The neuronal control of dragonfly flight

The mechanical action and innervation of the major flight muscles of aeshnid dragonflies are described. All flight muscles investigated are innervated by at least three motoneurons, and one by as many as fifteen. Cell bodies of motoneurons that innervate the same muscle are clustered together, and have similar, widespread dendritic branching patterns. Motoneurons of leg muscles have greater variety in cell body size and position than the major flight motoneurons. Striking similarities between the organisation of motoneurons in dragonflies and other insects raise interesting questions about the evolution of insect nervous systems. Intracellular recordings have been made from identified motoneurons, singly and in pairs, in tethered flying and non-flying dragonflies. During flight large rhythmical fluctuations in membrane potential, usually accompanied by several spikes, occur in flight moteneurons. The frequency of these fluctuations varies in the same motoneuron at different times, and can be different in different motoneurons simultaneously. Phase relations between fluctuations in different motoneurons can change. Several interneurons have inputs onto flight motoneurons, and each motoneuron receives inputs from a separate set of interneurons. Motoneurons have inputs to central neurons, forming feedback loops. Many receptors provide information about wing movements to the central nervous system. As in the locust, populations of non-spiking and spiking interneurons drive the flight motoneurons. In the hard cuticle of the basal region of one wing vein there are groups of small, elongate innervated indentations called crevice organs. They differ in structure from any previously described insect mechanoreceptor and probably monitor cuticular strains during wing movements