The ommatidium of the dorsal eye of Cloeon as a specialization for photoreisomerization

Abstract

The turbanate dorsal eyes of Cloeon have thin biconvex corneal facets, large crystalline cones, and long retinula cells which cross the clear zone. There is no large-scale movement of screening pigment upon adaptation. The combination of anatomical features suggests that the eye operates with high visual acuity because the corneal lens is focused on the tip of the cone, from which light is guided by the retinula cell column across the clear zone. By this means the eye could function near the diffraction limit. The suggestion that the main visual path is by light guides has some experimental support from examination of embedded shoes of the eye, but leaves no function for the clear zone, as there is negligible movement of pigment upon light adaptation. A clue to the function of the clear zone is provided by the unique appearance of a distal collection of rhabdom microvilli which are formed by all seven retinula cells around the tip of the crystalline cone. This distal rhabdom necessarily acts as a filter for light which crosses the clear zone. It is suggested that this light is utilized in the photoregeneration of the visual pigment after the effective visible wavelengths have been reduced. Filtering at this level could prevent unfocused regenerative rays, which cross the clear zone, from interfering with the angular sensitivity of the receptors. In other clear zone eyes, screening pigment around the cone tip could similarly serve a double function by acting as the aperture of the light guide and at the same time transmitting photoregenerative rays which cross the clear zone outside the light guides.