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Compound eye

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Dragonfly compound eyes
An image of a house fly compound eye surface by Scanning Electron microscope

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]

The general picture

Eyes with resolving power have come in ten fundamentally different forms, and 96% of animal species possess a complex optical system.[2] Image-resolving eyes are present in molluscs, chordates and arthropods.[3]

Almost all insects and arachnids use both simple and compound eyes.

Types of compound eye

Compound eyes fall into two groups: apposition eyes, which form multiple inverted images, and superposition eyes, which form a single erect image.[4] Compound eyes are common in arthropods, and are also present in annelids and some bivalved molluscs.[5]

Compound eyes, in arthropods at least, grow at their margins by the addition of new ommatidia.[6]

Good fliers

Good fliers like flies or honey bees, or prey-catching insects like praying mantis or dragonflies, have specialized zones of ommatidia organized into a foveal area which gives acute vision. In the acute zone the eyes are flattened and the facets larger. The flattening allows more ommatidia to receive light from a spot and therefore higher resolution.

Other types

Another version is the pseudofaceted eye, as seen in Scutigera. This type of eye consists of a cluster of numerous ocelli on each side of the head, organized in a way that resembles a true compound eye.

The body of Ophiocoma wendtii, a type of brittle star, is covered with ommatidia, turning its whole skin into a compound eye. The same is true of many chitons.



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