The Central Nervous Control of Flight in a Locust

That proprioceptive feedback is important in the regulation of locomotory patterns has been widely demonstrated. In fact, reflexes based on such peripheral loops have been suggested as adequate to produce the pattern and rhythm of movements in arthropod walking and flight. The present study characterizes some of the sensory input during flight of the locust which could be expected to be involved in these reflexes. However, it will be shown that this input is unnecessary for the establishment of a patterned oscillation in the thoracic ganglia which appears to be the same as that which produces the flight movements. The feedback from the moving parts of the flight system modifies considerably the frequency and details of ordering of a pattern which, nevertheless, is inherent in the ganglia. The elicitation and maintenance of this pattern depends on some source of excitatory input which need not be phased to the flight movements. The desert locust Schistocerca gregaria Forskal is a particularly favourable object for such studies for a number of reasons. It can be induced to fly in the laboratory for extended periods of time with almost constant, cyclical and strictly co-ordinated movements of its two pairs of wings (Weis-Fogh, 1956a). The fundamental oscillation of each wing is up and down, but superimposed upon this is an active twisting of the wing surface, the pronation, starting at the beginning of the downstroke. The reverse supination is mainly passive-elastic and prevails during the upstroke. Each wing is moved up by the contraction of a medial row of vertical elevators and moved down partly by elasticity (Weis-Fogh, 1959), partly by a dorsal longitudinal depressor muscle and, finally, by a lateral row of vertical depressor muscles which also control the twisting (two basalar and one subalar on each side in each segment). Each of these muscles consists of one to four fast-type motor units which are activated zero, one, or two times per wing stroke (Weis-Fogh & Wilson, 1961). The typical motor output then consists of a rather simple score of nerve impulses strictly correlated in time with the observed wing movements and repeated at the same frequency (Weis-Fogh & Wilson, 1961). It is this remarkable quantitative reduction in number of units and activities of the wing system that makes it worthy of study. The studies of Weis-Fogh (19566) on sensory mechanisms in locust flight and the hypothesis on the role of these in the production of the motor command formed the basis for the experiments described here. Since the behavioural experiments showed