Black swallowtails, Papilio polyxenes, like all butterflies, pass through four developmental stages: egg, caterpillar, chrysalis, and adult. Different predators seek out these stages when hunting. But this butterfly has strategies during each of its life stages that help it survive in a hostile environment.
Egg Defenses
The eggs are round, light green, and small. Placed on the lower surfaces of leaves of the parsley family (Umbelliferae), the eggs are often overlooked by searching wasps, beetles, and bugs.
Caterpillar Defenses
The first two instar stages of black swallowtail caterpillars are dull: brownish black with subdued reddish orange spines over their backs. These nondescript caterpillars often sit alongside the veins of the leaves they are eating and blend cryptically into the background. Their spines are irritating to the mouths of some birds and mammals and deflect the probing mouth parts of insects.
The third instar stage remains rather dull, but has a whitish band around the center of the caterpillar.
The fourth and fifth instars are brightly colored with alternating cream and chartreuse bands spotted with black and yellow (see image). Some caterpillars are very dark, but still have cream, chartreuse, and yellow markings. Caterpillars of this age have two lines of defense:
- They gradually accumulate a quantity of bitter tasting chemicals from their food plants (parsley, fennel, anise, dill, carrot, and relatives) that makes the caterpillars distasteful to some birds and mice. The bright colors of the caterpillar provide aposematic coloration that informs predators of this characteristic.
- There is a bright orange, fleshy, two part projection, the osmeterium, hidden in the dorsal neck folds of the caterpillar. When the caterpillar is disturbed, it exposes the osmeterium which then emits foul-smelling terpenes, possibly derived from those in their food plants. The combination of the sudden presence of a large, brightly colored projection and the sharp odor startle predators and are often sufficient for the caterpillar to escape being eaten.
These defensive characteristics work fairly well against visual and olfactory predators that are larger than the caterpillars and would normally disturb them. But for parasitoid wasps and flies, there are no defenses. These insects lay eggs on the surface of the caterpillar. The larvae burrow under the skin and eat the imaginal disks composed of the stem cells that should produce the adult butterfly. The caterpillar eats and grows normally, but the potential to mature beyond that stage is removed. When the insect begins to metamorphosize, there is nothing available to develop, the parasitoids pupate and instead of a butterfly, adult parasitoids hatch out of the practically empty chrysalis.
Chrysalis Defenses
The black swallowtail chrysalis does not look like an edible object. Rather, it has the appearance of a twig or blemish on the surface of the plant to which it is attached. Its lack of movement and color cause it to be overlooked by many visual predators.
Adult Defenses
Adult black swallowtails have similar coloration to the related pipevine swallowtail, Battus philenor, a species that, like the monarch butterfly, Danaus plexippus, stores poisons in its fatty tissues. Large irridescent blue patches on the hind wings combined with a black overall color makes It difficult to distinguish the female black swallowtail from a pipevine swallowtail. This condition of an edible or tasty organism looking like a poisonous or dangerous organism is called Batesian mimicry. Another relative, dark phase female tiger swallowtails, Papilio glaucas, also take advantage of the pipevine's nature to obtain protection from predators.
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