Fig. 6. The activity of the principal planta retractor muscle (PPRM) is highly correlated to that of the ventral muscles. (A) Electromyogram (EMG) recordings from the origin of PPRM and the insertion point of VIL in abdominal segment A5 were made in a restrained larva. Cycles of retraction (gray regions) and adduction were evoked by mechanically stimulating the planta hairs and the medial hairs, respectively. The activity index for each muscle was calculated by demeaning and rectifying the EMG (no smoothing), then integrating the voltage using a 200 ms bin. In most cases, activity in the two muscles was coincident but not identical. (B) A cross-correlation analysis (MatLab, function xcorr) of the activity index for each muscle shows that they are highly correlated with little or no response lag.