The compound eye of many ants, including a Camponotus I imaged this morning on the new focus-stacking rig, has two colors of ommatidia. Most are black, but the peripheral facets are lighter. I’ve seen this for years but had no idea why this might be the case.
A quick search on Google scholar suggests an orientation function related to light polarization:
Can anyone who knows more about insect vision than me weigh in? I’m genuinely curious.
Update: Micheal Bok has the answer!