Superposition images are formed by reflection in the eyes of some oceanic decapod Crustacea

Abstract

LIKE moths, the eyes of many pelagic and bottom-living decapods appear to glow brightly when viewed from the direction of illumination¹. It has generally been assumed that the optical system involved is similar to that in nocturnal insects, that is, the superposition mechanism of Exner². The essential feature of Exner's theory is that the optical apparatus of the cornea and crystalline cone refracts light through twice the angle at which it was incident on the cornea (Fig. 1a). Light from many facets then combines to form a single image on the receptor layer, and if this is backed by a tapetum the light is reflected out through the same facets that it entered, causing the glow. There is now no doubt that Exner's mechanism is present in some insects³. Recent accounts have, however, cast doubt on whether it occurs in Crustacea^4,5. This is largely because the high refractive indices and inhomogeneities required of the optical components have been shown not to be present. In this paper I show that superposition images are formed in the eyes of a midwater shrimp, but in a different. manner (Fig. 1b).