In the comparative physiology of photoreception by the Protista and the invertebrates two aspects are emphasized: (1) the diversity of visual processes in these groups and (2) their bearing upon general mechanisms of photoreception. Invertebrates have evolved a far greater variety of adaptations than vertebrates modifications aiding survival in the remarkably different biotopes they occupy. The number of species in itself suggests this multiformity; each of them has peculiarities of its own, in morphology as well as in physiology and behavior. But these special adaptations are variations on a few great themes. Although the catalogue of invertebrate species is immense, the literature concerning them nearly rivals it in extent-even if one considers only that fraction dealing with visual physiology. Taxonomy proceeds by grouping the species, categorizing them in genera, families, orders, and progressively larger units. Similarly, comparative physiology aims at an analogous, more or less compre hensive, classification. This Part A of Volume VII/6, like Part B that follows it, emphasizes the broad questions that concern groups larger than the individual species; in some cases these questions have general applicability. The middle course between approaches that are too specialized and those that are too general is often elusive, but here we attempt to follow it. The vast number of special adaptations-probably, as we have said, as large as the number of species-is beyond the range even of a handbook."

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. We have neither the expertise nor the ambition to cover the whole literature or to compile- all the knowledge about vision in flies. We are not even in a position to present, with the appropriate weight and breadth, the pioneering contributions of those who established this field of research. Our account mirrors our own intersects. We are selecting and interpreting other peoples' work from this subjective point of view, although we try to present it in its own right. For several years we have been fascinated by the first glimpses of the "inner life" of an insect and we hope to convey this fascination to those who are not discouraged by the scien- tific procedure.