eye, compound

Eye, compoundclick for a larger image
Fig. 157 Eye, compound . (a) Apposition image. (b) Superposition image.

eye, compound

a type of light-receptor organ, found particularly in insects and crustaceans, that is formed from numerous OMMATIDIA, each of which can form a separate image. Movement of the pigment between the ommatidia forms either
  1. an apposition image, where light enters each ommatidium parallel to its long axis, so forming a mosaic image; or
  2. a superposition image, where pigment is withdrawn, allowing in light to the sides of the ommatidia and giving a brighter, less sharp and overlapping series of images. See Fig. 157 . The latter occurs in nocturnal insects, the former in diurnal insects, and change from one to the other gives rise to dark adaptation. In many insects the image is either apposition or superposition and there is no dark adaptation.

Ommatidia are larger than rods and cones, so fewer can be packed into the same space. Consequently the detail of the image is less good than in the vertebrate eye. For example, the honeybee has a visual acuity than is 1% of human capacity, and most other arthropods are worse than this. However, the compound eye of insects is capable of detecting movement over a large field, and since the reaction time is small, insects are capable of reacting to movement rapidly.

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