1. Swiss Federal Institute of Technology Zurich Experimental Ecology ETH‐Zentrum NW 8092 Zurich Switzerland
2. Department of Biology University of Joensuu P.O. Box 111, SF‐80101 Joensuu Finland
3. Université Libré de Bruxelles Laboratoire de Biologie Animale et Cellulaire Avenue F.D. Roosevelt 50, B‐1050 Bruxelles Belgium