BEHAVIORAL AND NEURAL BIOLOGY47, 151-157 (1987) The Effect of Light on the Sexual Behavior of Drosophila affinis SCOTT P. MCROBERT AND LAURIE TOMPKINS 1 Department of Biology, Temple University, Philadelphia, Pennsylvania 19122 Drosophila affinis is a species whose mating is facilitated by light. Although sexually mature D. affinis males court D. affinis males and females vigorously in the light, they do not perform any of the courtship behaviors in the dark. This lack of courtship is not caused by the males and females being unattractive in the dark, since D. affinis males and females elicit high levels of courtship from Drosophila melanogaster males in light and in darkness. Fertility tests confirmed previous observations that mating occasionally occurs in the dark, suggesting that some D. affinis males are capable of courting in the dark. © 1987 Academic Press, Inc. Drosophila species have been divided into three categories on the basis of their ability to copulate in the dark. Light-independent flies mate in the dark as readily as they mate in the light; light-facilitated flies can mate in the dark, but do so at a substantially lower rate than they do in the light; and light-dependent flies cannot mate at all in the dark (Grossfield, 1971). Drosophila affinis is a species whose reproductive behavior is facilitated by light (Curtright & Miller, 1979). It is unclear, however, why D. affinis males and females cannot mate as well in the dark as they can in the light. A male may spend less time courting in the dark or may not perform all of the courtship behaviors, which could be due to the female's being less attractive than she is in the light or to the male's inability to find or maintain contact with the female in the dark. Even if the female does stimulate the male to perform as much courtship as he does in the light, copulation still may be unlikely if the female cannot easily be sexually stimulated in the dark or if she provides an acceptance signal that males must usually see before mating can take place. Correspondence should be addressed to Scott P. McRobert at the Department of Biology, Temple University, Philadelphia, PA 19122. 151 0163-1047/87 $3.00 Copyright © 1987by Academic Press, Inc. All rights of reproductionin any form reserved.