Fig. 2. Muscle field potential recordings, instantaneous frequencies and mean frequencies. (A) Examples of larval activity at different ages (h.p.h.). Solid bars indicate dark periods (of 5 s). (B) Plots of instantaneous frequency of tail contractions (Hz) vs time (s) from the traces shown in A. Every tail contraction is represented by one dot in the chart. Solid bars under plots and traces indicate the 5s dark period, imposed by the automatic shutter. There was an exact correspondence between the beginning of the light off-period and the beginning of swimming activity. Series of tail flicks, with a mean frequency of about 10 Hz, can be seen to precede or follow swimming bursts. Tail flicks are of larger amplitude than the potentials during swimming periods. Tail flicks can also be seen at the beginning of the shadow response that developed at 1.5 h.p.h. The frequency of potentials during the shadow response increased during larval aging and from 2 h.p.h. it was always higher than spontaneous swimming frequency. The 3.5 h.p.h. plot is also an example of how an active larva could change frequency of swimming if stimulated by a step-down in light. tf, tail ficks; ss, spontaneous swimming; sr, shadow response. (C) Mean frequency of muscle field potentials generated by tail flicks, spontaneous swimming and shadow response at different larval ages (h.p.h.). Frequency values (Hz) are means ± s.d.. Open triangles, tail flicks; grey circles, spontaneous swimming; black circles, shadow response.