ABSTRACT
The optomotor system of insects has been often regarded as inflexible. The neuronal responses to movement of a striped pattern illuminated continuously with constant intensity were recorded extracellularly throughout the preparation's lifetime. There are two types of endogenous cyclic sensitivity changes which differ in their periods: circadian and ultradian variations. Ultradian sensitivity changes in the visual system have been reported in another invertebrate: the ERG-amplitude in Procambarus fluctuates with a period of approximately 2 h. The sensitivity changes are mainly due to hormonal and/or neuronal effects on the neurones themselves or on preceding interneurones. Proprioceptors influence the sensitivity of the movement detectors: when the neurones are insensitive, spontaneous bursts of nerve impulses sometimes occur. In locusts non-rhythmic, short-term sensitivity changes have been demonstrated in certain visual interneurons. The sensitivity of these interneurones can be increased by external stimuli.
