Compound eyes have ∼750 facets, with 8 photoreceptors per facet

A fly's face is dominated by its eyes (Fig. 7.1). Each of the two compound eyes is a honeycomb matrix of ∼750 “ommatidial” subunits. Each subunit, in turn, has 8 photoreceptors or “R” cells (R1–R8) for a total of ∼6,000 receptors per eye. At this pixel density, flies see grainier images than humans, who have ≥ 100,000 receptor cells in the fovea alone. Because fly and human eyes appear to have had a common evolutionary origin, the obvious “One Eye or Many? Riddle” is: Did our common ancestor have a simple or a compound eye? If the former, then why/how did insects multiply it? If the latter, then why/how did chordates reduce it to a solitary remnant? Of course, there is a third possibility. Our common ancestor might have had only a primitive light detector, and we chordate or arthropod descendants then built our own versions of eyes based on the genes that were active at those spots on our face.

The epithelium of the eye disc is a monolayer (as is true for all discs; cf. Ch. 4), but the epithelium of the adult eye is stratified. Above the bundle of 8 R cells, each adult ommatidium has 4 “cone” cells that secrete the lens (no relation to vertebrate cones). Between the bundles are pigment cells that prevent blurring by absorbing scattered photons: 2 PPCs, 6 SPCs, and 3 TPCs (primary, secondary, and tertiary pigment cells) per ommatidium.