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The compound eye of insects is composed of units called ommatidia. 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 and thus 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 Archaeognatha and Thysanura to several hundred in larger Diptera.
Each ommatidium is hexagonal in cross section, and is ten times longer than its diameter. 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 which acts to further focus the light. The cornea and pseudocone form the outer 10% of the length of the ommatidium.
The inner 90% of the ommatidium contains six to seven (depending on the species) long thin R cells. The R cells are the photosensing cells. They 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 certain diptera (the mosquitoes and flies) the rhabdom has separated into seven independent rhabdomeres. This has required the rewiring of the eye such that each ommatidium now has seven axons leading from it. The advantage to this arrangement is that it increases the number of picture elements by a factor of seven, without increasing the number of ommatidia.
Since an image from the compound eye is created from the independent picture elements produced by ommatidia, it is important for the ommatidia to react only to that part of the scene directly in front of it. To prevent light entering at an angle from being detected by the ommatidium it entered or by any of the six neighboring ommatidia, six pigment cells are present. The pigment cells line the outside of each ommatidium. Each pigment cell is situated at the apex of the hexagons and thus lines the outside of three ommatidia. Light entering at an angle passes through the thin cross-section of the photoreceptor cell, with only a tiny chance of exciting it, and is absorbed by the pigment cell, before it can enter a neighboring ommatidium. In many species, in low-light situations, the pigment is withdrawn, so that light entering the eye might be detected by any of several ommatidia. This enhances light detection but lowers resolution.
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