a tale of two Swallowtails

During the last week, I’ve seen several Eastern Tiger Swallowtail butterflies in the backyard.  They seem to be especially attracted to the Cup Plant (the same plant the Goldfinches have been tearing apart).

eastern tiger swallowtail butterfly

The easily recognized yellow and black striped color morph of the eastern tiger swallowtail. Bright blue almost iridescent scales at the tip of the hind wings indicate this is a female.

eastern tiger swallowtail butterfly

Nearby, the other color morph of the female eastern tiger swallowtail — all black with the same iridescent blue scales on the hind wing — nectars on the same plant.  There are no traces of the black or yellow stripes; the wings are entirely melanistic.

eastern tiger swallowtail butterfly

There were actually several of these butterflies on the Cup Plants, two of which sort of cooperated for a group shot. Can you see a Goldfinch between the two butterflies?

The male color pattern is entirely yellow and black, but without the iridescent blue highlights), so why are there two color morphs for the female?

As I have described in an earlier post, several species of Swallowtail butterflies, and in particular, the female Eastern Tiger Swallowtail, imitate the color pattern of the distasteful Pipevine Swallowtail (a different species entirely).


Pipevine Swallowtail, the model species for the mimicry complex, and dark color morph of the Eastern Tiger Swallowtail.  (Photo by Bob Moul)  Larvae of these butterflies are specialists on Pipevine plant species, which produce organic compounds that are toxic carcinogens and urologic poisons.

Predators usually remember what tastes bad or makes them sick, so mimicking something that predators avoid is a good survival strategy.  However, that requires that the predators actually have experience with the model (the one being copied), and so far, Pipevine Swallowtails are rare strays in Minnesota because the Pipevine food plants on which their larvae develop are absent here.

We now know that the melanistic (dark form) Eastern Tiger Swallowtail females produce almost all melanistic daughters, and the yellow color morph females produce almost all yellow daughters.  So perhaps our melanistic females are offpspring of Tiger Swallowtail butterflies that hatched in nearby Iowa or Wisconsin, where Pipevine Swallowtails have been sighted along with their required food plants.

eastern tiger swallowtail butterfly

Joe Pye weed offers lots of nectar to the butterflies in the garden as well.  The fact that these butteflies have beautifully intact wings (including the swallow “tails”) and scales probably means they have only recently emerged from their pupal cocoons.

It’s interesting to note that there is no melanistic female color morph in the Eastern Tiger Swallowtail’s very similar sister species — Canadian Tiger Swallowtails, although the Pipevine Swallowtail has been found in several places in Ontario.

3 thoughts on “a tale of two Swallowtails

  1. A butterfly landed on my hand and was dropping clear liquid from its behind and mopped it up with his feeler? Or his tongue? Idk what you’d call it. What does that mean?

    • I’m not sure what that means, Heather. I suppose when they get full of nectar, their excreta might be clear liquid, but why it would then probe the fluid with its antennae, I can’t say.

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