What Is the Life Cycle of Swallowtail Butterflies? Egg to Adult
What is the life cycle of swallowtail butterflies? Swallowtails (family Papilionidae) go through four stages: egg, larva, pupa, and adult, a process called complete metamorphosis. The whole cycle can take as little as a month in warm weather or stretch across a winter if the chrysalis overwinters.
The Four Life Cycle Stages
Every swallowtail passes through the same four stages, though timing varies by species, temperature, and latitude. Species in the southern United States can complete two to three generations a year, while northern populations may fit in only one.
1. Egg Stage
A female swallowtail lays her eggs singly rather than in a cluster, usually on the top or underside of a host-plant leaf. The Missouri Department of Conservation notes that black swallowtail females deposit eggs singly on suitable food plants, and a single female may lay several hundred eggs over her lifetime, not all at once. The eggs are pale yellow or green, round, and roughly the size of a pinhead.
Host plants: Each species favors a narrow set of plants for egg-laying, since that plant becomes the caterpillar's only food source:
- Black swallowtail (Papilio polyxenes): parsley, dill, fennel, carrot, and other members of the carrot family, plus wild Queen Anne's lace
- Eastern tiger swallowtail (Papilio glaucus): tulip tree, wild black cherry, and sweetbay magnolia
- Giant swallowtail (Papilio cresphontes): citrus trees and prickly ash
Duration: Eggs typically hatch in 4 to 10 days, faster in warm weather.
2. Larva (Caterpillar) Stage
The newly hatched caterpillar eats its own eggshell first, then moves on to the host plant. Young black swallowtail caterpillars are black and white and resemble bird droppings, a disguise that fades as they mature into the familiar green, black, and yellow banded pattern.
Growth and molting: A caterpillar passes through five instars, molting its exoskeleton between each one to accommodate its growing body. Most of this stage is spent eating; a caterpillar gains a substantial amount of body mass before pupating.
Defense, the osmeterium: When disturbed, a swallowtail caterpillar rears back and everts the osmeterium, a forked orange or yellow gland behind its head. The USDA Forest Service describes the osmeteria as bright orange glands that produce a foul-smelling blend of defensive acid secretions, wiped onto the attacking animal. The structure also looks like a snake's forked tongue, which may startle a would-be predator on its own.
Duration: The larval stage generally runs 3 to 4 weeks, depending on species and temperature.
3. Pupa (Chrysalis) Stage
Once a caterpillar reaches its final instar, it stops eating, wanders away from the host plant, and finds a sturdy twig, stem, or wall to attach to. It spins a silk pad and a silk girdle to hold itself upright, then molts one last time into a chrysalis.
Camouflage: Chrysalises are usually mottled brown or green, matching bark or foliage closely enough that birds routinely miss them.
Overwintering: A summer brood's chrysalis may hatch in 9 to 14 days, but a chrysalis formed in fall typically overwinters and does not emerge until the following spring.
4. Adult Butterfly Stage
The adult emerges through a split in the chrysalis, usually in the morning. It hangs upside down while hemolymph pumps into its crumpled wings, then waits one to three hours for them to harden before its first flight.
Wingspan and flight: Eastern tiger swallowtails have a wingspan of roughly 3 to 5.5 inches (8 to 14 cm), among the largest butterflies in North America.
Feeding and mating: Adults feed on nectar and, in some species, mud puddles for salts and minerals. Common nectar sources include milkweed, coneflower, butterfly bush, and thistle. Most adult swallowtails live only a few weeks, just long enough to mate and lay the next generation of eggs.
Swallowtail Facts Worth Knowing
Species Diversity
Papilionidae contains over 550 species worldwide, concentrated in the tropics. North American species include the eastern tiger swallowtail (Papilio glaucus), black swallowtail (Papilio polyxenes), giant swallowtail (Papilio cresphontes), and spicebush swallowtail (Papilio troilus), each tied to a different set of host plants.
Mimicry
Female eastern tiger swallowtails in the southern part of their range often have a dark form that mimics the pipevine swallowtail, a species toxic to birds because its caterpillars feed on poisonous pipevine. Predators that have learned to avoid the toxic model leave the mimic alone too.
Broods and Range
Swallowtails do not migrate the way monarchs do, but the number of broods a species produces shifts with latitude. The eastern tiger swallowtail typically produces two broods in the north and up to three broods in the southeastern states.
Conservation Pressure
Habitat loss and herbicide use along roadsides and field edges reduce the host plants swallowtail caterpillars depend on. Planting parsley, dill, fennel, or native spicebush in a garden gives local females a place to lay eggs and can support a full generation from egg to adult.
Watching the Cycle at Home
Anyone who plants a swallowtail host plant can watch the full cycle firsthand: eggs appear within days of the first adults visiting, caterpillars are visible feeding within a week, and a chrysalis usually forms somewhere nearby rather than on the host plant itself. Checking the same few plants daily during spring and summer is enough to catch all four stages in a single season.
