The eyes of insects and many other arthropods are compound, each composed of up to several thousand individual visual organs called ommatidia. The surface of each ommatidium is a hexagonal lens, below which is a second, conical lens. Light entering the ommatidium is focused by these lenses down a central structure called the rhabdom, where an inverted image forms on light-sensitive retinular cells. Pigment cells surrounding the rhabdom keep light from other ommatidia from entering. Optic nerve fibers transmit information from each rhabdom separately to the brain, where it is combined to form a single image of the outside world.