Here in the DMV, butterfly season is off and running at full force. A visit to a pollinator garden or meadow is rewarded with an array of several beautiful butterflies whose dominant wing coloration is based on the theme of a black background with series of patches and spots in shades of white, orange, and blue. Butterflies participating in this cabal include the dark form of the eastern tiger swallowtail, the pretty red-spotted purple, the eastern black swallowtail – eater of parsley and dill, and the clever spice bush swallowtail whose larvae mimic serpents. Why have these beauties converged on a rather common color scheme?
As caterpillars feed and grow, they accumulate noxious compounds and take on a startling visage, complete with fleshy dangling appendages and orange bumps advertising their distastefulness. Image: Paula Shrewsbury, PhD
Caterpillars of the butterfly that we meet today, the pipevine swallowtail, dine on members of the birthwort family, Aristolochiaceae, plants that include vines commonly known as pipevines. Members of this family produce a class of compounds known as aristolochic acids which are known to be mutagenic, carcinogenic, and toxic to kidneys of mammals. As they consume pipevine leaves, these swallowtail caterpillars store aristolochic toxins in their tissues, and then retain them as they pupate into adult butterflies, which then pass them along to their eggs. Aristolochic acids sequestered by caterpillars may help protect them from attack by parasitic wasps. Adult butterflies are rendered distasteful to vertebrate predators such as birds by virtue of these noxious compounds. It is believed that birds attempting to eat butterflies whose larvae consumed pipevines have a nasty experience that teaches them not to mess with darkly colored butterflies.
Leaves of the pipevine contain noxious and toxic compounds that help protect pipevine caterpillars and swallowtails from their enemies. Image: Paula Shrewsbury, PhD
Other species of butterflies capitalize on the lesson taught by the pipevine swallowtail and have evolved color patterns to mimic their appearance, thereby gaining protection from visually astute predators. This type of mimicry in which warning colors of a distasteful species like pipevine swallowtails act as a model copied by other palatable butterflies is called Batesian mimicry. The great English naturalist Henry Bates first described this form of mimicry while studying butterflies in Brazilian rainforests. We met other species of Batesian mimics such as harmless flower flies that mimic stinging bees and wasps.
This gorgeous resting pipevine swallowtail will soon seek pipevines on which to deposit brightly colored orange eggs that hatch into tiny caterpillars. As these very hungry caterpillars feed and grow, they accumulate noxious compounds and take on a startling visage complete with fleshy dangling appendages and orange bumps advertising their distastefulness. Other darkly colored butterflies, including the dark female form of the eastern tiger swallowtail, the spice bush swallowtail, the eastern black swallowtail, and the pretty, red-spotted purple are believed to mimic the color pattern of the pipevine swallowtail to gain protection from hungry predators.
Adult butterflies are not the only creatures that mimic pipevine swallowtails. Recall that the caterpillars are also laced with aristolochid acids. Scientists believe that a cyanide producing millipede with deep red coloration mimics the color and pattern of the pipevine caterpillar, thereby gaining protection from predators. We have seen this mimicry before with several orange and black insects that eat milkweed including monarch caterpillars, milkweed bugs, and milkweed tussock moth caterpillars. This convergence on a similar, easily recognizable color pattern by two or more nasty-tasting insects is called Müllerian mimicry, named for the famous German naturalist Fritz Müller. As this spectacular butterfly season rolls along, try to grab a glimpse of one of these dark winged beauties and ponder the question – model or mimic?
Acknowledgements
Bug of the Week thanks observant Dr. Shrewsbury for spotting adults and larvae of the pipevine swallowtail and providing images featured in this episode. “Secret Weapons” by Thomas Eisner, Maria Eisner, and Melody Siegler, and “Featured Creature Common name: pipevine swallowtail, blue swallowtail scientific name: Battus philenor (Linnaeus 1771) (Insecta: Lepidoptera: Papilionidae: Papilioninae: Troidini)” and “Common name: Polydamas swallowtail, gold rim swallowtail, tailless swallowtail scientific name: Battus polydamas lucayus (Rothschild & Jordan) (Insecta: Lepidoptera: Papilionidae: Papilioninae: Troidini)” by Donald Hall provided valuable insights into the clever ways of this week’s star.