ommatidium

thumb|right| Ommatidium: A – cornea, B – crystalline cone, C & D – pigment cells, E – rhabdom, F – photoreceptor cells, G – membrana fenestrata, H – optic nerve thumbnail|Ommatidia of a krill The compound eyes of arthropods like insects, crustaceans and millipedes are composed of units called ommatidia (: 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 arthropod and range from as low as five as in the Antarctic isopod Glyptonotus antarcticus, or a handful in the primitive Zygentoma, to around 30,000 in larger Anisoptera dragonflies and some Sphingidae moths.

==Description== 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 further focuses the light. The cornea and pseudocone form the outer ten percent of the length of the ommatidium.