The threshold for pymetrozine action on the femoral chordotonal organ is 10–8 mol l–1. The threshold for pymetrozine action on the femoral chordotonal organ is 10–8 mol l–1. Response of the middle leg femoral chordotonal organ (fCO) was monitored during ramp-and-hold stimulation of the receptor apodeme [ordinate; spike counts during elongation (filled circles) and relaxation (filled squares) ramps, baseline activity subtracted]. Different concentrations of pymetrozine were applied, starting with 10–9 mol l–1 and increasing in steps of 5-fold or 2-fold (abscissa) every ∼25 min until stimulus-related responses had completely vanished. 10–5 mol l–1 was applied as the final control concentration. Numbers of measurements per data point (n=N) range from 6 (above-threshold concentrations) to 12 (threshold area). Median values and 25% percentiles are shown. Graph for relaxation stimuli is offset to the right by 20% for better visibility of percentiles. Note that calculation of medians from data obtained in several individuals obscures the fact that in any given animal pymetrozine had an all-or-none effect, although at slightly different concentrations; for details, see text. Jessica Ausborn et al. J Exp Biol 2005;208:4451-4466 © The Company of Biologists Limited 2005