There were several of these on some Common Milkweed, Asclepias syriaca. Naturalist John Connors, of the North Carolina Museum of Natural Sciences, noted that early instar Milkweed Tussock Moths were skeletonizing some of the leaves. This or, another, Anchor Stink Bug moved into the batch of caterpillars and started to devour them. Apparently it was not much bothered by the presumed chemical protection of the caterpillars--see linked image. (After looking at these images carefully, I believe they are of the same individual.)