The optical structures of animal eyes

The ability to respond to light is common to many forms of life, but eyes themselves — structures that break up environmental light according to its direction of origin — are only found in animals. At its simplest, an eye might consist of a small number of light-responsive receptors in a pigmented pit, which shadows some receptors from light in one direction, and others from a different direction (Figure 1A). This definition distinguishes an eye from an organ with a single photoreceptor cell, which may indeed be directional because of screening pigment, but which does not allow for spatial vision — the simultaneous comparison of light intensities in different directions. An alternative starting point for an eye would be for each receptor to have its own pigmented tube (Figure 2A), the assemblage forming a convex cushion. In these two proto-eyestructures we have the beginnings of the two mutually exclusive ways of building an eye: the single-chambered range of eyes, often misleadingly called ‘simple’, and the compound eyes.