The mites known to parasitize the lower respiratory tract of birds are discussed and a host list of the various species is given.
Sternostoma tracheacolum Lawrence, 1948, is redescribed and the variations in specimens from various hosts and localities are analysed. The synonymy of S. castroae with S. tracheacolum is confirmed and S. meddai and Agapornyssus faini are made new synonyms of S. tracheacolum. The specimens found in canaries vary widely according to the origin of the host. Those from South African and Italian canaries are closely related morphologically and present relatively long chelae, while specimens from Belgian and North American canaries have much smaller chelae. An intermediate form is found in a Brazilian canary. S. tracheacolum is thought to use wild birds as its normal hosts since it has been found repeatedly in birds in North America, as well as in other parts of the world, and because its presence in the trachea and lungs in these birds seems to be much better tolerated than in the canaries. Canaries are thus probably infested secondarily, and it seems that Passer domesticus has served in the transfer of the mites between the two groups. The specimens from wild birds present the same variability as those of the canaries, and one can also distinguish three different groups on the basis of the length of the chela. The origin of these variations is discussed. Geographical or biological isolation of the host probably plays a more important role than the host itself in the production of variation.
Hypopi representing probably two species of the genus Falculifer have been found in the air-sacs and lungs of two central African birds. Also Speleognathus poffei, S. striatus and Boydaia sp. have been taken from the lungs or trachea of their hosts in Ruanda-Urundi or U.S.A.
Collection of most North American material was carried out under a research grant (G-11035) from the National Science Foundation.
The authors wish to thank the following for their co-operation and assistance in this study by making available various collections of Sternostoma tracheacolum: E. W. Baker, U.S. National Museum, Washington, D.C.; G. Owen Evans, British Museum (Natural History), London; D. P. Furman, University of California, Berkeley; S. Grétillat, Laboratoire Central de l'Elevage, Dakar; R. F. Lawrence, Natal Museum, Pietermaritzburg; and G. Lombardini, Rome.