On the Role of Light in the Mating Behavior of Drosophila subobscura

Unlike other related species wild flies of Drosophila subobscura require light for successful ma­ting. Two explanations for the influence of light on the reproductive behavior could be suggested: 1)Light increases sexual activity. 2)Light mediates the transmission of visual courtship elements. In order to decide this question the mating success of wild-type and mutant flies was tested at light of different wave-lengths. It turned out that mutant flies with reduced ommochrome compo­nent of the eye pigment were not able to copulate at wave-lengths above 570 nm whereas the other strains still copulated at wave lengths close to the limit of their spectral sensitivity. Since spectral sensitivity of the photoreceptors is assumed to be the same in all strains tested the difference in mating success must be ascribed to effects caused by the lack of pigment. Thus an investigation of the optomotor response was carried out. It revealed that mating success is clearly correlated with the ability to discriminate contrast. From this result it can be concluded that the exchange of visual signals and not merely the perception of light is essential for the courtship of Drosophila subobscura. Optical stimuli may be important at two different stages of mating behavior: at the begin­ning, when the male is activated by the sight of a female, and during the wing dance, where the movements of the partners are coordinated optically.