Fig. 4. Representative examples of K⁺ microelectrode voltage difference at single sites in the lower (A), main (B) and distal (C) segments of Malpighian tubules of Drosophila melanogaster. Bathing medium K⁺ concentration was 20 mmoll^-1 in all experiments. The electrode was vibrated through an amplitude of 100µm and positioned so that its point of closest approach was approximately 10µm from the tubule surface (open horizontal bar) at each site. Hatched horizontal bars denote the background signal measured when the vibrating electrode was positioned 1000µm from the tubule surface. Note that positive voltages denote increases in [K⁺] of the unstirred layer relative to the bath and negative voltages denote decreases in [K⁺] of the unstirred layer relative to the bath.