Natural History of the Spicebush Swallowtail 

The Spicebush Swallowtail Butterfly, Papilio troilus L., is probably the most commonly observed swallowtail. The adult of this predominantly black butterfly favors large light gaps in forested areas and, as the name implies, the larvae are found primarily on the leaves of spicebush (Lindera benzoin L.). This family of insects (Papilionidae) contains the largest (Papilio homerus Fabricius, Jamaica's Giant Swallowtail) and perhaps some of the most beautiful species of butterflies. Several species of swallowtails are common in the eastern United States, including a few species with color patterns that make them difficult to distinguish from the Spicebush Swallowtail, especially in flight. In this area, these include the Black Swallowtail (P. polyxenes asterius Stoll), the Pipevine Swallowtail (Battus philenor L.) and the black phase of the Tiger Swallowtail (P. glaucus L.). 

An adult Spicebush Swallowtail is most easily differentiated from other black swallowtails by a double row of seven to eight small whitish to gold spots along the margins of its otherwise black front wings. The hind wings
have a double row of somewhat larger half-moon shaped, whitish-to-gold spots along their lateral margins, and a single bright orange spot along the inner margins. There is also a large bright orange spot at the front margin
of each of its hind wings, but these spots are only visible on a specimen that has its wings spread. The center parts of the hind wings have a broad patch of color, ranging from blue to gray in females, and yellow to green in
males, that blends into black. Each hind wing also has a distinctive tail-like elongation, which is a diagnostic family trait. 

Like other holometabolous insects (those that go through complete metamorphosis at each stage), phases of a Spicebush Swallowtail's life cycle include: egg, larval instars (caterpillar stages), pupa (chrysalis), and adult. Although they must be abundant considering the size of the local population of both adults and larvae, the singly laid eggs are very difficult to find. They are flattish round, approximately 1 mm in diameter, off-white in color, and look somewhat like single pale radish seeds laid on the upper surface of spicebush leaves. 

The fifth instar stage of the spicebush swallowtail:   The color of the first-instar larva when it emerges from the egg is dark bronze. Microscopic examination of this tiny larva reveals an elaborate array of sharp spines projecting off lobes on its prothorax (segment bearing the first pair of legs) and last abdominal segment . . . presumably, these are "egg-bursting spines." The feeding pattern of this first instar is distinctive, and provides the most useful visual aid finding larvae. First, the larva makes a lateral cut from the leaf margin, approximately 45 degrees, to the mid-vein. It then lays down a silken track across the cut portion of the leaf by moving its head from side to side. As it dries, the silk causes this section of the leaf to fold, forming a cavity to which the larva repeatedly returns between nighttime feeding bouts
on the surrounding leaf tissue. The silk also helps the larvae grip onto the otherwise smooth surface of spicebush leaves. Like the first instar, subsequent instars initially lay down a silk bed across the leaf surface,
which causes a leaf to fold in half and provide a hiding place for the larva. 

Each of the five larval instars of the Spicebush Swallowtail has a bright orange, forked osmeterium or scent gland at the upper end of the prothorax, which is characteristic of swal lowtail butterflies. This gland extends and
releases a pungent odor that smells like concentrated spicebush when a larva is disturbed. The second instar is blackish green and white, somewhat bumpy, with black spots on its prothorax. It resembles a bird dropping. The next two instars are smooth, lighter green with some white areas, and the black spots on the prothorax are more prominent. The last (fifth) larval instar is velvety smooth and bright green with two very large black spots on its prothorax that look like the eyes of a snake (see photo). The color pattern of this fifth instar is thought to be a defensive mechanism against birds, its main predator. The forked osmeterium, which resembles a snake's tongue, may also be part of this mimicry. In addition, a larva assumes a pose similar to a snake's striking posture when it is disturbed. 

When the last larval instar is ready to pupate, it evacuates its gut contents and changes color from bright green to bright yellow. In captivity, this prepupal stage typically moves off the host plant and finds a hard, vertical
surface on which to attach itself by silken threads. The chrysalis, which is not encased in a silken cocoon, can be brown to green and has two forks pointing upward. 

The Spicebush Swallowtail is multivoltine (has more than one development cycle per season) and several generations are produced each summer. The population at SERC seems to be at its peak in late August. It takes approximately one month from egg to pupa during midsummer. Adults seem opportunistic and keep laying eggs at the end of the season past any hope of complete larval development. During the summer, the pupal stage lasts about 3 weeks. The Spicebush Swallowtail overwinters as a chrysalis.