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compound eye
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Horseshoe crab (Limulus polyphemus) with a hard, rounded shell and long pointed tail lying on sandy ground. Close-up of a trilobite fossil embedded in brown rock, showing the segmented body and ridged exoskeleton.
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compound eye

anatomy
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Horseshoe crab (Limulus polyphemus) with a hard, rounded shell and long pointed tail lying on sandy ground.
Horseshoe crab The horseshoe crab (genus Limulus) has compound eyes that appear to have evolved independently from the trilobites and the myriapods. It is thought that the single-chambered eyes of spiders and scorpions are descended from chelicerates such as Limulus.

A compound eye is an eye composed of many individual visual units, known as ommatidia. This type of eye is characteristic of insects and crustaceans, though it has also evolved independently in mollusks and annelids. Each ommatidium contains its own lens and photoreceptor cells, which together capture light from a small region of the visual field.12

The numerous ommatidia are arranged on a convex surface, creating a faceted appearance. These individual units work together to form a mosaic image, which is perceived by the insect's brain as a single, erect picture. The resolution of a compound eye generally improves with a greater number of ommatidia, though it is typically less sharp than that of a human eye. Some compound eyes are adapted for low light conditions by allowing light to overlap between ommatidia, creating a brighter but less distinct image.23

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5 Britannica Sources
  1. compound eye
  2. photoreception - Differences in resolution
  3. photoreception - Image formation
  4. photoreception - Neural superposition eyes
  5. insect - Eyes

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