpart Plagiorhynchus cylindraceus, also a parasite of Turdidae and other passeriform birds. I was stimulated into considering the conspecificity of those two forms when presented with two juvenile specimens of P. formosus collected in an Australian pigeon by Dr. J. H. Arundel, University of Melbourne, Victoria. Through the kindness of Professor Y. J. Golvan, who redescribed P. cylindraceus, (1956, Ann. Parasitol. 31: 350384), I was able to examine numerous specimens of that species from his collection. Careful comparison of all taxonomic characters generally used revealed no difference whatever between American P. formosus and P. cylindraceus from Europe, Israel, and Australia. Hook numbers, arrangements, and sizes broadly overlap in these specimens, as do other characteristics. In their description of Plagiorhynchus (Prosthorhynchus) taiwanensis, Schmidt and Kuntz (1966, loc. cit.) incorrectly stressed the constancy of lemniscus length in this genus. Subsequent examination of a longer series of P. formosus specimens, as well as the European worms, shows variability of the lemnisci that overlaps in both groups. As the length of the lemnisci was the only character separating P. taiwanensis from P. formosus, the former also must fall as a synonym of P. cylindraceus. The life cycles of P. cylindraceus and P. formosus also support their conspecificity. Sinitsin (1929, J. Parasitol. 15: 287) reported cystacanths of P. formosus in the terrestrial isopod Armadillidium vulgare. Schmidt and Olsen (1964, J. Parasitol. 50: 721-730) subsequently presented a detailed description of P. formosus's development in this host. Dollfus and Dalens (1960, Ann. Parasitol. 35: 347349) reported P. cylindraceus from the same species of isopod. As this crustacean was originally European (Van Name, 1936, Bull. Am. Mus. Nat. Hist. 71: 1-535), it seems possible this acanthocephalan became established in North America, and maybe Australia and the Far East as well, as the range of the sowbug was extended. The synonymies of this species are as follows: