Swanflower is a small herb that plays an interesting role in the environment along Zarzamora Creek: it’s interesting because it’s toxic. Ironically, it was given to women in childbirth.
The plants in the genus Aristolochia are known as pipevines, so named because the flowers looked like deeply curved tobacco pipes to European settlers.
The species that’s common along San Antonio’s creeks is A. erecta, or Swanflower. A. reticulate, common on the Red River, is known as Texas Ducthman’s pipe. (The deeply curved pipes, such as Meerschaums, may have been associated with German immigrants, or “Dutchmen.” While Aristolochia flowers looked like tobacco pipes to European settlers, they look like orchids to botanists.)
Atistolochia is the prototypical genus of Aristolochiacea, the birthwort family. “Wort” is the old word for herb, so the common name reflects the Greek words aristos and locheia for “excellent” and “childbirth.” Pipevines are still used by midwives in various cultures, although the toxins in these plants have generally put them out of favor in the West.
Many plants in this family have aristolochic acid, which can induce genetic mutations. The evidence mounted that the acid can lead to kidney damage and cancer, prompting warnings from the U.S. Food and Drug Administration.
The toxins are a key to the pipevines’ ecology, and the plants are associated with a butterfly.
Pipevine swallowtails, Battus philenor, are big black swallowtails with hindwings of iridescent blue. While other animals, including humans, get sick from the pipevines’ toxins, Pipevine swallowtails and their relatives thrive on Aristolochia.
The adult female lays her eggs on the pipevine, and then the hatching caterpillars go to work. They are voracious. They tend to eat the entire plant, then go in search of the next.
Birds are the main predators of butterfly caterpillars. But the Pipevine swallowtail caterpillars have absorbed so much toxin that birds that eat them get sick. The result: birds eat other caterpillars, but avoid Pipevine swallowtails.
Biologists say that’s an example of adaptation. The Pipevine swallowtails evolved by developing an immunity to the toxins that make them unpalatable to predators.
Some biologists have suggested that Aristolochia erecta is itself evolving to mimic trailing grass. The advantage is that plants that don’t look like a typical pipevine to a Pipevine swallowtail caterpillar wouldn’t get devoured.
The plant is associated with the whole lifecycle of Battus philenor. The adult males stake out the pipevines and wait for the females. A male then hovers above the female, fanning her with pheromones.
The female lays her red-orange on the pipevine’s stems, usually in a place that is open to sunlight. If the eggs were tiny pool balls, they’d be stripes. Aristolochia butterflies excrete a stripe of material on each egg. It’s nourishing — and the first thing the caterpillar eats when it emerges from the egg. The caterpillar then eats the remainder of the egg and then goes to work on the plant.
Most Pipevine swallowtail caterpillars are brown-black. But in areas with higher temperatures, including Texas, most caterpillars are red. (Generally, Pipevine swallowtails produce two generations a year, one of which overwinters as pupae.)
The caterpillars are about 2 inches long and have orange spots in rows along their bodies.
Caterpillars go through phases and then pupate. The pupae are green or brown, with a distinctive wing-shaped widening along the sides. These “wings” usually are streaked with purple.
The butterfly that emerges is striking. Swallowtails are large butterflies — their wingspan can reach 5 inches. The iridescent blue on the hindwings is hard to miss.
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