In the compound eye of invertebrates such as insects and crustaceans, the pseudopupil appears as a dark spot which moves across the eye as the animal is rotated. This occurs because the ommatidia that one observes `head-on` (along their optical axes) absorb the incident light, while those to one side reflect it. The pseudopupil therefore reveals.....
Found on
http://en.wikipedia.org/wiki/Pseudopupil
(from the article `photoreception`) ...eye appears darker than other areas because these ommatidia are the ones that are best aligned to absorb light coming from the direction in which ...
Found on
http://www.britannica.com/eb/a-z/p/127
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