As the name rightly suggests, the characteristic feature of a zebra swallowtail butterfly is the black and white stripes on its wings. This butterfly has tail-like extensions on its hind wings, hence the name "swallowtail." The hind wings may also have two blue markings at the base and a red spot close to the body. The margins of the hind wings are scalloped.
The wingspan of these butterflies ranges between 7 to 11 centimeters. There are two seasonal forms of zebra swallowtail butterflies: spring forms and summer forms (adult butterflies emerging from the pupa stage during spring or summer). The spring forms are smaller and whiter compared to the summer forms. In spring forms, the tails are small and black with white tips, whereas summer forms have longer tails with a prominent white color.
The zebra swallowtail butterfly (Protographium marcellus) is mainly found in the southeast parts of the United States and certain regions of Canada. It is the official state butterfly of Tennessee.
Life Cycle and Lifespan
Male zebra swallowtail butterflies fly around the host plants (pawpaws) in search of females. After mating, the female butterflies lay spherical eggs on the leaves of the pawpaw trees. The color of the eggs changes from light green to golden brown after three to four days.
The larvae are black in color on hatching, but as they grow, stripes develop. There are two types of stripe patterns in older larvae, which are hairless. The most common form is green with white and yellow horizontal stripes. They have a band of three colors (yellow, black, and bluish-white) between the thorax and the abdomen. The second type, which is rarely seen, is black with white and orange stripes.
These caterpillars live and pupate underneath the leaves of pawpaw trees. The pupa is usually green or brown in color, and is small and compact compared to that of other swallowtails. They hang with their heads upwards, binding to a stem or leaf with the tail. They also have a silk ring around their thorax.
A period of around one month is required for the eggs to develop into zebra swallowtail butterflies. Some pupae spend the entire winter in hibernation before emerging as butterflies. Once they emerge, there is no further growth stage, they will start reproducing right away. After emergence, a butterfly may live up to six months in its natural habitat.
Caterpillar Diet, Pawpaws
The habitat of zebra swallowtails includes those regions where pawpaw trees grow abundantly. These butterflies are seen in open woodlands near swamps and rivers, grasslands, savannas, and southern pine woods.
The tender leaves of pawpaw trees form the staple diet of zebra swallowtail caterpillars. For this reason, the adult females deposit eggs on the young leaves of pawpaw trees. Only a single egg is laid per leaf. The caterpillars are voracious feeders who may turn cannibalistic if they encounter other caterpillars. The "single egg per leaf" system is specifically to prevent cannibalism.
They feed on the eggshells soon after hatching. After that, the caterpillars move to the leaves of the pawpaw plant, small trees or shrubs with long, droopy leaves that produce an offensive, kerosene-like odor when bruised.
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Butterfly Diet
Flower Nectar: Adult butterflies do not feed on pawpaw leaves. Unlike caterpillars, butterflies lack jaws. Instead, they sip liquid nectar from flowers using a straw-like organ called the proboscis. Zebra swallowtail butterflies feed on the nectar and pollen of flowers like blueberry, blackberry, lilac, redbud, verbena, and common milkweed. Sometimes they collect pollen using the proboscis and digest it to absorb proteins. The extra energy from these proteins helps them to mate and lay eggs for longer periods. Their mating season can extend up to six months. The proboscis is not especially long in zebra swallowtails, so they are mostly found feeding on the nectar of flatter flowers.
Puddling: The males may also indulge in puddling, wherein they assemble together on moist ground to gather nutrients like salt and amino acids. These nutrients are transferred to the body of the female butterflies during mating. It is believed the nutrients help to maximize the survival rate of eggs. While some species derive nutrients from soil, others opt for sources like urine, dung, dead animal matter, tears, and even blood.
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Defense Mechanisms
Pawpaw Toxins: Their affinity for pawpaw leaves has another interesting reason. Pawpaw leaves contain toxic chemicals called acetogenins. While most insects avoid pawpaw leaves, the larvae of zebra swallowtails feed exclusively on them. These chemicals are retained in the body of the caterpillars throughout their life. Even adult butterflies have some amount of acetogenins in their body, though they no longer eat pawpaw leaves. The presence of these toxic chemicals may discourage predators like insects and birds.
Osmeterium: The distinctive feature of swallowtail butterflies (a forked gland called the osmeterium) is found behind the head of the caterpillar in this species too. This structure, which is usually hidden, can be pushed out to release a smelly secretion to deter predators. When the caterpillar feels threatened, the osmeterium fills with blood and emerges to emit the secretion. The foul-smelling secretion may deter small predators like ants and spiders.
Usually, the caterpillars never leave the pawpaw trees until they transform into adult butterflies. In some cases, a caterpillar may leave the tree and pupate in leaf litter on the ground, mostly as a defense mechanism to avoid predators. In short, zebra swallowtails have some striking features that make them distinct from others. They are also known as pawpaw butterflies and kite swallowtails.
Fun fact for today!The zebra swallowtail is Tennessee's official state butterfly!