THE NECK MOTOR SYSTEM OF THE FLY CALLIPHORA-ERYTHROCEPHALA .1. MUSCLES AND MOTOR NEURONS

1. The organization of muscles that move the head was analysed from serial-section reconstructions of whole animals. Muscle innervation was resolved by reduced silver and ethyl gallate staining or by cobalt or Lucifer Yellow fills from single motor terminals at identified muscles. 2. The organization of cuticular attachments for neck muscles, and the arthrology of joints between the rear of the head and cervical sclerites of the prothorax, was analysed from cleared exoskeletons. 3. As a rule muscle blocks consist of two or more single muscles each supplied by only one motor neuron. Motor neurons supplying associated muscles of a block share the same peripheral motor nerve and have dendrites in a common area of neuropil. 4. The complex shape of motor neuron dendrites suggests that a variety of sensory fibres and interneurons coverge on them. This is confirmed by observations of inputs from the halters and prosternal organs, from hair receptors fields on the notum, and from chordotonal organs in the prothorax. 5. Cobalt coupling reveals that certain neck motor neurons receive input either from visual interneurons or from descending neurons that relay visual information.