Most insects have a view of the world that is very different from ours, because their eyes are built on a different plan. Insects such as the housefly, the hornet, the butterfly, and the beetle, have what we call compound eyes. These eyes are made up of many separate units. Some, like the dragon fly, have as many as thirty thousand units per eye, each with its own lens. Others, like the ant, have only six.
With a compound eye, the insect sees a mosaic image. This looks something like the highly magnified dots of a newspaper photograph. Because the lenses in the insect’s eyes have a fixed focus, and can’t be adjusted for distance, insects see shapes poorly.
On the other hand, compound eyes are excellent for detecting motion. Houseflies and dragon flies have eyes that cover most of their head. This gives them almost 360 degree vision, enabling them to see predators coming from any direction. That’s why a house fly usually escapes our attack, unless we hit it quickly from behind using a fly swatter.
Most insects can see some colour. While our eyes see a full spectrum of wave lengths from red to violet, many insects see a limited range of colours. The colours they detect are the ones most useful for finding food and shelter.
So next time you stalk a fly, remember it does have eyes in the back of its head.