The pigment cells line the outside of each ommatidium. The cornea and pseudocone form the outer ten percent of the length of the ommatidium. The specific composition of ommatidium varies between different organisms. The outer part of the ommatidium is overlaid with a transparent cornea. Each ommatidium feeds into a single nerve fiber. These "R cells" tightly pack the ommatidium. Each butterfly ommatidium contains nine photoreceptors with generally each photoreceptor utilizing a single visual pigment. An ommatidium contains a cluster of photoreceptor cells surrounded by support cells and pigment cells. A compound eye may consist of thousands of individual photoreceptor units or ommatidia (ommatidium, singular). Each ommatidium contains eight photoreceptor cells (R1-8), support cells, pigment cells, and a cornea.