This poor caterpillar has been parasitized by a fly and the fly larva is now emerging from the caterpillar's body. Rather gruesome, but it a common occurrence, especially for caterpillars of all types who are attacked by both parasitic flies and parasitic wasps. When I saw this beautiful caterpillar a week earlier, it seemed fine, and then when I returned to look at it again, I saw this. Thanks to Bug Guide for the information on what is going on, as I was not sure what was what.