Butterfly Weed. As I expected it might be, it was a Monarch Butterfly caterpillar, and it wasn't alone. There are at least three in the same area.
Black Swallowtail caterpillar from a tiny thing, through the chrysalis and metamorphosis to an adult. We decided not to try that here. But we are glad to see the Butterfly Weed serving as host to the Monarchs. That's one of the reasons for planting it.
Butterfly Bush was full of Eastern Tiger Swallowtails this morning. There were a bunch of Silver Spotted Skippers, and one transient Zebra Swallowtail as well (not pictured)