This book is written for our friends in Germany and abroad to give an account of our main interests during the last years. We gratefully take the opportunity provided by Springer Verlag with the Series Studies of Brain Function to present some of our recent work on vision in Drosophila in a coherent form and in a broader context than publications in periodicals would permit. We want to picture the visual system of the fly fitting it into the natural history from which our own vision emerged. In this perspective genetics seems to be a useful ap proach. Our study is a rather personal exercise....