The dynamic behaviour of photoreceptor cells in the fly in response to random (white noise) stimulation at a range of temperatures.

1. Photoreceptor cells in Calliphora stygia were stimulated with randomly fluctuating green light while the resulting fluctuations in membrane potential were recorded with intracellular micro‐electrodes. 2. Fourier analysis was used to obtain the frequency response functions between the light intensity fluctuations and the membrane potential fluctuations at a range of different temperatures. 3. The results show that for small light fluctuations the transducer function can be modelled by a cascade of five identical linear exponential filters whose time constants decrease as the temperature of the cell is increased. 4. The time constants of the linear filters and their rate of change with temperature are similar to the electrical behaviour of cell membranes. However, a series of chemical reactions with similar activation energies could also explain the observed behaviour. 5. Evidence is presented that the total light response is a linear summation of discrete waves of depolarization (bumps), which become longer in duration but of constant area as the temperature is reduced.