Papilio glaucus shows sex-limited polymorphism. Males are consistently yellow with black tiger stripes. Females occur either in that same yellow form or in a dark morph in which the yellow ground is replaced by sooty black, with the tiger pattern still faintly visible from beneath. The determining factor is inherited on the W chromosome, so only females can express the dark form.
The dark morph resembles Battus philenor, the pipevine swallowtail, whose larvae sequester aristolochic acids from pipevines and whose adults are unpalatable to birds. This is a textbook case of Batesian mimicry, in which a harmless species gains protection by resembling a defended one. The frequency of the dark morph is highest in the southeastern United States, where the pipevine swallowtail is abundant, and falls away northward where it is scarce.
What mimicry costs is less often noted. Dark females may be less attractive to males, so the morph's advantage against predators is offset by mating disadvantage, and the balance between the two is still being measured.