On first glance, I thought the butterfly shown above was a late-flying Eastern Black Swallowtail.
After a closer look, I decided the unknown visitor might be a Pipevine Swallowtail. (I don’t have any photos of Pipevine Swallowtails because I’ve never seen one in person. Here’s a link with photos.) But how could it be any kind of swallowtail, without the characteristic “tails” on its hind wings?
As always, I turned to the internet for answers. Searching for “butterflies that look like Eastern Black Swallowtails” led me to the Swallowtail Butterfly Comparison page on a site called Butterflies at Home. There I discovered that my unknown butterfly is a Red-spotted Purple, which explains why it doesn’t have tails on its hind wings. It isn’t a swallowtail at all. Instead it belongs to the family of brush-footed butterflies. (As an aside, I’m now fascinated with name “brush-footed”.)
But why do all of these butterflies look so similar? What is so special about a combination of blue highlights and reddish spots? Obviously the pattern carries some sort of selective advantage, something deeper than aesthetic appeal for camera-wielding writers.
It seems that the story starts with Pipevine Swallowtails, which lay their eggs on the poisonous pipevine plant (also known as Dutchman’s Pipe.) As the caterpillars feed and grow, they ingest and store a toxin called aristolochic acid, which lingers in their bodies as the caterpillars mature into adults. So the butterflies, as well as all stages of the caterpillars, are poisonous. Even their eggs are poisonous.
All in all, it’s an elegant and effective defense against predators. So effective, in fact, that it conveys a measure of protection for any butterfly with black wings, blue highlights, and reddish spots. Selective advantage, indeed.
Now, if only I could find a Pipevine Swallowtail to photograph…
For more information, check out a few of these articles:
- “Spicebush Swallowtails and Pipevine Swallowtails” (side-by-side photos of a Spicebush Swallowtail and a Pipevine Swallowtail)
- “Don’t Eat ‘Em; They’re Poisonous” (article about Pipevine Swallowtails and caterpillars)
- Battus philenor at Art Shapiro’s Butterfly Site (where we get a hint at how biologists know that the mimics arose after the Pipevine Swallowtail, not the other way around)
- “Butterfly gardeners beware of toxic pipevine species” (in which we learn that some species of South American pipevine are too toxic for North American Pipevine Swallowtails)