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 arthropod and range from as low as 5 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. 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. The specific organ of ommatidia, or eye units, vary between different organisms. The butterfly compound eye consists of multiple ommatidia, each of which consist of nine photoreceptor cells (numbered from R1–R9), primary and secondary pigment cells. Nymphalid butterflies have the simplest eye ommatidium structure, consisting of eight photoreceptor cells (R1–R8) and a tiny R9 cell organized into a different tier. These "R cells" tightly pack the ommatidium. The portion of the R cells at the central axis of the ommatidium collectively form a light guide, a transparent tube, called the rhabdom. Although composed of over 16,000 cells, the Drosophila compound eye is a simple repetitive pattern of 700 to 750 ommatidia, initiated in the larval eye imaginal disc. Each ommatidium consists of 14 neighboring cells: 8 photoreceptor neurons in the core, 4 non-neuronal cone cells and 2 primary pigment cells.