What are ommatidia?
The compound eyes of insects, crustaceans and millipedes are made up of units called ommatidia .
Each ommatidium contains a cluster of photoreceptors, support, and pigment cells. The outer part has a cornea.
Each has one axon of a nerve cell and gives the brain one part of the picture.
The brain forms an image from all the elements. The number of units range from a handful in primitive insects to around thirty thousand in dragonflies.