Contents

03 February 1989
Vol 243, Issue 4891

About The Cover

A caterpillar Nemoria arizonaria next to the catkins (staminate flowers) of Emory oak. The spring brood of caterpillars develops into mimics of the catkins; the fall brood develops instead into mimics of oak twigs. This developmental polymorphism is triggered by the larval diet. See page 643. [Photo by Erick Greene, Department of Avian Sciences, University of California, Davis, CA 95616]