Fig. 5. Representative spatial scans of K⁺ microelectrode voltage difference along the lower (MT1–3), main (MT4–6) and distal segments (MT7–9) of the Malpighian tubules (MT) of Drosophila melanogaster. Segments were divided into 4–8 sites 100µm apart, with site number 1 in all cases closest to the ureter. 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.