A couple of days ago I posted a photograph of a female eastern tiger swallowtail. I should have been a little more specific in my description–it was the yellow form.
Eastern tiger swallowtails have two different forms (or morphs) of females–one is yellow and black, as are all males of the species. The other form is all black, like the one shown here.
It can look very black, or stripes can appear, depending on how the light hits it.
Scientists explain it in terms of mimicry.