The Life Cycle of Black Swallowtail Butterflies

A few years ago, after seeing a bunch of caterpillars on the parsley plant in our backyard get eaten immediately after hatching, my mom decided to keep a few of the remaining ones safe in a mesh butterfly cage.

Those caterpillars turned out to be eastern black swallowtails (Papilio polyxenes asterius), a native butterfly species that uses parsley as a host plant. Unlike the ones in the garden bed, the caterpillars in the butterfly cage were protected from danger, and grew into full adult butterflies. Witnessing them grow was incredibly fascinating, and it allowed me to better appreciate the metamorphosis they undergo.

Black swallowtails lay their eggs on plants from the Apiaceae family (more commonly referred to as the carrot family). The eggs are typically pale yellow and spherical in appearance. While black swallowtails can lay hundreds of eggs, very few of them actually survive long enough to hatch.

The caterpillars that do emerge are extremely tiny and covered in small spikes. After eating their eggshells for nutrients, they begin to feed on the host plant. The toxins they absorb from the host plant cause them to taste bad, a defense mechanism against predators. The caterpillars also have a pronged orange organ that protrudes from their heads when threatened. Called the osmeterium, this adaptation releases a foul odor, and aids in warding off potential threats.

Gallagher, J. (n.d.). Young black swallowtail caterpillar. In Flickr.

Caterpillar larvae go through multiple instars (stages between molts). When they’re young, they’re mostly black with a white stripe and some orange-tinged spikes. They molt a few more times before reaching their fifth and final instar as caterpillars, where they’re green and black banded with yellow spots. During this stage, they eat voraciously in order to prepare for their upcoming transformation.

Tabata, A. (2021). Older black swallowtail caterpillar.

Eventually, the caterpillars will enter a “wandering” phase where they’ll start crawling around looking for a place to form their chrysalis. They typically try to distance themselves from their host plant. Once they’ve found a spot, they hang in a “J” shape. Black swallowtails create a silk girdle to anchor themselves to their spot. Caterpillars might hang in the “J” for several days on end, but once they begin the actual process of pupating (forming the chrysalis), it only takes a few minutes. (Side note: unlike a cocoon, which is spun from silk and plant matter, the chrysalis is fully part of the butterfly. It’s a hardened exoskeleton that protects them as their insides transform within.)

The chrysalides of black swallowtails vary in shades of brown or green, depending on environmental factors. Shortly before the butterfly is going to emerge, the chrysalis turns a darker green, and the patterns of the butterfly’s wings are visible from the outside. The pupa stage normally takes about 9-18 days, but depending on the circumstances of their surroundings, some black swallowtails overwinter in their chrysalis and emerge in the spring.

Hall, D. (n.d.). Green pupa of the eastern black swallowtail. In University of Florida.

Finally, the adult butterflies emerge. After drying their wings for a few hours, they’re ready to take flight. The species exhibits sexual dimorphism in its wing coloration, with males displaying more prominent yellow spots and females displaying more prominent blue spots. All black swallowtails have distinctive bright orange spots on their outer hindwing. Their wingspans range from 6.9-8.4 cm.

Black swallowtails spend a relatively short amount of their life as adults. Most only live for one to two weeks after emerging. During that time, they play an important role in the environment as pollinators, feeding on the flowers of all kinds of plants. They also mate and seek a host plant to lay their eggs, where they start the cycle anew.

Tabata, A. (2021). Female adult black swallowtail.

Like other butterflies, the many forms of the black swallowtail all contribute to its broader role in the ecosystem. Seeing one offers a fascinating glimpse into the complex and interconnected world that surrounds us. So the next time you’re near a plant from the Apiaceae family, keep an eye out for black swallowtails!

Sources

NAVA TABATA

Editor at The City Voice

Nava is currently a senior at City. She has been writing for The City Voice since her sophomore year and now serves as an editor. Although she enjoys covering a wide range of subjects, her favorite topics to write about are history and ecology. Outside of the newspaper, you can usually find her playing violin in City’s pit orchestra, volunteering at her local library, or going on walks to observe the antics of her neighborhood squirrels.