Characterisation of Large Second-Order Ocellar Neurones of the Blowfly Calliphora Erythrocephala

1. Blowflies have twelve large, second-order ocellar neurones (L-neurones) with axons in the single ocellar nerve. These neurones have fairly restricted arborizations in the posterior slope neuropile of the protocerebrum and cell bodies in the nerve, near to the fused ocellar retinae. 2. Like ocellar L-neurones of other insects, or large second-order neurones of the fly compound eye, blowfly L-neurones hyperpolarise in response to increases in light intensity and depolarise in response to decreases in light intensity. Both polarities of response have a strong phasic component. Adaptation to sustained illumination shifts the intensity­response curve, with little change in its gradient. 3. The maximum responses of blowfly L-neurones to sinusoidal changes in light intensity occur at stimulus frequencies of 5­10 Hz. 4. Hyperpolarising an L-neurone with small currents causes an increase in input resistance. Larger hyperpolarising currents cause oscillations in the membrane potential. The amplitude of the oscillations increases with current strength. Repolarisation generates brief rebound spikes of variable amplitude. 5. Injection of small hyperpolarising currents increases the amplitude of a response to a subsaturating pulse of light. This effect is not seen for saturating responses to light and is likely to be due to the increase in membrane resistance caused by hyperpolarisation.