One of the coolest things about female Eastern Tiger Swallowtail butterflies (Papilio glaucus) is that they come in two distinctly different colors, black and yellow. According to Wikipedia, that characteristic makes them dimorphic. I’m no scientist, so I had to do some research to see what “dimorphic” means. If I understand it right, it means that there are two different phenotypes (called morphs) that exist in the same population of a species and they have to be in the same habitat at the same time to qualify.
I was thrilled yesterday to observe and photograph both variants at a local garden. The yellow ones resemble the male, although the male is only yellow and black and has no additional orange and blue markings. I have seen a lot of the yellow female swallowtails this summer. The black swallowtails, which have black bodies as well as black wings, seem to be more rare, or at least I have seen them only rarely this summer.
Which one is more beautiful? I’ll leave that call to each of you.
Female Eastern Tiger Swallowtail butterfly (black variant) UPDATE in 2020: I think that I misidentified this as an Eastern Tiger Swallowtail and now think it might be a Black Swallowtail, though the angle keeps me from being sure about the identification
© Michael Q. Powell. All rights reserved.
Mike Powell 
I wonder what determines which color morph develops?
Sue, I would have to defer to Dan Proud or some others of a scientific background to answer that question. Now you’re causing me to wonder too.
Very nice photos to compare this dimorphism! I think I’m going to pick the black form as my favorite. I like going for the underdog! Just to throw another bit of info at ya – polymorphism is when there’s many forms. I can’t think of any butterfly examples, but there are some spiders that exhibit polymorphism like the Hawaiian happy face spider (family Theridiidae) which has more than 20 different forms! I know this thanks to my friend Darko who studies them!
Dimorphism, polymorphism–it’s really complicated. To make things worse, according to one source, “P. glaucus is one of a few species of papilionids known to produce gynandromorphs.” The diversity of nature is astounding.
[…] glaucus) set it apart from other photos I have taken of the same species. (See my postings from 8 August and 21 July for other shots of the Eastern Tiger […]
[…] I had ever encountered was the black variant of the female Eastern Tiger Swallowtail. (Check out my posting from last year to see the two variants of the female Eastern Tiger Swallowtail, a characteristic […]
[…] you are interested in seeing photos of both female variants, check out a 2012 posting entitled “Swallowtail ladies are not all the […]
Now I’m confused. I recently took a pic of a dark butterfly. I tried using the app, Seek, on it and it ID’d it as a Tiger Swallowtail. So I looked around the net a bit to try to find a page that would give info about how to tell what the various dark swallowtails are and came up with this https://www.butterfliesathome.com/images/butterflies/swallowtails/dark-swallowtail-butterfly-ventral-comparison.jpg which says there are no spots on the body of the tiger swallowtail. One of the pics I took showed that the one I was photographing indeed had no spots on the body. However the pic on your page labeled Female Eastern Tiger Swallowtail butterfly (black variant) shows a butterfly with lots of spots on its body.
Part of the issue is the question of where you live. There are different species found in different parts of the US. For example, there is the Eastern Tiger Swallowtail (Papilio glaucus) and a Western Tiger Swallowtail (Papilio rutulus) that are different species. As far as I know, there is no generic Tiger Swallowtail, so I am confused by some of the photos on the page you referenced. For example, as far as I know there is no dark variant of either a male Eastern Tiger Swallowtail or a male Western Tiger Swallowtail. This blog post is one of the best that I have seen for helping to sort out the dark swallowtails. http://louisiananaturalist.blogspot.com/2009/06/four-dark-swallowtails.html
Kathy, I have done some more checking and think that you are correct in questioning my identification of the dark butterfly in the 2012 posting. I was a whole lot less knowledgeable back then and think I was unaware how many different dark swallowtail species were in my area. I can’t tell for certain what it is because of the angle, but if I had to guess today, I’d say it looks most like a Black Swallowtail.