AN EXPERIMENTAL TEST OF NATURAL SELECTION FOR PUPATION SITE IN SWALLOWTAIL BUTTERFLIES.

Abstract

The pupae of some swallowtail butterflies are dimorphic-usually green or brown-while those of others are monomorphic. We have suggested (West and Hazel, 1979) that one important factor determining whether a species evolves dimorphism or monomorphism is the type of pupation site that the species uses. The use of a narrow range of cryptic sites in the leaf litter, for example, would favor monomorphic brown pupae, while the use of a variety of sites on exposed stalks, tree trunks, etc. would favor the ability to develop green or brown pupae in response to such substrate characteristics as color or texture (Hazel and West, 1979). This suggestion immediately raised the question why different species prefer different kinds of pupation sites to begin with. Clarke and Sheppard (1972) successfully selected for changes in pupation site preference in Papilio polytes, thereby revealing genetic variation in the trait, and we have shown (West and Hazel, 1979 and unpubl.) that there are some striking differences in natural pupation sites among six eastern North American swallowtails. Among these there are two species which, despite the similar forest habitats of their immature stages, prefer very different pupation sites: Papilio glaucus in the leaf litter and Battus philenor well off the ground on exposed surfaces like tree trunks and cliffs (West and Hazel, 1979). These species differ in two other important respects. Papilio glaucus lays eggs singly and widely spaced on trees (includ-