This butterfly is called a coral hairstreak. The flower this butterfly is on is butterfly milkweed. I have noticed that many of the published pictures of coral hairstreak show it on butterfly milkweed (and butterfly milkweed is not all that common). Why is that? Well, one reason is that butterfly milkweed is a spectacular flower. Any time you get a bright orange-red flower in a picture, it is going to make an impact. Another reason could be that butterfly milkweed is most often seen in prairie patches, as is the coral hairstreak. But quite possibly the main reason is the time that butterfly milkweed blooms--usually within a couple of weeks of the fourth of July around here. That is also, coincidentally, the time that the adults exit their pupae and start to fly.
Hairstreaks are smallish butterflies, and are typically blue, brown, or gray, but can be many other colors. Hairstreaks get their name from a thin, hair-like tail that projects from the back of the hind wing. Supposedly, this acts to distract predators, which attack the tail. The coral hairstreak is one of a few hairstreaks that does not have the tail.
The food plants of the larvae are plants in the Prunus genus--black cherries, choke cherries, wild plum, and the like.
These caterpillars are also reported to be associated with ants.