Definitions for ommatidiumˌɒm əˈtɪd i əm; -ˈtɪd i ə
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any of the numerous small cone-shaped eyes that make up the compound eyes of some arthropods
One of the conical substructures which make up the eyes of invertebrates.
Origin: From the stem form of ‘eye’ + -idium.
The compound eyes of arthropods like insects, crustaceans and millipedes are 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 around 30 thousand in larger Anisoptera dragonflies and in some Sphingidae among 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 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 6 to 9 in the case of some butterflies long and thin photoreceptor cells 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.
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