Ommatidium

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The compound eye of insects is composed of hundreds of unit eyes called ommatidia. An ommatidium contains a cluster of photoreceptor cells surrounded by support cells and pigment cells. The outer part of the ommatidium is overlayed with a transparent cornea.

Ommatidia of Antarctic krill eye - click for higher resolutions

Cross-section of a compound eye - top: low light condition, with separating pigments contracted, so that the light reaching multiple ommatida focus on a single nerve; bottom: bright condition, with pigments expanded

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