The Compound Eye

Abstract

How do we know that the faceted compound eyes of insects are eyes? No pupil, no iris, no eye ball, no lid, no brow. It is difficult to have eye contact with a hornet even if it is watching you. Whatever the answer may be, once we take for granted that insect eyes are indeed eyes, we realize that they must have many properties in common with our own. First of all, visual space for all organisms is a sphere with the viewing subject in the center. Humans can see only half of it at a time; insects may see all around. But for insects, as for ourselves, the distance between two points in the visual sphere is given as an angle with its apex in the viewing subject. Likewise the area of the projection of an object is given as a solid angle. The same projection may correspond to an object which is close and small or large and far away. Its distance from the viewing subject can be derived only from secondary cues (disparity, motion parallax, previous knowledge, etc.). Second, insects and humans exploit roughly the same spectral region of electromagnetic radiation. Both types of eye contain densely packed arrays of photosensitive cells which transform the local properties of the light in the visual sphere into an array of neuronal signals in the brain. A description of all the known similarities would fill many pages.