
Contents
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Origins Origins
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A little history: apposition and neural superposition A little history: apposition and neural superposition
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Basic optics Basic optics
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Imaging mechanisms Imaging mechanisms
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Resolution Resolution
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Diffraction and eye size Diffraction and eye size
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Sensitivity Sensitivity
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Light and dark adaptation Light and dark adaptation
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The pseudopupil The pseudopupil
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Ecological variations in apposition design Ecological variations in apposition design
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The forward flight pattern The forward flight pattern
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Acute zones concerned with prey capture and mating Acute zones concerned with prey capture and mating
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Horizontal acute zones Horizontal acute zones
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The anomalous eyes of strepsipterans and trilobites The anomalous eyes of strepsipterans and trilobites
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Summary Summary
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7 7 Apposition compound eyes
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Published:March 2012
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Abstract
Apposition compound eyes are made up of ommatidia. In conventional apposition eyes, the receptive rod (rhabdom) acts as a detector that measures the average brightness of a small region of space, typically about 1° across. The overall erect image seen by the animal is the mosaic formed by these adjacent fields of view. In dipteran flies, the inverted image in each ommatidium is resolved by seven separate receptors. However, the responses of these are combined in the lamina (first synaptic layer) in a way that pools their signals, giving enhanced sensitivity without loss of resolution, an arrangement that has been called ‘neural superposition.’ Because individual facet lenses are very small, the images they produce are severely limited by diffraction, so that the minimum resolvable angle is rarely better than 1°. To improve on this requires an eye of unsupportable size. Arthropods do achieve enhanced resolution by having local regions of enlarged facets, but at the expense of resolution elsewhere. The pseudopupil – the small dark spot that appears to move across the eye as the observer moves around it – can be useful in determining resolution. Acute zones are found frontally in many flying insects, and are involved in the capture of other insects on the wing. Some arthropods that live in a flat environment, such as crabs on a beach, or bugs which hunt in the surface film of ponds, have an acute zone around the eye's horizon.
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