Ommatidia are small units which make up the compound eye of animals like insects. Many ommatidia work together to give mosaic images (mosaic vision). This compound eyed are highly sensitive but their resolution is low.
Related Theory
The compound eye of the cockroach is composed of approximately 2,000 ommatidia that are separated from one another by a pigment sheath extending to the basement membrane of the eye, where nerves and tracheae are clearly seen.