According to Butterflies and Moths of North America, there are 926 species of butterflies known to live on the continent. Jeff Trahan – with help from Rosemary Seidler, Terry Davis, and me – has found 97 of them in Caddo Parish.
Four of those species are commonly called “the dark swallowtails,” and those four species can be tricky to tell apart. They are the Pipevine Swallowtail, Black Swallowtail, Spicebush Swallowtail, and the dark morph female Eastern Tiger Swallowtail.
To distinguish between them, let’s look at them at the same time.
Okay, look at them again. This time, focus on the field marks, or things that define each species.
The top left is a Black Swallowtail. Males are unmistakable because instead of white spots, they have yellow spots, and they have an extra row of them. The female is pictured here. Male and female Black Swallowtails can always be identified by the black dot inside the orange dot. There are also some differences in the size and shape of the white spots on the forewing, and some other distinctions, but "pupil-in-the-iris" orange/black spot is a dead giveaway.
The top right is the female Eastern Tiger Swallowtail, dark morph. Notice the white spots on her forewings are really dashes. Also notice the white spots on her hindwings have a little orange in them, especially the ones on either end of the row. Sometimes you can still see a little yellow and maybe even a tiger stripe, but the dashes and the extra orange is always there. She's a bit bigger than the other dark swallowtails, too.
The bottom left is a Spicebush Swallowtail. There are some differences between males and females, but both sexes always have those fingertip-shaped spots on the hindwing that are bluish-white.
The bottom right is a Pipevine Swallowtail. There are subtle differences between male and female Pipevine Swallowtails as well, but both sexes have blue tails. Also, the blue color on Pipevines does not seem to be compartmentalized the way it is on other dark swallowtails, but rather blended looking.
The pupil-in-the-iris is still visible when viewing the ventral side of the Black Swallowtail. There are pretty much two rows of orange spots, and the row closest to the body has a double spot.
"Tigers do not have spots!" At least female Eastern Tiger Swallowtails do not have spots on their bodies. All the other dark swallowtails do. Also, the dashes are still visible.
Often you can see some of the dorsal side when Spicebush Swallowtails' wings are closed, and when that happens, you can still see those bluish-white fingertips. With only the ventral side of the hindwing showing, you can see that Spicebush Swallowtails have two rows of orange spots, minus one. That absent orange spot is diagnostic.
Sometimes you can still see the blue of the Pipevine Swallowtail's tail on the ventral side. Sometimes you can tell that the body is a bit blue (the others are pretty much black). Regardless of whether you can see that blue or not, you can always identify a Pipevine Swallowtail with its wings closed by counting the rows of orange spots. There's only one row.
To test out these identification short cuts, grab your binoculars and head out to the park for some butterfly watching. Swallowtails fly fast and high, so you may have to let some go uncounted.
But it's the thrill of the hunt, right?