THE AQUATIC ORIBATID MITE GENUS MUCRONOTHRUS IN CANADA AND THE WESTERN USA (ACARI, TRHYPOCHTHONIIDAE)

The aquatic oribatid mite genus Mucronothrus (Trhypochthoniidae) is re described on the basis of characters of adults and immatures and its f amilial placement is reviewed. It is represented in North America by t wo species. Mucronothrus nasalis, a globally widespread species, is re described on the basis of all active instars. in Canada it is now know n from Newfoundland, New Brunswick, Ontario, Quebec, Saskatchewan, Alb erta, and British Columbia and in the U.S.A. from Oregon and Colorado. Ancillary new distribution records include England and Cape Horn. Mis interpretations of homologies of leg setae in the literature are corre cted. A population near Toronto, Ontario, was examined for variation i n genital and leg setation; this conforms well with previously describ ed geographic patterns (J. Trave). 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. C urrent records are from California, Oregon, and Montana, where it inha bits riffles in streams. Only females are known and, like M. nasalis, the species appears to be thelytokous.