Muscle activity during flight in some large Lepidoptera.

In order to investigate the physiological mechanisms which generate behaviour, it is reasonable to examine first the movements which constitute the behaviour and next the patterns of activity in the muscles which cause the movements. It is then possible to consider the central nervous processes which produce the excitatory output to the muscles. This type of analysis has been extensively applied in studies on the flight of locusts (Wilson, 19640) and flies (Wilson & Wyman, 1963; Wyman, 1965, 1966). In the present paper this approach is applied to the flight of Lepidoptera. Flying insects are favourable objects for the investigation of central nervous interactions by which patterned motor outputs are produced because of the relative simplicity and rhythmic nature of the flight movements and because of the relatively small number of motor units involved. The muscles and skeletons of insects from different orders have many similar features, and it seems probable that these similarities represent homologies (Snodgrass, 1927, 1935; Pringle, 1965). Morphological evidence suggests that there is also a high degree of homology among the components of the nervous system (Schmitt, 1962). Therefore, if insect flight has evolved only once, it is reasonable to expect similarities among the flight-control systems of various insects, and any differences present should be explicable as evolved modifications of a primitive mechanism. Proceeding from this point of view the author began a comparative study on the Lepidoptera. Such a study should help to define those properties which are common to the flight-control systems of all insects and could also uncover differences from which new ideas about the central nervous mechanisms can be derived. Lepidoptera were chosen because the group includes animals which differ widely in size and in wingbeat frequency and because preliminary work by M. Konishi (unpublished) indicated that there are interesting variations in the patterns of muscle activity in different species of this group. The first study of muscle activity during flight in the Lepidoptera was made by Roeder (1951). He showed that in the moth Agrotis there is synchrony between muscle potentials and wing movements. Contractions of the muscles are initiated by the motor impulses, and the alternating activity of antagonistic muscles is timed by the nervous system. This type of flight system is called 'neurogenic' or 'synchronous', in contrast to the ' myogenic' or ' asynchronous' flight system of flies and wasps. In the latter case, the wing movements occur at a greater frequency than the muscle potentials and phase