The function of the insect ocellus.

INTRODUCTION Many insects possess two types of eye: a compound eye and a simple eye or dorsal ocellus. Much work has been done on the powers of vision in insects, and it is known that these are almost entirely due to the presence and form of the compound eye. The perception of form and colour, and the appreciation of movement in the visual field, are its recognized functions. No such clearly defined functions have yet been assigned to the dorsal ocellus. The optical system of this organ is such that no image can be formed on the retina. Its occurrence within the class is extremely erratic and bears no apparent relation to the mode of life or the systematic position of those insects in which it occurs. On the other hand, insects deprived of their ocelli show certain peculiarities of behaviour, which suggest that the ocellus is in some way concerned in the response of the animal to light. The object of the present research was to obtain more knowledge of the function of the ocellus, primarily by recording the nervous response to ocellar stimulation. The migratory locust (Locusta migratoria migratoroides) provides a preparation just large enough for electrodes to be placed on the ocellar nerve and electrical changes occurring there to be amplified and recorded. It has also proved possible to record from the circum-oesophageal commissure and thus to investigate, at a point nearer the motor system, the effects of stimulating the ocellus and cutting the ocellar nerves. Information has also been obtained on the distribution of the ocellus among insects, the optical system of the ocellus, and the histology of the ocellar nerve.