1. Department of Biological Sciences, Smith College, 44 College Lane, Northampton, MA 01063, USA
2. Program in Organismic and Evolutionary Biology, University of Massachusetts, 611 North Pleasant Street, Amherst, MA 01003, USA
3. Present address: Department of Chemistry and Biochemistry, University of Colorado, 215 UCB, Boulder, CO 80309, USA
4. Department of Biology, University of Florida, PO Box 118526, Gainesville, FL 32611, USA