1. 1Zoological Laboratory, University of Groningen, P.O. Box 14, 9750 AA Haren, The Netherlands
2. 2A.R.C. Research Group on Photoperiodism and Reproduction, Department of Zoology, University of Bristol, BS8 1UG, U.K.
3. 3Zoological Laboratory, University of Groningen, P.O. Box 14, 9750 AA Haren, The Netherlands