This week yet another arctic blast pummeled the eastern half of the US with bone-chilling cold and blizzard misery. Time to escape once again to the tropical rainforests of Central America where we recently met fungus gardening ants, chemically defended termites, guardian ants of the bullhorn acacia, and amorous brush-footed butterflies. This week we visit the gorgeous Polydamas swallowtail and some of its kin and learn how they protect themselves from voracious predators in the rainforest.
Potent chemical defenses sequestered from pipevine protect the Polydamas swallowtail, enabling it to bask with impunity in the sunshine in full sight of potential predators.
This tale begins with the food plant of the Polydamas swallowtail, members of the Dutchman’s pipe family, Aristolochia. These plants evolved a class of highly toxic alkaloidal compounds known as aristolochic acids to dissuade the many vertebrate and invertebrate herbivores wishing to consume their tender nutritious leaves. However, caterpillars of the Polydamas swallowtail and their cousins in the genus Battus turned the table on their noxious host plant. Battus caterpillars not only tolerate these toxic compounds, they actually ingest, sequester, and then employ these toxins as defenses against their own predators.
In a fascinating study with the pipevine swallowtail, Battus philenor, scientists demonstrated that as caterpillars develop into adult butterflies, they carry along nocent aristolochic acids and advertise their own toxicity with contrasting colors of red and black in much the same way orange and black monarch butterflies warn would-be predators of their distastefulness. But transfer of nasty chemicals does not stop with the adult butterfly. No, the female Battus actually transfers some of the potent aristolochic acids to her eggs, presumably to help protect them from tiny ova- loving predators.
There is no doubt that swallowtail butterflies and their caterpillars are masters of chemical warfare, but the larvae of swallowtail butterflies have one more secret chemical weapon to unleash against predators. Just behind the head of the caterpillar is a specialized structure called the osmeterium. This forked, orange colored appendage is usually tucked beneath the skin out of sight. When the swallowtail larva is threatened, the caterpillar extends the osmeterium in the direction of the disturbance. This glandular organ is coated with foul smelling chemicals which in some species is reminiscent of rancid butter. The disturbing visual and olfactory display likely discourages hungry predators from wanting to dine on this beautiful caterpillar. Despite their stunning appearance as adults and larvae, many swallowtail butterflies may be the Lucrezia Borgias of the butterfly world.
When harassed by a predator or nosy bug geek, swallowtail caterpillars like this magnificent Mimoides evert the osmeterium and chemically assault their tormentor.
We thank the hearty crew of BSCI 339M, ‘Mayan Culture and the Interface between Tropical Rainforests and Coral Reefs’, for providing the inspiration for this Bug of the Week. We thank Lepidoptera gurus David Wagner and Lee Dyer for providing most excellent advice in identifying the caterpillars and butterflies featured in this episode. The wonderful articles “Phenological Variation in Chemical Defense of the Pipevine Swallowtail, Battus philenor” by James A. Fordyce, Zachary H. Marion, and Arthur M. Shapiro; and “Secret Weapons: Defenses of Insects, Spiders, Scorpions, and Other Many-Legged Creatures” by Thomas Eisner, Maria Eisner, and Melody Siegler; and the “Butterfly Fun Facts website “ (http://www.butterfly-fun-facts.com/butterfly-species/butterflies-f-j/gold-rim-butterfly-battus-polydamas/) were used as references for this episode.