Compound eyes consist of many photoreceptor units or ommatidia. Each 'ommatidium' (singular) is an individual 'eye unit'.
The image perceived is a combination of inputs from ommatidia on a convex surface. Thus they point in slightly different directions.
Compared with simple eyes, compound eyes possess a wide-angle view. They can detect fast movement and, in some cases, the polarization of light.[1]
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