Females reflect ultraviolet light, but the wings of male Checkered Whites contain an ultraviolet-absorbing pigment. This pigment is easily extracted, however, by dipping the wings in a dilute ammonia solution. Such treatment made male wings reflective of ultraviolet, like female wings, without altering any other markings. I built lifelike models from ammonia-treated wings and then, using stiff angling wire, presented the specimens to butterflies in the field. Clearly, both female and male Checkered Whites make use of sexual differences in color in order to discriminate potential mates from individuals of their own sex.
Some female butterflies are also picky about color when choosing a mate from among many suitors.
Diane C. Wiernasz of the University of Houston investigated this behavior in the Western White , P. occidentalis , a butterfly closely related to the Checkered White. She released virgin females into a field and captured males that successfully courted them. These males had darker markings at the tips of their forewings than did rejected suitors. And Wiernasz was able to make males unattractive to virgin females by using white paint to reduce the size of the crucial dark markings. This is the only study of its kind that we have, but it demonstrates that some females discriminate among males on the basis of subtle differences in color.