The image perceived is a combination of inputs from the numerous ommatidia (individual “eye units”), which are located on a convex surface, thus pointing in slightly different directions. Compared with simple eyes, compound eyes possess a very large view angle, and can detect fast movement and, in some cases, the polarisation of light.
— https://en.wikipedia.org/wiki/Eye#Compound_eyes
Vision has a price. Something like 20 per cent of our energy goes to run our brains, and in humans, half of that goes to vision. So in an environment where vision is unnecessary, it’s a huge energy drain. Light-poor environments are also likely to be nutrient-poor as well. So in these environments, natural selection will not favor vision, except maybe for crude sensors to tell when the organism is approaching the surface. Earthworms don’t have eyes. Many cave fish, insects, and naked mole rats (probably the most grotesque mammals imaginable) also lost their eyes.
— Steven Dutch, “What Good Is Half a Wing?”
