Chapter 96 - Eyes and Vision

Abstract

This chapter discusses eyes and vision in insects. Insect eyes are of two basic types: compound (or multifaceted) and simple (or single chambered). Despite the major differences in their form and construction, compound and simple eyes perform essentially the same job of splitting up the incoming light according to its direction of origin. Each facet of the eye produces an inverted image, even though the geometry of the eye as a whole dictates that the overall image is erect. The inverted image is focused onto the distal tip of the rhabdom. Having a slightly higher refractive index than its surroundings, the rhabdom behaves as a light guide, so that the light that enters its distal tip travels down the structure, trapped by total internal reflection. Any spatial information in the image that enters the rhabdom tip is lost, scrambled by the multiple reflections within the light guide, so that the rhabdom itself acts as a photocell that averages all the light that enters it. Compound eyes are of two distinct and optically different kinds: apposition eyes, in which each receptor cluster has its own lens; and superposition eyes, in which the image at any point on the retina is the product of many lenses.