Neural mechanism by which controlling inputs influence motor output in the flying locust.

The wing motion during locust flight is caused by alternating contractions of elevator and depressor muscles. These muscle contractions are initiated by bursts of impulses which occur alternately in the elevator and depressor motor neurons (Wilson & Weis-Fogh, 1962) (see Fig. 1). The isolated thoracic ganglia can produce this pattern of impulse bursts in the absence of any patterned input, or even with abnormally patterned input (Wilson, 1961). Thus the thoracic central nervous system is relatively autonomous in the generation of the basic flight pattern. On the other hand, inputs to the thoracic ganglia must be able to produce variation in the pattern of activity in the flight motor neurons so that the flight behaviour will change appropriately as circumstances differ. That is, there must be mechanisms for the regulation of the system generating the flight pattern. In this paper two different techniques are used to analyse these mechanisms. The first technique is to stimulate a flying locust with relevant inputs and then to study the resulting changes in motor output pattern. The second technique is to analyse other kinds of variation present in normal output patterns (especially the short-term variation) and then to infer from the characteristics of the variation what factors may have caused it. The first technique has been used by previous workers. When the pitch, or body angle, of the locust is changed, sensory structures produce altered input to the thoracic ganglia (Gettrup, 1966). As a consequence, the amount of activity in particular motor neurons changes so that the wing motion is adjusted appropriately (Wilson & WeisFogh, 1962). These changes develop slowly, over 4-6 sec. (about 70-100 wingbeat cycles) (Gettrup, 1966). Similarly slow is the response to stimulation of nerves from the stretch receptors of the wing hinges; wingbeat frequency increases with a time constant of about 2 sec. (Wilson & Wyman, 1965). Both of these responses occur with little change in the relative timing of activity in different units. Basically similar observations are presented in this paper for the motor output patterns during the production of roll torques in response to asymmetric lighting and for the responses to stimulation of the wing nerve. Thus there is a variety of controlling inputs which produce slow changes in the average parameters of the flight motor pattern. In addition to these slow changes in average parameters there is more rapid variation of some parameters. There are no known inputs which produce the cycle-to cycle changes in the flight pattern. Hence the first technique of analysis (the manipulation