Pigment movement and the crystalline threads of the firefly eye

Abstract

The compound eye of the firefly is the one outstanding example which shows evidence of an image which is formed by the combination of rays that have entered by several facets. The original account by Exner1 includes a description and photograph of the erect image that can be found behind the cleaned cornea of the firefly Lampyris. This image has been confirmed by others2 and a theoretical explanation of it supposes that an inverted first image is formed within each corneal cone by the curvature of its front surface and that an erect second image is then formed by the curved surface of the proximal tip of the corneal cone3. The erect superposition image, however (which undoubtedly exists if the cornea is well cleaned), does not lie in the place of the receptors4 and there is no evidence that it has a functional significance. In fact, the superposition theory depends on the unlikely existence of an optically homogeneous and transparent region between the cornea and the layer of receptors. Despite its importance to the theory, the anatomy of the firefly retina remains unknown. The following account, based on Photuris versicolor, points to the conclusion that the superposition theory is applicable only to a cleaned cornea because the actual anatomy suggests quite a different optical system.