The Function of the Insect Ocellus

Abstract

ABSTRACT Little light is thrown on the function of the insect ocellus by blinding experiments or by a study of its distribution within the class. The ocellus cannot, on optical grounds, receive an image, but can only be affected by changes in intensity of illumination. The ocellar nerves are characterized by a small number of constituent fibres, and by the large diameter of the latter. When the ocellus is darkened, the electrical response in the nerve consists of a decrease in potential near the ocellus, relative to the cut end of the nerve. No impulses were detected in the ocellar nerve. When the ocellus is darkened, or one of the lateral ocellar nerves cut, there is a continuous discharge of nerve impulses in the commissures. This ceased as soon as the ocellus was reilluminated. It is suggested that darkening the ocellus causes depolarization which spreads down the ocellar nerve and depolarizes a ganglion in the brain, thereby inducing the discharge of impulses down the commissures. It is pointed out that this might facilitate reflexes at lower centres, and so accelerate the animal’s response to shadows.