Papilio polyxenes, the (eastern) black swallowtail, American swallowtail or parsnip swallowtail, is a butterfly found throughout much of North America. An extremely similar-appearing species, Papilio joanae, occurs in the Ozark Mountains region, but it appears to be closely related to Papilio machaon, rather than P. polyxenes. The species is named after the figure in Greek mythology, Polyxena (pron.: /pəˈlɪksɨnə/; Greek: Πολυξένη), who was the youngest daughter of King Priam of Troy. Its caterpillar is often called the "parsleyworm" because the caterpillar feeds on parsley.
The Papilio polyxenes demonstrates polyandry and a lek mating system, showing no male parental care and display sites. Females are therefore able to choose males based on these sites and males are the only resource the females find at these sites.
Taxonomy Papilio polyxenes is part of the tribe Papilionini of the swallowtail butterfly family, Papilionidae. The members of this tribe all have tails on the hindwings, and therefore include species named swallowtail. P. polyxenes is part of the genus Papilio, which is the biggest group of the family Papilionidae. Subspecies polyxenes, once found in Cuba, is now considered likely extinct. Members of this genus typically feed on plants of the family Lauraceae, Rutaceae and Umbelliferae.
Distribution Papilio polyxenes are found from southern Canada through to South America. In North America they are more common east of the Rocky Mountains. They are usually found in open areas like fields, parks, marshes or deserts, and they prefer tropical or temperate habitats.
Morphology
Eggs and larvae Eggs are pale yellow. Young larvae are mostly black and white with a saddle, and older larvae are green with black transverse bands containing yellow spots.
Caterpillar and chrysalis This caterpillar absorbs toxins from the host plants, and therefore tastes poor to bird predators. The parsleyworm has an orange "forked gland", called the osmeterium. When in danger, the osmeterium, which looks like a snake's tongue, everts and releases a foul smell to repel predators.
The pupae may be green or brown, but not depending on surroundings or the background on which they have pupated. The color of the chrysalis is determined by a local genetic balance that ensures the majority of pupae will blend in. A section of the green pupae will turn a much darker green at the very end of the pupae stage. This color change occurs a few hours to a full day before hatching. Unusually, this butterfly's chrysalis is girdled with a loop, with its feet attached to the silk pad, which helps the butterfly to unfold its very large wings when crawling out.
Sexual dimorphism
The black swallowtail has a wingspan of 6.9–8.4 cm, and females are typically larger than males. The upper wing surface is black with two rows of yellow spots – these spots are large and bright in males and smaller and lighter in females. Females have a prominent blue area between these two rows, while males have a much less prominent blue area. These differences give rise to effective Batesian mimicry of Battus philenor seen in females. Both sexes show a red spot with a black bullseye on the inner hind margin of the hindwings and an isolated yellow spot on the front edge of the wings. The ventral side of wings of males and females are essentially identical: forewings have two rows of pale yellow spots, and hindwings have rows of bright orange spots separated by areas of powdery blue. The ventral side also acts as an effective mimic for both males and females for protection against predators.
Mimicry Female markings are similar to those of B. philenor, allowing females to engage in dorsal mimicry to reduce risk of predation by birds that preferentially prey on the black swallowtail. Females have evolved dorsal mimicry because they spend more time revealing their dorsal wing side during oviposition. The ventral wing surface of the black swallowtail also mimics that of B. philenor, so both males and females are protected when their ventral wing surface is displayed.
Intrasexual selection Male black swallowtails can sometimes mimic the female wing-back pattern, and therefore succeed in reduced predation as well. However, males of the typical coloration are more successful in intrasexual competition for mating territories compared to the males who mimic the female wing pattern. Females have no preference based on wing markings, and are equally likely to mate with a typical versus an alternative coloration. Therefore, male-male intrasexual selection is of greater importance than female mate choice in maintaining the classic male wing-back coloration and pattern.
Life cycle
Females lay single eggs on host plants, usually on the new foliage and occasionally on flowers. The eggs stage lasts 4–9 days, the larval stage 10–30 days, and the pupal stage 18 days. The duration of these stages may vary depending on temperature and the species of the host plants. Feeny et al., 1985 finds that survival of eggs and larvae of P. polyxenes varies depending on the location (near Ithaca, New York) where the eggs were laid. Eggs laid on wild carrot or parsnip plants were more likely to survive to the fifth instar than were eggs laid on poison hemlock. Eggs laid on plants at elevations of 300 to 435 metres (984 to 1,427 ft) above sea-level were more likely to survive than eggs laid at 120 meters above sea-level. There were also significant differences in survival rates between early and late broods in a year, and between years. In all cases, the primary cause of mortality was predation.
Emergence Winter is spent in the chrysalis stage, and adults will emerge in the spring to seek out host plants. Adults will emerge in the mornings on a daily basis. First brood adults will fly from mid-May until late June, second brood adults will fly from early July until late August, and occasionally a partial third brood will occur that will emerge later in the season.
Life expectancy Members of the black swallowtail are long lived compared to other butterflies that inhabit temperate zones. They encounter little predation and are quick and agile if they are disturbed. However, mortality from predators will occur during roosting and during unfavorable weather due to the associated increase in predation. Adult butterflies are at the highest risk for predation when they are incapable of flight or are starved from poor weather.
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