The Peripheral Photoreceptor in Hornets: Microanatomy and Functions

Our study deals with the ultrastructure of the peripheral photoreceptor of the Oriental hornet (Vespa orientalis, Hymenoptera: Vespinae), with special attention paid to those on the frons plates of the forehead. Photoreceptors are distributed on the entire cuticular surface of the hornet and upon outside view each photoreceptor appears recessed within a pore. The photoreceptor cells are comprised of a base located above the basal membrane from which arises a process (bulge) that penetrates into a cup-like cavity extending between the layers of the cuticle up to the epicuticle. The process of the photoreceptor resembles an oblong egg and between it and the surrounding exocuticle there is a gap of about 30-50nm. The process connects to the body of the photoreceptor cell by a narrow neck-like region. An axon enwrapped in glia cells infiltrates between the process and the cuticle to form a synapse with the membrane of the process at about two-thirds of its length. The photoreceptor cells contain horseshoe-shaped microlamellae, while the process boasts an outer layer of microfilaments that are arranged perpendicular to the periphery and underneath this layer-two layers of a vitreous body which is composed of a very loose network of widely scattered connective tissue bearing very delicate fibers. We conjecture that in the Oriental hornet, which is a strictly daytime flier, these photoreceptors function as directional eyes or light sensors during foraging flights. Furthermore, the process, extending beneath the cuticular surface and contained within a cavity resembling the clapper of a bell, may possibly function also as an accelerometer. Conceivably, all the photoreceptors in the hornet's body together with their neuronal connections comprise an organ composed of numerous individual micro-gyroscopes. These are sensitive to light, to acceleration from different directions and to differences in compression during daytime flight of the hornet, and via the axons transport information regarding the pitch, roll and yaw of the entire body.