Visual system of the european hummingbird hawkmothMacroglossum stellatarum (sphingidae, lepidoptera): Motion-sensitive interneurons of the lobula plate

Abstract

The European hummingbird hawkmoth Macroglossum stellatarum is a member of the sphingid family, denoted by their ability to hover in place in front of flowers. The fast and acrobatic flight behavior is thought to be under the control of the optomotor system both during normal, forward flight and during stationary, hovering flight. Electrophysiologic and neuroanatomic studies presented here reveal large-field, motion-sensitive neurons in the lobula plate. Anatomic and physiologic features characterize these nerve cells as horizontal and vertical motion-sensitive neurons. Horizontal cells react best to horizontal movement of vertical gratings moving in either direction and show no response to vertically moving horizontal gratings. The dendritic arborizations of horizontal cells are found characteristically in the two outer layers of the lobula plate. In contrast, vertical cells react best to upward or downward motion of horizontal gratings. They are not sensitive to horizontal-moving gratings. The dendritic arborizations of the vertical cells are restricted to the two innermost lobula plate layers. These anatomic and physiologic features, as well as a number of other characteristics, such as tonic responses and cosine-shaped tuning curves, are most reminiscent of neurons that comprise the horizontal and vertical systems described from the fly, thus suggesting comparable roles in the control of optomotor behavior. The present study also identifies a small group of neurons that exhibit phasic responses and reactions to specific orientations of the pattern. These cells are discussed with regard to their possible roles in control tasks, such as edge-orientation detection or the detection of sudden displacements of the visual surround.