Ultraviolet Differences between the Sulphur Butterflies, Colias eurytheme and C. philodice, and a Possible Isolating Mechanism

Abstract

Two common North American “sulphur” butterflies, Colias eurytheme and C. philodice, hybridize widely over much of the United States. The planting of dense agricultural stands of larval foodplants, together with habitat destruction starting about 1850, are believed to have affected the species' geographical ranges and abundances, causing extensive sympatry over much of the eastern United States¹, setting the stage for introgression. Taxonomists have tried to separate the species on the basis of visible wing coloration, but this varies continuously from orange (pure eurytheme) through yellow–orange (eurytheme and many hybrids) to yellow (philodice and rarely eurytheme). Female polymorphism and numerous described infraspecific taxa compound the taxonomic confusion. Although some of this variation has been attributed to genetic (refs. 2 (and references therein) and 3) and environmental factors⁴, species determinations are often guesswork. We wish to point out two correlated qualitative characters that can be used to separate these species. One character, ultraviolet reflexion, may be more than a taxonomic tool—it may be the butterflies' own carte de visite.