The neck motor system of the fly Calliphora erythrocephala - II. Sensory organization

1. The electrophysiology of the four paired nerves innervating the fly's neck muscles were examined by extra- and intracellular recording and staining with Lucifer Yellow. 2. Units that responded to moving black and white gratings and that were, therefore, termed 'visual' were recorded in all four neck muscle nerves. These visual motor neurons were direction-selective responding most vigorously to a single direction by excitation. 3. The preferred direction and the field of view of visual motor neurons was specific for each of the four neck muscle nerves and generally reflected an organization into horizontal- and vertical-motion-sensitive units. 4. The response characteristics of visual motor neurons corresponded to the structural relationship between their dendrites and visual interneurons in the lobula plate or terminals of descending neurons that were themselves associated with specific lobula plate neurons. 5. Visual units were only a small fraction of all neck motor neurons. A non-visual motor neuron responding to mechanical stimulation of wing and antenna and four mechanosensory fibre tracts associated with the prothoracic neck motor neuropil are described in detail.