ABSTRACT One of the large descending brain neurones in each half of the central nervous system of a locust is probably important in regulating the attitude of the locust’s flight in the pitch plane. This function is suggested by a consideration of the stimuli which excite the interneurones, and of the muscles whose motoneurones they excite. Each of these interneurones has its cell body in the protocerebrum and its axon descends the ipsilateral connective and has branches in all of the thoracic ganglia. The interneurones are excited by currents of air directed at the front of the head. An increase in the intensity of light falling on the median ocellus causes a reduction in the frequency of spikes produced by stimulation of the wind-sensitive hairs, and a reduction in the intensity of this light causes an increase in the frequency of these spikes. Each interneurone makes excitatory connexions with motoneurones of the subalar and dorsal longitudinal flight muscles ipsilateral to it.