This tiger swallowtail caterpillar (larva) is not the usual green one would expect. That is because it is pre-pupal, just about ready to form a pupal case. When I came back, 30 minutes later, the caterpillar was gone. I hope it had found a place to pupate and hadn't been eaten by a bird! To see what this caterpillar looks like when it becomes a butterfly: http://www.pbase.com/laroseforest/image/143642688