What does "dark form" mean? And what would tell me the difference between a female and male? Thank you so much for your time!

Female Eastern Tiger Swallowtails can be either dark, like this one, or Yellow. Males are only found in yellow, so it must be a female.

In this case, the color and some deduction (along with knowing about the dark form), it's easy to tell. However, the tip of the abdomen shows clearly in your photo, and even if it were a yellow female (or all you could see was the tip of the abdomen), you could still tell it's a female. The male has a pair of "claspers" there that are clearly visible, and the female doesn't. Often, depending on the species of buttefly, you can tell by differences in patter, color, size, and/or shape too.

David,
In my rush to judgment (for those old enough to remember the book), I failed to notice the claspers on



Now, I see the light. I figured you were right. I just could see it. Thanks
Robert

The backlighting in the other two photos lets the underlying pattern show through - lit from overhead (like this photo), these females can look solid black. All three photos together show how the dark form really is just a variation on the yellow form.

Very instructive, the lighting and the insect came together just right!