The Swallowtail family, known scientifically as Papilionidae, includes some of the world’s largest and most visually striking butterflies. Their common name comes from the tail-like extensions often present on their hindwings. Before transforming into these magnificent fliers, the caterpillars undergo several distinct visual stages, or instars, each with unique camouflage strategies. This guide focuses on the specific physical characteristics that define the appearance of these fascinating caterpillars throughout their development.
Appearance of Early Instar Larvae
The first few larval stages, typically the first to third instars, present a different appearance than the mature caterpillar. Newly hatched larvae are small, often measuring less than half an inch in length. Their primary survival strategy during this vulnerable time is visual deception known as “bird dropping mimicry.”
The body of the young caterpillar is generally dark, usually black or dark brown, with a prominent white or cream-colored patch or saddle across the middle segments. This mottled, contrasting coloration is highly effective at making the larva appear to a predator as an unappetizing bird dropping. Many early instars, such as the Black Swallowtail, also possess a slightly spiny or bristly texture. These small, dark forms often rest exposed on the upper surfaces of leaves, mimicking where a bird dropping would naturally land.
Characteristics of Mature Larvae
As the caterpillar grows through its final stages (the fourth and fifth instars), its appearance changes in a shift away from camouflage. The larva rapidly increases in size, often reaching lengths of one and a half to two inches before pupating. The body transitions from the dark, mottled appearance to a smoother, plump, and cylindrical form.
A common pattern among many species, such as the Black Swallowtail, involves a bright, smooth-skinned body of pale green or yellow-green with distinct transverse black bands. These black bands are frequently punctuated by rows of bright yellow or orange spots. Other species, like the Eastern Tiger Swallowtail and Spicebush Swallowtail, adopt a uniform green color with a slightly swollen thoracic segment. This final stage coloring is thought to help the caterpillar blend into the dense foliage of its host plant, where dappled sunlight helps obscure it.
Specialized Identifying Features
The most remarkable identifying feature of the swallowtail caterpillar is the osmeterium. This soft, fleshy organ is typically hidden within a fold just behind the head. The structure is Y-shaped or forked, and when deployed, it rapidly everts, presenting a bright color, usually vivid orange or yellow.
The sudden appearance of this horn-like structure is a startling visual component of the caterpillar’s defense mechanism. Many mature larvae also possess two large, round markings on the swollen thoracic area known as false eyespots. These spots are often ringed in black with a yellow or orange center, creating the illusion of a much larger creature, such as a snake or small reptile. The false eyespots and the everted osmeterium work together to complete a visual threat display that often deters smaller predators. The osmeterium also releases a foul-smelling chemical cocktail, providing a chemical defense.
Appearance of the Chrysalis
The final visible stage before the butterfly emerges is the chrysalis, which is distinct from the silken cocoons produced by many moth species. Swallowtail chrysalises are described as being “naked,” meaning they lack a protective outer layer of silk or leaves. Their shape is typically angular and somewhat rough, often resembling a broken twig or a piece of bark. The color of the chrysalis is determined by the environment and the time of year.
A pupa formed during the summer months on a green stem will usually be green to match its surroundings. Conversely, chrysalises formed in the late fall or winter, which are intended to overwinter, are typically brown or gray to blend in with bare branches or leaf litter. The caterpillar secures its chrysalis by attaching a small silk pad at the base and a silken girdle, or loop, that wraps around the middle section of the body, holding the pupa firmly against its substrate.