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.[1] This occurs because the ommatidia which one observes "head-on" (along their optical axes) absorb the incident light, while those to one side reflect it.[2] The pseudopupil therefore reveals which ommatidia are aligned with the axis along which the observer is viewing.[2]