How do compound eyes work in butterflies?

A butterfly's compound eye does not work like a human eye. Instead of a single lens focusing an image on a retina, the compound eye is an array of thousands of tiny, independent light-detecting units called ommatidia (singular: ommatidium).

You can think of each ommatidium as its own tiny, basic "eye."

1. Each ommatidium has its own lens (corneal lens and crystalline cone) that focuses light onto a small group of photoreceptor cells.
2. Each ommatidium is pointed in a slightly different direction from its neighbors and captures one "pixel" of the world from a very narrow angle.
3. The butterfly's brain receives all these thousands of individual signals simultaneously.
4. The brain then assembles these signals into a mosaic image—a single, wide-angle picture of the world, pieced together from thousands of dots.

This type of eye is not as "high-resolution" as a human eye, but it is exceptionally good at detecting motion, which is why it's so hard to catch a butterfly. It also provides an extremely wide field of view.