Red-Spotted Purple

Limenitis arthemis astyanax — the red-spotted purple — is one of the most-cited examples of BATESIAN MIMICRY in North American butterflies and a flagship case study in evolutionary biology of mimicry. The species is widespread across the eastern and central US from southern New England south through the eastern US to northern Florida and west to the Great Plains. Adults are 7-10 cm wingspan with predominantly dark brown-to-black wings and brilliant BLUE IRIDESCENT HINDWINGS marked with rows of red-orange marginal spots and red basal spots. The dark-and-blue coloration is a flagship case of BATESIAN MIMICRY of the toxic pipevine swallowtail (Battus philenor), which contains aristolochic acids from its larval host plant and is unpalatable to bird predators. The red-spotted purple is one of several pipevine-mimic species in eastern North America (along with spicebush swallowtails, black-form female tiger swallowtails, female Diana fritillaries, and male promethea moths — all converging on the same toxic-pipevine model). The species' major evolutionary feature is the SUBSPECIES STRUCTURE. The species Limenitis arthemis exists in TWO STRIKINGLY DIFFERENT FORMS across North America: (1) Limenitis arthemis arthemis — the WHITE ADMIRAL — black butterfly with broad WHITE BANDS across both wings, found in the northern US and southern Canada (where pipevine swallowtails are absent and there is no mimicry pressure); (2) Limenitis arthemis astyanax — the RED-SPOTTED PURPLE — dark butterfly with brilliant BLUE iridescent hindwings, found in the southern and central US (where pipevines are common and mimicry is selectively advantageous). The two subspecies INTERBREED FREELY where their ranges overlap (a hybrid zone across the northeastern US — New York, Pennsylvania, Massachusetts) and produce intermediate phenotypes — demonstrating that the dramatic phenotypic difference is the result of natural selection on mimicry pressure rather than reproductive isolation. The hybrid zone is one of the most-cited cases in evolutionary biology of geographic variation in mimicry and is featured in essentially every modern textbook discussion of Batesian mimicry. Larvae feed on willow, poplar, cherry, and birch — the larvae are bird-dropping mimics in early instars and become more cryptic in late instars. The species is harmless to humans and a major beneficial pollinator.