Eastern Tiger Swallowtail (Papilio glaucus)
Spring arrived, and along with it Papilio glaucus or the Eastern Tiger Swallowtail, one of our most spectacular butterflies. While adults feed on nectar of a wide variety of flowers, their caterpillars feed on leaves of Tulip trees (Liriodendron tulipifera), Black Cheeries (Prunus serotina), among many others. Choosy they are not!
The common name tells it all: this large butterfly, with a wingspan of up to 5.5 inches, belongs to the family of Swallowtails or Papilionidae (with over 570 species worldwide), and can be found in Eastern North America from Vermont to Florida, and show bold black stripes on their yellow wings (“tiger”), along with elegantly elongated tips at the end of their hindwings (“swallowtail”). They can be observed in a variety of habitats, including woodlands, fields, and in your garden, assuming you planted some butterfly-friendly (and hopefully native) flowers! For more information on how to do that, please see at the reference list.
After emergence from tiny green eggs, which the butterfly mom conveniently placed on their host trees, the caterpillars get right to business: first, they eat their egg shell (no waste in nature!), and then they use their large mandibles to chew up the leafs of their host tree. The caterpillar, which eventually will become about 1 ¼ inches before it pupates, is, unlike so many other swallowtail caterpillars, dull and green in appearance. The head itself has two rows of simple eyes, short antenna, and mandibles. The body has 13 segments, of which the first three form the thorax with a pair of true legs on each of them. Five of the other ten remaining segments have fleshy outgrowths which function as legs, keeping them stable on stems and leaves.
Given the essentially unlimited food supply, caterpillars get bigger by the minute. Like all arthropods, Eastern Tiger Swallowtails have their skeleton outside (“exoskeleton”), and thus have to replace this sturdy shell with an even bigger one, if they want to continue to grow. They do this by shedding their outer skin (“cuticula”) through a process called molting. Molting is orchestrated by a variety of insect hormones, including ecdysone and juvenile hormone. Once a caterpillar reaches a certain size, ecdysone is released, and in the presence of juvenile hormone it molts into another, larger caterpillar. However, with each molt, less and less juvenile hormone is released. And if the caterpillar was a good baby, and ate everything Mother Nature provided, the levels of juvenile hormone will eventually drop to such low levels, that when ecdysone rises once again, the caterpillar starts to pupate, instead of becoming another, even larger caterpillar.
And so it goes: the caterpillar, with lots of ecdysone, but little juvenile hormone, has one more bit of a tulip tree leaf, ejects whatever was not digested, and wanders to a safe spot where it molts into a pupa (“chrysalis”). Depending on the season, the pupa will metamorphose into our beloved Eastern Tiger Swallowtail within a few weeks (this species has up to three broods per year), or it overwinters, until spring returns.
Upon emergence, the careful observer can easily distinguish between males and females. While the hindwing of the male (figure above) has a dark black band along the edges, females have the same band with elegant blue spots, and usually also carry a slightly thicker “tail”. Interestingly, females come in two forms (“morphs”), depending on the region. In addition to the morph described above, females also have a dark morph in certain regions of the USA, and are thought to mimic the toxic Spicebush Swallowtail (Battus philenor), which is avoided by avian predators. As females refuse to mate with males who try get away with the same disguise, males are stuck with the conspicuous tiger pattern, attractive to female Tiger Swallowtails, humans, and birds alike.
You might also be able to observe (young) males congregating at puddles, mud, and even dung and carrion, a behavior which is called “puddling”. They do this to extract additional sodium ions and amino acids, as nectar is full of sugar, but not much else. And, like good gentlemen, they present this gift to their lady to give their offspring a head start. In fact, it has been shown that males who are able to provide more sodium, will allow the female to have more offspring.
Next time you see one, ask yourself: is it a male or female? Which morph? And if you have a little garden available, why not welcome them with a native flower bouquet!
Belth, J. E. 2013. Butterflies of Indiana – A Field Guide. Indiana University Press, Bloomington.
NABA-North Jersey Butterfly Club. 2017. New Jersey Butterflies – Eastern Tiger Swallowtail (http://www.naba.org/chapters/nabanj/butterflies/eastern_tiger_swallowtail.html)
NABA-North Jersey Butterfly Club. 2017. Creating a butterfly garden (www.naba.org/chapters/nabanj/gardening.html)