Multiple Facets of Compound Eyes

Compound eyes, found in insects and some marine animals, are the type most often observed in nature. The world's oldest eye, preserved as a fosilized rock, has a compound structure much like the eye of a modern horsefly.

Compound eyes comprise hundreds of relatively long tubes bunched together like a handful of soda straws. At the external tip of each tube is a fixed lens that focuses light rays toward a group of light-sensitive cells at the tube's innermost end. Since the tubes fan out slightly, the eye structure is rounded, giving an extremely broad field of vision.

Each tip, or facet, or a compound eye picks up a tiny image of the section of the world in front of it, and transmits this fragment to the brain as a nerve impulse, there to be fused with signals from other facets into an overall mosaic picture. Since facets cannot change focus, compound eyes are unable to form precise images. A wasp cannot tell the difference between a fly on a wall and a nailhead. But compound eyes are extremely efficient at detecting movement. Honeybees always head toward flowers swayed by a light breeze; predactory dragonflies are able to make precise calculations of the speed of smaller insects darting through their visual fields.

The simple eyes of most spiders are like separated facets of a compound eye. They are usually arranged in clusters along the spider's back, in such a way that they can register the movement of images passing in sequence from one eye to the next. Spiders which sit quietly in their webs are not so dependent on their vision as hunting spiders, and therefore can get along with very nearsighted eyes.

Life Science Library - Light and Vision