Alternatives to superposition images in clear-zone compound eyes

Abstract

The clear zone between the cones and the receptor layer in dark -adapted eyes of insects that are active in dim light has formerly been explained as a space to allow formation of a superposition image. Although erect images have been seen in Ephestia (Lepidoptera) and Hydrophilus (Coleoptera), new experiments show that they are accompanied by scattered light and that the angular sensitivity of individual receptors must be wide in the dark-adapted state. Alternatives to the superposition theory are examined, and it is concluded that in eyes with crystalline cones the clear zone (in general, in the numerous shapes and sizes of eyes of nocturnally active insects) enables light entering by many facets to sum upon individual receptors on the far side of the clear zone. In addition to the scattered light effect, light is carried across the clear zone in crystalline tracts or retinula cell columns, which provide a separate optical pathway for each ommatidium also in the light-adapted state.