Swallowtails can show a wide range of color both in their caterpillar and adult stages. As caterpillars, North American swallowtails are often dark to light green, with a variety of colorful markings that differ from one species to the next. Some members of the genus Papilio often show black stripes with yellow dots, but others have black and gray or plain green bodies.
Swallowtail species found in the Unites States are part of the genera Battus and Eurytides, but Papilio is the most numerous group, with the majority of the species. Common species include the black swallowtail; the anise and the indra swallowtails, which attack plants of the parsley family when in the caterpillar stage; the Eastern tiger swallowtail; and the giant swallowtail.
Swallowtails are the only caterpillars to have an organ on their heads called an osmeterium. This fork-shaped organ secrets a strong-odored substance when the animal feels threatened. Swallowtail caterpillars have segmented bodies and legs that help them move. They have three pairs of legs on the first three segments, about four pairs of legs on the middle segments and another pair of legs on the last segment. Many species have urticating hairs, which release skin-irritating substances when touched.
Like other larvae, caterpillars are often associated with agricultural loss and damage. Most swallowtail caterpillars are agricultural and forest plagues, attacking a wide range of plant species. Some swallowtail caterpillars have developed resistance to insecticides. The biological control of these caterpillars include the use of certain types of bacteria that can kill the caterpillars without harming other beneficial animals and insects, such as bees. Although irritating to the skin if touched, these caterpillars are not toxic to humans.