Listen to Austin 360 Radio

Following the swallowtails

Distinctive, colorful butterflies are out in full flutter, but they're fascinating as caterpillars, too

Jim and Lynne Weber

The flowers that bloom in late spring and early summer are often visited by large, colorful butterflies of the swallowtail family.

These insects are so named because the tails on their hindwings resemble the forked tails found on birds in the swallow family. They are even more distinctive when in the caterpillar stage because they possess a hidden structure behind their heads called an osmeterium, a fork-shaped organ that is exposed when under threat (or forced out with a gentle squeeze) and can emit a smelly and bad-tasting secretion. Because of this, swallowtails are distasteful to many predators in both their caterpillar and adult butterfly forms.

The smallest and darkest swallowtail seen this time of year is the pipevine swallowtail (Battus philenor). Its upper side has blackish-brown forewings and hindwings that have an iridescent metallic blue sheen. The undersides of its hindwings have bright orange dots surrounded by black and blue, with subtle white markings.

Pipevine swallowtail caterpillars, which are reddish-brown with rows of fleshy red tubercles rising up from their backs, normally feed in small groups on plants in the pipevine family. It is these plants that give the insects their poisonous quality.

Often found in urban and suburban gardens, the black swallowtail (Papilio polyxenes) flies spring to fall, with the male more colorful than the female. The male is mostly black with two rows of yellow spots along the wing edge, while the female has only one row of reduced yellow spots on her forewing and blue scaling on her hindwing, mimicking the pipevine swallowtail.

Black swallowtail caterpillars feed on plants in the parsley family and are distinctively patterned with light green or white bands alternating with black bands spotted with six yellow dots.

One of our bigger and brighter swallowtails is the two-tailed swallowtail (Papilio multicaudata). Growing to more than 5 inches wide, its distinctive yellow wings have black, tigerlike striping, with a row of bright blue spots along the trailing edge on both sides of the hindwings. Each hindwing also has two primary black tails, giving this insect its common name.

The two-tailed swallowtail caterpillars are carrot-orange with a pair of pale yellow eyespots on top of the head and a pale yellow band behind the head, making them resemble a small snake. In our area, these caterpillars feed on the leaves of the wafer ash or hop tree.

Our largest butterfly is also a swallowtail, aptly named the giant swallowtail (Papilio cresphontes), with a wingspan of more than 6 inches. Its wings are dark brown to black with yellow bands and a yellow spot in the center of each tail on the hindwings. Their young caterpillars look like bird droppings, which makes them much less appealing to predators. Giant swallowtails have distinctive flight patterns, with the females tending to beat their wings slowly but moving quickly over long distances and the males having a more rapid, jerky flight because of their slightly smaller wings.

You may find an adult swallowtail butterfly missing some or all of its tails. Though the true purpose of the taillike extension on the hindwings of these butterflies is unknown, it is thought they trick predatory birds into biting off this expendable part of their wing, giving the swallowtail a chance to fly yet another day.

About Nature Watch