Male monarch butterfly, Danaus plexippus,
on its host plant, Asclepias curassavica.
Notice the pheromone glands in the two dark spots,
one on each hindwing near the abdomen.
This Monarch was in very good condition so it is difficult
to know whether it was migrating since there is a small,
nearly-year-round population near Houston, Texas,
where this photo was taken.
I am receiving so many gorgeous pictures from you, dear readers, that I am overwhelmed by the beauty of the images and the creatures and places in them. If you have a high-resolution digitized nature image (I prefer JPG format) that you'd like to share with your fellow readers, feel free to email it to me, along with information about the image and how you'd like it to be credited.
You seem to have confused the butterfly with the foodplant. Or perhaps the sentence has a typo and was meant to say: "Male monarch butterfly on Asclepias curassavica." The scientific name you give is for a type of milkweed; the butterfly is Danaus plexippus. See http://en.wikipedia.org/wiki/Monarch_butterfly for more.