Fig. 3. Sal-SeR microelectrode voltage recorded over an excursion of 100 µm in Drosophila saline containing 0.5 mmol l^-1 salicylate. Each point indicates the differential signal recorded between the excursion limits of 100 µm, using the `move, wait, sample' protocol described in Materials and methods. In the absence of any gradients in salicylate concentration the voltage is indicative of the noise of the electrode and measurement system. Variation in the signal is reduced when individual measurements (0-20 min) are compared with averages of three (30-45 min) or five (52-60 min) measurements.