Fig. 6. Sample recording showing the change in electrical potential of a salicylate-selective microelectrode positioned in droplets of secreted fluid or calibration solutions. Microelectrode voltage was sampled at 3 Hz by the data acquisition system. The labels 5, 0.5 and 0.05 refer to calibration solutions containing 5, 0.5 or 0.05 mmol l^-1 salicylate, respectively, in Drosophila saline. d1 indicates the voltage recorded in a droplet that was secreted over 30 min prior to the addition of salicylate to the bathing droplet and indicates the background voltage recorded due to endogenously secreted compounds. d2 indicates the voltage recorded in a droplet secreted 30 min after the addition of 0.05 mmol l^-1 salicylate to the bathing saline.