These might have been cloudless sulphur caterpillars. Then again they might have been another sulphur. I'm not sure. They look different from previous cloudless sulphur cats that I posted here and here, but they tend to vary in their colors and spots as they grow, so I don't know.
To further complicate matters, I wasn't able to get enough pictures of them. The first time I saw them (at the arboretum), there were several individuals, but I didn't have my camera with me. Then when I went back a day or two later with my camera, I couldn't find most of them any more. I'll go back and check again in a couple of days.