The flowers of the white-veined pipevine, Aristolochia fimbriata, resemble some strange sea creature with an array of tentacles surrounding a rugose head and dark maw. The “mouth” won’t consume anything larger than the small flies that it attracts as pollinators, but the flowers aren’t the only thing about this plant that is vaguely Lovecraftian. A. fimbriata is also a host plant for the sinister, spiky caterpillars of the Pipevine swallowtail butterfly (Battus philenor).
It has been suggested  that pipevine swallowtail caterpillars mimic tropical Onychophorans (velvet worms), and the two horns do give it vague resemblance. I’m not sure the theory makes sense, though. The caterpillars’ diet of toxic Aristolochia leaves probably makes them less palatable than the velvet worms they are supposedly mimicking for protection, and some other caterpillars that feed on toxic plants (e.g. Monarch and Gulf Fritillary larvae) also have spikes or tendrils as part of their warning to predators.
The heart shaped leaves that the caterpillars feed on are as beautiful as the flowers are weird:
Many Aristolochia species are large, climbing vines, but the stems of A. fimbriata stay small, no more than a foot or two long, and instead of twining, they creep along the ground around other garden plants. This is a plant that plays well with others.
Seed production is quite prolific in my garden, and in some flowerbeds the plants have started to form a very pretty summer ground-cover. Nevertheless, A. fimbriata doesn’t seem to be an invasive species; the seedlings surround the original mother plants and haven’t spread to other parts of the garden. And in any case, biological control comes on blue and black swallow-tailed wings.
A. fimbriata is a subtropical species native to southern Brazil, Bolivia, Paraguay, Uruguay, and Argentina. Its foliage turns to mush at the first frost, but the tuberous roots are completely hardy here in the piedmont. Seed also overwinters in the mulch of my flowerbeds and sprouts in the spring.
The butterflies arrived early this year, and the first crop of caterpillars have reduced many plants to stubs. Now, there are chrysalids scattered around the garden. When ready to metamorphose, the caterpillars will travel a considerable distance to find a good spot:
The butterflies will probably emerge and lay the next generation of eggs just about the time the foliage starts to look good again, but hosting such beautiful insects in the garden is well worth a little leaf damage.
 “It has been suggested”–The passive voice here is a weaselly way of indicating that I have read this in various places but haven’t been able to track down the original source.