Pseudopupil

[1] 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.

[2] The pseudopupil therefore reveals which ommatidia are aligned with the axis along which the observer is viewing.

[2] The pseudopupil analysis technique is used to study neurodegeneration in insects like Drosophila.

An adult Drosophila eye consists of nearly 800 unit ommatidia which are repeated in a symmetrical pattern.

[3] By visualizing and counting the intact rhabdomeres, degradation level can be measured.

The head of a mantis showing the black pseudopupil in its compound eyes
The eye of a mantis shrimp has three regions, each with its own pseudopupil.