Fig. 4. (A) Representative scan showing spatial and temporal heterogeneity of salicylate flux at seven locations along the main segment of the Drosophila Malpighian tubule. At each location, the position of the Sal-SeR microelectrode at the epithelial surface is indicated by the arrowhead of the left-most arrow. The arrows corresponding to 11 additional scans made at intervals of 10 min at the same seven sites are indicated by the adjacent arrows, which have been offset to the right for clarity. ASET software calculated the salicylate-specific signal differences between the two limits of microelectrode excursion by subtracting the voltage at the outer limit of the excursion from that measured at the inner limit. The length of each arrow corresponds to the mean differential signal based on three measurements at each time interval. (B,C) Plots of differential signal of the Sal-SeR microelectrode as a function of time at four sites in the main segments of two Malpighian tubules. The positions of sites 2-4 relative to site 1 are indicated in the legend. Each point is the mean of three measurements.