Eyes are essential organs of the visual systems. They detect light and convert it into electro-chemical impulses carried by neurons to the brain, which is then turned into images. The animal kingdom is full of amazing eyes that appear and function in drastically different manners. All eyes can be categorised into two groups: “simple eyes”, with one concave photoreceptive surface, and “compound eyes”, which comprise a number of individual lenses laid out on a convex surface. Note that “simple” does not imply low level of complexity or acuity.
Simple eyes. The human eye is an example of a simple eye, which uses a single lens to focus images onto a light sensitive membrane lining the inside of the eyeball called the retina.
Compound eyes. The type usually found in insects and arthropods, compound eyes are made up of many individual lenses. In dragonflies, for example, a single compound eye can have as many as 10,000.
Some compound eyes process an image in parallel, with each lens sending its own signal to the insect or arthropod’s brain. This allows for fast motion detection and image recognition, which is one reason why flies are so hard to swat.