1. Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, UK
2. Current address: Department of Biology, City College of New York, City University of New York, Convent Avenue at 138th Street, New York, NY 10031 USA
3. Department of Biology, Lund University, Sölvegatan 35, SE-223 62 Lund, Sweden