This is a Monarch Butterfly Caterpillar feeding on Swamp Milkweed. I shot this very early in the morning at the Edwardsville Watershed Nature Center. Monarch butterflies are some of the most unique butterflies in the world due to their annual migration from the eastern half of the United States to central Mexico.
Monarchs feed entirely on the various species of Milkweed plants. Milkweed is poisonous to other creatures, and this makes the Monarch poisonous for them to eat. This affords them a pretty good level of protection from many other creatures that would normally eat them.
Adult Monarch Butterflies will continue to feed on Milkweed plants and other nectar plants as well. I photographed this feeding adult at the Edwardsville Watershed Nature Center. I have attached a link to a website that has extensive information covering the Monarch Butterfly. These fascinating creatures are a joy to observe and photograph.