The Functions of the Ocelli of Calliphora (Diptera) and Locusta (Orthoptera)

Abstract

1. By determining the appropriate physical constants of the ocellus it is shown that in both Calliphora and Locusta the focal plane lies much deeper than the retinal space so that the eye cannot perceive form with any accuracy. 2. The large visual fields afforded by their position on the head and the position and shape of their retinae suggest that the ocelli are efficient light-gathering units. 3. The ocelli alone are unable to mediate phototactic responses to light of the intensities employed in these experiments. 4. The efficiency of phototactic reactions mediated by the compound eyes is, under certain conditions, significantly greater when the ocelli are intact than when they are occluded. 5. In Calliphora this stimulatory influence of the ocelli is not manifest at a low light intensity. 6. In Locusta nymphs, ocellar occlusion produces an effect only when the compound eyes are light-adapted and orientation depends on light stimulating the anterior part of the eye. 7. In both species, the experimentally observed behaviour is explicable on the theory that ocellar occlusion causes a decrease in the sensitivity to light of the anterior part of the compound eye.