ABSTRACT
In a variety of Diptera there are five motomeurones which innervate the wing depressor muscle (DLM) (Ikeda, Koenig & Tsuruhara, 1980; King & Wyman, 1980). During flight these motorneurones fire in a repeating cycle. Each motorneurone fires once per cycle. The firing times of the different motorneurones are separated so that no neurone fires immediately before or after any other (Wyman, 1965, 1969a, b). Wyman and co-workers have proposed that this spacing is due to mutual inhibition between the motorneurones (Wyman, 1969a, b;Harcombe & Wyman, 1977; Tanouye & Wyman, 1981). Koenig & Ikeda (1980) have now challenged this theory. They have presented data on the interspike intervals during flight that they believe do not fit the mutual inhibition hypothesis: they suggest that an ‘excitatory effect’ causes the spacing. The present communication argues that the two sets of data are consistent with each other and with the mutual inhibition theory.
It should be noted that while randomly placed stimuli are expected to fall equally at all phases of a cycle, they are not expected to fall equally at all latencies. This is because a short latency stimulus can occur in an interspike interval of any length, but a long latency stimulus can occur only in a long interspike interval. Thus, there should be fewer stimuli at long latencies. See fig. 5 in Wyman (1965) for a complete explanation.
