To the human eye, many butterflies appear the same, but the butterflies themselves can often identify each other quite easily using ultraviolet markings. The ultraviolet patches on some butterflies are directionally iridescent, so that they appear to flicker in flight. This flickering is thought to have an important role in butterfly behavior and communication.

https://tucson.com/saddlebag/butterfly-vision/article_17df38a0-dee4-11e8-8ceb-07b913cdb028.html