Ommatidium

The compound eyes of arthropods like insects, crustaceans and millipedes are composed of units called ommatidia (singular: ommatidium). An ommatidium contains a cluster of photoreceptor cells surrounded by support cells and pigment cells. The outer part of the ommatidium is overlaid with a transparent cornea. Each ommatidium is innervated by one axon bundle (usually consisting of 6-9 axons, depending on the number of rhabdomeres) and provides the brain with one picture element. The brain forms an image from these independent picture elements. The number of ommatidia in the eye depends upon the type of insect and ranges from just a handful in the primitive Archaeognatha and Thysanura to around 30 thousand in larger Anisoptera dragonflies and in some Sphingidae moths.

Ommatidia are typically hexagonal in cross section and approximately ten times longer than wide. The diameter is largest at the surface, tapering toward the inner end. At the outer surface there is a cornea, below which is a pseudocone that acts to further focus the light. The cornea and pseudocone form the outer ten percent of the length of the ommatidium.