The larvae of American swallowtails are attracted to the oils of plants such as dill, parsley, celery, and carrots. These plants produce specific chemicals that repel insects that try to eat them. American swallowtail larvae are resistant to these chemicals, so when they feed on the plants, the chemicals make them bad-tasting to bird predators.
Black Swallowtails are a fairly common but very pretty butterfly with a wingspan of about a 2.5 to 4 inches wide. They are easy to attract to host plants to lay eggs, and the caterpillars are easy to raise also.
When Black Swallowtail caterpillars are ready to pupate they tend to wander quite a distance from the plant they were eating. Before they wander off however, they defecate/poop one last time. They empty out their whole digestive tract so this last poop/frass is kind of runny or diarrhea like.