The compound eye of insects is composed of hundreds of unit eyes called ommatidia. An ommatidium contains a cluster of photoreceptor cells surrounded by support cell, and pigment cells. The outer part of the ommatidium is overlayed with a transparent cornea. The photograph to the right shows a cluster of photoreceptor cells from a single ommatidium from Drosophila melanogaster (magnified 400X). It is about 85 μm in length and 5-10 μm in diameter. The nucleus and the rhabdomere from a photoreceptor cell are visible.