The aquatic oribatid mite genus Mucronothrus in Canada and the western U.S.A. (Acari: Trhypochthoniidae)

The aquatic oribatid mite genus Mucronothrus (Trhypochthoniidae) is redescribed on the basis of characters of adults and immatures and its familial placement is reviewed. It is represented in North America by two species. Mucronothrus nasalis, a globally widespread species, is redescribed on the basis of all active instars. In Canada it is now known from Newfoundland, New Brunswick, Ontario, Quebec, Saskatchewan, Alberta, and British Columbia and in the U.S.A. from Oregon and Colorado. Ancillary new distribution records include England and Cape Horn. Misinterpretations of homologies of leg setae in the literature are corrected. A population near Toronto, Ontario, was examined for variation in genital and leg setation; this conforms well with previously described geographic patterns (J. Travé). This population exhibits previously unknown variation in cheliceral form. A second species, Mucronothrus willmanni n.sp., is proposed on the basis of adults and tritonymphs. Current records are from California, Oregon, and Montana, where it inhabits riffles in streams. Only females are known and, like M. nasalis, the species appears to be thelytokous.