ABSTRACT
Locusts, like most other insects, possess three ocelli, or simple eyes, in addition to two large compound eyes. Focussed images are not formed on the retinae of the ocelli (Parry, 1947; Wilson, 1978a) and ocellar neurones are influenced by changes in light intensity occurring over wide areas of the visual field. Some of the second-order neurones which extend from the retina of each ocellus to the brain have large axons -amongst the largest found in the locust central nervous system. They are expected to convey information about changes in light intensity to the brain with some urgency. At least two different ways in which these large second-order ocellar neurones (L-neurones) could influence the behaviour of a locust have been suggested. First, by monitoring changes in the orientation of the visual horizon they could contribute to the stability of flight (Wilson, 1978a; Stange & Howard, 1979; Simmons, 1980a). Second, they could facilitate the responses of some central neurones, such as those driven by the compound eyes, when light intensity is reduced (Parry, 1947) – for instance, when a shadow falls over the locust. The observations reported here show that the L-neurones boost the responsiveness of a pair of well-characterized central neurones when the overall light intensity is rapidly decreased.
