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The eye of Insects is of two types: simple eyes or ocelli and compound eyes. Typically, the two types coexist, but one, the other or both may be lacking. The ocelli are divided into dorsal and lateral ocelli. The former normally coexist with the compound eyes and are three in number arranged in a triangle. The lateral ocelli, in varying numbers, are the only ones present in the larvae of holometabolous insects. The compound eyes are always lateral and are formed by several optical units, or ommatidia, each of which corresponds to a lens and a retina. Each ommatidium is isolated from neighbours through pigmented collars and light passes directly to the nerve fibres to the base. Only the rays that arrive perpendicularly to the lens can reach the nerves, so each ommatidium has a very limited field of view. The image of the outside world is perceived as a mosaic of points and, despite not having a good definition, it works great for the perception of motion.
Photography: eye of a domestic fly (Musca domestica, Linnaeus 1758)