1. Department of Biology, University of Western Ontario, London, Ontario N6A 5B7, Canada
2. Current address: Fish and Wildlife Branch, Saskatchewan Ministry of Environment, 3211 Albert St., Regina, Saskatchewan S4S 5W6, Canada
3. Harold W. Manter Laboratory of Parasitology, W 529 Nebraska Hall, University of Nebraska State Museum and School of Biological Sciences, University of Nebraska, Lincoln, Nebraska 68588-0514, USA