Two mechanisms of screening pigment migration in superposition compound eyes are described. The first is the radial pigment migration mechanism in which screening pigment migrates proximally from the dark adapted position around the distal crystalline, cones into the clear zone of the eye when light adapted. This mechanism is able to substantially attenuate light intensity at the retina for small migrations of screening pigment away from the dark adapted position, and is found to be correlated with arthropods which can experience a wide range of light intensities in their habitats. The second mechanism is the cone pigment migration mechanism in which screening pigment migrates proximally from the dark adapted position around the distal crystalline cones to a position which occludes all but a small region at the tip of the crystalline cone when light adapted. This mechanism is not able to attenuate light intensity at the retina as substantially as the first mechanism (for a given pigment migration), but has a much finer control. Consequently, this mechanism is found to be correlated with arthropods which are active at one light intensity only.
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