Red-spotted Purple – The dark blue and black butterfly that isn’t a swallowtail

For most of us in the eastern U.S., there are four common species of dark blue and black butterflies. Three of those species are swallowtails, but one of them isn’t. At first glance, the red-spotted purple (Limenitis arthemis astyanax) looks similar to the dark blue and black swallowtails. However, if you give it a second glance, it is easy to tell apart because it lacks the swallowtails!

Often the red-spotted purple is dismissed as one of the dark blue and black swallowtails, but it lacks the swallowtails which makes it easy to identify – it’s the “dark swallowtail” that doesn’t have swallowtails. Photo credit: John Flannery, cc-by-sa 2.0

Although the red-spotted purple used to be considered its own species, it is now considered a subspecies and has been combined with the white admiral (Limenitis arthemis arthemis). The species as a whole is sometimes called the red-spotted admiral (not to be confused with the red admiral which is a different species). However, the two subspecies look very different and are typically still referred to by their original common names.

Most of us don’t have to worry about the white admiral. It is found primarily in Canada and north into Alaska. The red-spotted purple is the subspecies found throughout most of the eastern U.S. However, there is an area of overlap between the two subspecies that some of our readers need to be aware of. This area of overlap roughly corresponds to New England and the Great Lakes region. Where the two subspecies overlap, they hybridize and you get a variety of different wing patterns that are intergrades between the two subspecies.

Red-spotted purples are medium-sized butterflies – roughly the size of a viceroy or fritillary. When their wings are open, they are black with a blue wash on the hind wings. When their wings are closed, they are mostly black with a band of blue near the edge, a row of orange spots next to the blue, and then a few more orange spots near the body.

The adult red-spotted purple is a mimic of the pipevine swallowtail which is inedible to birds. However, in addition to lacking the swallowtails, when its wings are closed the red-spotted purple also has a cluster of orange spots near the body – a feature that none of our swallowtails have. Photo credit: David Hill, cc-by 2.0

The coloration of the red-spotted purple mimics that of the pipevine swallowtail (Battus philenor) which is unpalatable to birds. Because the red-spotted purple looks similar to an inedible butterfly, many birds will leave it alone. In fact, the red-spotted purple’s range overlaps almost exactly with that of the pipevine swallowtail. The northern portions of the pipevine swallowtail’s range are where you find the intergrades between the red-spotted purple and the white admiral. Once you move completely out of the pipevine swallowtail’s range, you are into white admiral territory because the mimicry only works if the pipevine swallowtail is also present.

Even the caterpillars of the red-spotted purples are amazing mimics. They look like bird poop. Swallowtail caterpillars will also often look like bird droppings when they are young, but swallowtail caterpillars lose this characteristic as they get older. However, red-spotted purple caterpillars always look like bird droppings. In much of their range, their primary host plants are wild black cherries and other members of the rose family. They are also sometimes found on willows, poplars, cottonwoods, aspens, viburnums, and a few other trees. (The white admiral caterpillars primarily feed on willows, poplars, cottonwoods, and related plants.)

Many species of caterpillars hide on the underside of a leaf or roll a leaf up and hide inside of it when they want to rest. But not the red-spotted purple caterpillar. It gathers its frass (scientific word for insect poop), creates silk to connect the frass into a stick, attaches the frass stick to the tip of the leaf that it has been eating, and then crawls out to the end of the frass stick. Which I guess is a pretty good strategy. How many predators are going to crawl out on a poop stick to eat a pile of bird droppings?

Red-spotted purple caterpillars look like bird droppings. Photo credit: Mike Boone, cc-by-sa 2.5

Each year there are typically 2 generations of butterflies throughout most of the red-spotted purple’s range. In the southern portion of their range, the late summer / early fall generation is often the largest. However, all of the adult butterflies will die by the end of fall. It is the caterpillars that will overwinter and produce the next generation of butterflies in the spring.

The caterpillars create hibernacula in which they hibernate through the winter. The hibernaculum consists of a leaf that the caterpillar rolls up and attaches to a twig of its host tree. The hibernaculum looks like any other curled up dead leaf hanging on the plant through winter. The caterpillar then crawls inside its leafy sleeping bag and hibernates until spring. When the tree begins putting out fresh, new leaves again, the caterpillar will emerge and continue feeding before forming a chrysalis. In keeping with the caterpillar’s seeming fascination with poop and mimicry, even its chrysalis looks like a big pile of bird droppings hanging from a twig.

Even the chrysalis of the red-spotted purple looks like bird droppings. Photo credit: Mariko, cc-by-nc

Although we often associate butterflies with open areas with lots of flowers, that’s not the best place to look for red-spotted purple butterflies. They prefer open woods, woods edges, and suburban areas that have a good number of mature trees. They can also be found along wooded creek banks, mud puddles in dirt roads, and similar locations – this is one of the species that likes to puddle or gather moisture and nutrients from mud.

Red-spotted purples also aren’t as attracted to flowers as many of our more familiar butterflies. They will drink nectar from flowers occasionally, but they prefer to get their nutrients from the sap of oak trees, overripe fruit, piles of dung, and decomposing animals. Understandably, most people don’t to want to add piles of dung or roadkill to their butterfly gardens. However, that doesn’t mean you can’t attract these beautiful butterflies to your yard.

Try putting out overripe fruit that you didn’t eat in time, watermelon rinds, or similar items. (Don’t put too much out at once or you’ll be more likely to attract raccoon, opossums, or maybe even bears depending on where you live.) Other butterfly species that aren’t as attracted to flowers may also come to the overripe fruit. Another option is to create a mudding or puddling spot if you have a bare spot of ground that you can keep moist.

The red-spotted purple is an interesting butterfly for many reasons. Too often it gets dismissed as “one of those confusing dark blue and black swallowtails,” especially when people are just starting to learn how to identify butterflies. I get it. For a long time, I did the same thing. However, once you realize that it’s the one that doesn’t have the swallowtails, then it becomes easy to identify. And in my opinion, it’s well worth learning. Not only is it a beautiful butterfly, but it has a fascinating life history too.

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