Asclepias tuberosa, sullivantii, and incarnata (red and 'Ice Ballet') all have caterpillars. Even the mystery weed I yanked out of a viburnum rootball this spring and stuck in a 15 gallon pot with leftover dirt has turned out to be a milkweed, with 3 babies (and plenty of aphids). A total of 12 caterpillars so far, from the barely seen to 2" in length. One of the images above shows, I assume, the last molting--gobbled up soon after by the little guy / girl. It's my first year doing this, and it's quite a treat. Hope to have more pics soon of later stages. They grow FAST! My milkweeds are from Prairie Nursery, check them out.
Have some facts:
--Females lay 100-300 eggs in their short lifetime (just weeks), and the eggs hatch in 4 days.
--There are 5 stages for the caterpillar, or instars, that last 10-14 days. Each stage the skin gets shed.
--Then the pupal or chrysallis stage lasts 10-14 days.
--This last, late summer batch will be the ones that go to Mexico to overwinter. Hopefully.