The dorsal eye of the mayfly Atalophlebia (Ephemeroptera)

Abstract

The dorsal eye of Atalophlebia has two unusual features, the sensitivity only to ultraviolet (u. v.) light, and the candelabra-shaped rhabdom. In addition, the crystalline cone is surrounded to its tip by a yellow pigment, and the tip tapers gradually as a dense fibre. These details, particularly the pigment distribution, indicate that a superposition image cannot be formed by u. v. light. Also, there is no refracting or reflecting structure that could form a sharp superposition image. Instead, it is suggested that u. v. rays are sharply focused on the cone tip and conducted by the retinula cell columns acting as light guides across the clear zone. Light of longer wavelength, on the other hand, is partially focused through the yellow pigment, and, although it is not seen by the insect, it is available to photoregenerate the visual pigment. This method of boosting sensitivity is appropriate for a pure u. v. eye and does not require a sharp focus of the regenerative rays, although the clear zone is an essential part of the mechanism. The rhabdom has an extraordinary shape like a flat 5-armed candelabra in cross section, with five posteriorly directed arms which are formed by six retinula cells. There is also a 7th retinula cell without a rhabdomere. This cell penetrates laterally the rhabdom of the other six, and also forms a sheath around half of its own ommatidium and half of the the adjacent ommatidium. The exceptional relations between this cell, and the other six, together with the orientated candelabra pattern of the rhabdom, and the large size of the 7th retinula axon, is interpreted as a way of enhancing the current flow down the 7th axon which runs direct to the medulla, bypassing the lamina.