Butterflies lay their eggs on a very specific host plant that is necessary for their larva to feed upon. Pipevine Swallowtails lay their eggs on Pipevine (Aristolochia species) including Dutchman's Pipe (A. tomentosa and Virginia Snakeroot (A. serpentaria). We have been searching for pipevine plants with little success but apparently the butterflies are much better at it than we are as they are thriving.
Pipevine plants have a nasty tasting chemical that is retained by the caterpillar and the adult butterfly. A predator is likely to get sick from sampling one and apparently learns from the experience. Black Swallowtail and Red-Spotted Purple butterflies, which are non-toxic, have a similar appearance and predators avoid them as well, a benefit called mimicry. In Pipevine Swallowtail territory, Eastern Tiger swallowtail females develop a similar dark coloration, while the same females in other areas are light colored like the males.
All swallowtails come equipped "osmeterium", brightly-colored, horns which they extend from their heads when bothered. They can rear back and almost touch their tail, secreting a foul smell which turns off predators. (See Patrick Coin's picture with the orange horns.)
The Bill Roston Butterfly House is opening this weekend at Close Memorial Park. It is stocked with a wide variety of native butterflies with more added weekly. You can see all stages of their life cycle on most days as they lay their eggs and raise their caterpillars.
There is a fantastic set of pictures by Patrick Coin, showing the complete life cycle of the Pipevine Swallowtail butterfly at this Flickr.com web site. A wide range of his nature photographs are found at Flicker collections. Fair Warning: Addicting- don't enter unless you have time to spare!