Morphology and physiology are two fields which cannot be separated. This
statement needs to be amplified: purely factual results of a
morphological or physiological nature only have real value when they are
gained in the context of certain guiding, embracing questions. By
themselves they are mostly of little value, because only a guiding
hypothesis or theory is of any importance. Equally, a physiological
question will always raise questions as to the morphological substrate,
and vice versa. Thus, Wiedemann's discovery, for instance, that the
visual cells in each ommatidium of the dipterans have differing fields
of vision has revived the question as to what the optical properties of
individual visual cells, and the complete ommatidium, might be and how
neighboring ommatidia interact. These questions in turn led to that of
the morphological, neuronal wiring diagram of the visual cells in the
optical ganglia. Within the realm of invertebrates, the morphological
and physiological problems of visual perception have been resolved in
very different ways on various levels, from the photo- receptor to
higher centers; despite many investigations, however, there remain
unsolved problems. The first chapter of Vol. VII/6B deals with the
neuroarchitecture in the visual system of two groups: crustaceans and
insects. These systems are best known through recent investigations. The
second chapter is devoted to the neural principles in the visual system
of insects. It could not have been written without parallel research
into morphology.