1. a Swedish Museum of Natural History, Stockholm, and (postal address) Department of Zoology , University of Göteborg , Box 25059, Göteborg, S‐400 31, Sweden
2. b C.N.R. Istituto Italiano di Idrobiologia , 50–52, Largo Vittorio Tonolli, Pallanza, I‐28048, Italy
3. c Istituto di Zoología , Universita di Bologna , Bologna, 1–40126, Italy