Without being able to see the head clearly, these look like Imperial Moths. The colors in this species are yellow and that strange purple, and have highly variable patterns.

Definitely imperial moths. They are related to silkworm moths.

I'm no expert but think it could be a clouded yellow butterfly or a sulphur yellow butterfly , both are found in the midwest and northeast eastern united states

Today I was walking through the southern Illinois woods when I saw two insects, presumably butterflies, hanging onto a plant. If you look closely, they appear to be mating. Each butterfly's wingspan is approx 4-5 inches. Does anybody know what kind of butterfly this is?

Thanks!