This is the caterpillar of the giant swallowtail butterfly. A few weeks ago, I was fortunate enough to see a butterfly laying an egg on the Australian willow tree at the arboretum. I saved the leaf, and have been rearing the caterpillar. It was eating only sparingly from the leaves of the tree that its mother had chosen. It seemed to be taking forever to grow. I decided to try giving it the tender leaves of an orange tree, and it likes that much better.
These caterpillars hide in plain sight by mimicking bird droppings.