(You may be wondering what the little balls to the left of the caterpillar might be. They look like eggs, but are not. They are frass, otherwise known as caterpillar poo.)
This crazy looking caterpillar, dressed in black and covered with red spines and horns, looks pretty formidable. From everything I have read, the spines are not dangerous and are mostly for show. However, the caterpillar will develop toxins from eating the Pipevine plant, so if you were thinking about snacking on one, I would suggest you choose something else. The main toxin, Aristolochic acid, wards off predators, like hungry birds and mammals, and a good enough dose can cause kidney failure in humans.
This caterpillar will turn into a beautiful Pipevine Swallowtail butterfly, Battus philenor. Simply gorgeous black butterfly with one row of bright crimson dots on the hind wing. This is one we found that had just emerged earlier this spring. You can read that post, on my Indy Parks blog here.