Unlike humans, a fly has five eyes. Three simple eyes, with little more than an ability to distinguish light from dark and sense movement and two much larger, compound eyes.
Compound eyes are a feature unique to Insects. Instead of a single lens, insects see the world as a composite diorama, through many hundred or even thousands of smaller, individual lenses known as ommatidia. House flies, for instance, have a very well-developed visual system, capable of seeing motion, color and pattern of the objects in their environment, owing to their advanced compound eyes.
Compound eyes first appeared on Earth more than 500 million years ago. We know this because they can be seen on fossil trilobites dating from that era. Each Compound eye contains up to 10,000 to 30,000 facets and covers most of an insect’s head. In contrast to a human eye, each facet within the compound eye points in a slightly different direction and perceives light emanating from only that particular point in space, creating a mosaic of partially overlapping images.
There are two main types of compound eyes – Apposition & Superposition.
According to the structure and distribution of pigment between the ommatidia, the eye can form either apposit or superposit images.
In case of an apposit image, each ommatidium focuses only those rays that are almost parallel to its long axis, forming a tiny portion of an image of the much wider visual field. The resultant image, on the whole, is a combination of these part images.
In a superposit image, the sensory cells of an ommatidium pick up light from a much larger part of the visual field, with the resultant image overlapping as many as 30 neighboring ommatidia, gaining in brightness, yet losing sharpness in comparison to an apposit image.
Diurnal insects have apposition eyes, whereas nocturnal insects have superposition eyes. There are, however, many intermediate grades and in some animals, one type of eye may temporarily morph into another by movement of pigment between the ommatidia to adapt to light or dark conditions, in an environment.