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Pseudopupil

The pseudopupil in an arthropod eye is a dark spot which shifts depending on what angle you are observing the eye from. It is caused by the abosrption of light of the ommatidia being observed “head-on” from that viewing angle (the ommatidia viewed from other angles are reflecting light and thus not black). In fiddler crabs the pseudopupil often appears to be a long vertical band within the eye.

A close-up of the eyestalks and eye from the fiddler crab Uca vocans. The dark vertical band in the center of the eyes is the pseudopupil. Original photo by Chris Lukhaup. image
A close-up of the eyestalks and eye from the fiddler crab Uca vocans. The dark vertical band in the center of the eyes is the pseudopupil. Original photo by Chris Lukhaup.