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: eight photoreceptor neurons