Micromorphology of the dorsal ocelli of the Oriental hornet.

The development of vision in animals throughout evolution has been reviewed by Sir Stewart Duke-Elder, whose survey of the sense of sight ranges from lowly Crustaceans to mammals and man. According to Duke-Elder each ocellus is formed by the "fusion of two or more ocelli, each with its own retina and pigment cup". This process of 'ocellation' probably occurred independently in a number of phyla. So far as Hymenoptera are concerned, at least in Bombus, only the median ocellus has retained any evidence of its 'dual' origin. In hornets, there are three ocelli which are organized on the dorsal part of the head and receive their innervation from the optical lobes that are located in the protocerebrum. Proceeding from the exterior to the interior, the ocelli are generally composed of a cornea, followed by corneogenic cell layers and then by a layer of sensory cells from which emerges the ocellar nerve. Thus, in wasps and hornets the ocelli consist of a group of visual cells beneath a common lens and they possibly accentuate the response to light stimuli that are perceived by the compound eyes. Recently, the ocelli have been ascribed roles in orientation and navigation. In honeybees the sensitivity of the ocelli to light at various wavelengths was compared with that of the compound eyes, and was found to be different, to wit: in the ocellus there were two peaks--one of UV light at a wavelength of 335-340 nm and the other of green light at 490 nm, whereas in compound eye the peaks were at 350 nm and 540 nm. From all the foregoing, it would seem that the role of the ocelli in insects in general and in hornets in particular is not yet sufficiently clear. The present study was undertaken to elucidate the structure of the ocelli in the Oriental hornet and possibly also their function.