OCULOMOTOR CONTROL IN CALLIPHORID FLIES - ORGANIZATION OF DESCENDING NEURONS TO NECK MOTOR-NEURONS RESPONDING TO VISUAL-STIMULI

In insects, head movements are mediated by neck muscles supplied by ne rves originating in the brain and prothoracic ganglion. Extracellular recordings of the nerves demonstrate units that respond to visual stim ulation of the compound eyes and to mechanosensory stimulation of the halteres. The number of neck muscles required for optokinetic eye move ments in flies is not known, although in other taxa, eye movements can involve as few as three pairs of muscles. This study investigates whi ch neck motor neurons are likely to be involved in head movements by e xamining the relationships between neck muscle motor neurons and the t erminals visiting them from approximately 50 pairs of descending neuro ns. Many of these descending neurons have dendrites in neuropils that are associated with modalities other than vision, and recordings show that visual stimuli activate only a few neck motor neurons, such as th e sclerite depressor neurons, which respond to local or wide-field, di rectionally specific motion, as do a subset of descending neurons coup led to them. The results suggest that, like in the vertebrate eye or t he retinas of jumping spiders, optokinetic head movements of flies req uire only a few muscles. In addition to the importance of visual input s, a major supply to neck muscle centers by nonvisual descending neuro ns suggests a role for tactile, gustatory, and olfactory signals in co ntrolling head position. (C) 1995 Wiley-Liss, Inc.