Oh, I think you're right -- it looks like it must be dogbane! I hadn't seen that monarchwatch photo gallery before -- very helpful, thank you! If I can't find any common milkweed I will ask on the teacher network for help. I once had a monarch pupa on the outside of my kitchen windowsill so there must be milkweed around here somewhere (that's why I thought the dogbane was milkweed, because it's near that window)!
Thanks so much for your help! Caterpillar thanks you too.