Small-field neurons associated with oculomotor control in muscoid flies: cellular organization in the lobula plate

In muscoid flies, the lobula plate is the last station in the optic lobes for processing spectrally independent information from retinotopic afferents. Until recently, it was thought that most lobula plate neurons were color-insensitive wide-field tangential neurons that respond to direction-specific motion. It has been suggested that certain of these supply inputs to premotor descending neurons involved in the control of flight and head movements. The present account describes a Golgi and cobalt-silver analysis that reports evidence for additional lobula plate outputs, which are numerically complex and structurally elaborate. Beneath a retina with approximately 4,000 ommatidia, each of at least 15 populations of morphologically distinct small-field neurons comprises approximately 110-440 elements that contribute to an isomorphic neural assembly subtending the whole retina. Morphologically small-field efferents form three classes according to the origin of their axons and their arborization in the lobula plate and lobula. Neurons arising from the lobula plate, or shared by it and the lobula, visit dorsal descending neurons supplying the neck and flight motor in contrast to output neurons from the lobula, which project to ventral descending neurons supplying leg motor neuropils. The possible functional significance of small-field lobula plate outputs onto descending neurons in the dorsal deutocerebrum is discussed.