Rhabdom

The compound eyes of arthropods like insects, crustaceans and millipedes are composed of units called ommatidia (singular: ommatidium). An ommatidium contains a cluster of photoreceptor cells surrounded by support cells and pigment cells. The outer part of the ommatidium is overlaid with a transparent cornea. Each ommatidium is innervated by one axon bundle (usually consisting of 6-9 axons, depending on the number of rhabdomeres) and provides the brain with one picture element. The brain forms an image from these independent picture elements. The number of ommatidia in the eye depends upon the type of insect and ranges from just a handful in the primitive Zygentoma to around 30 thousand in larger Anisoptera dragonflies and in some Sphingidae moths.

Ommatidia are typically hexagonal in cross section and approximately ten times longer than wide. The diameter is largest at the surface, tapering toward the inner end. At the outer surface there is a cornea, below which is a pseudocone that acts to further focus the light. The cornea and pseudocone form the outer ten percent of the length of the ommatidium.

The inner 90% of the ommatidium contains 6 to 9 (depending on the species) long and thin photoreceptor cells in the case of some butterflies often abbreviated "R cells" in literature and often numbered, e.g. R1 through R9. 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.

In true flies, the rhabdom has separated into seven independent rhabdomeres (there are actually eight, but the two central rhabdomeres responsible for color vision sit one atop the other), such that a small inverted 7-pixel image is formed in each ommatidium. Simultaneously, the rhabdomeres in adjacent ommatidia are aligned such that the field of view within an ommatidium is the same as that between ommatidia. The advantage of this arrangement is that the same visual axis is sampled from a larger area of the eye, thereby increasing sensitivity by a factor of seven, without increasing the size of the eye or reducing its acuity. Achieving this has also required the rewiring of the eye such that the axon bundles are twisted through 180 degrees (re-inverted), and each rhabdomere is united with those from the six adjacent ommatidia that share the same visual axis. Thus, at the level of the lamina - the first optical processing center of the insect brain - the signals are input in exactly the same manner as in the case of a normal apposition compound eye, but the image is enhanced. This visual arrangement is known as neural superposition.

...
Wikipedia