Thermoregulatory significance of wing melanization in Pieris butterflies (Lepidoptera: Pieridae): physics, posture, and pattern

Abstract

Pieris butterflies use a novel behavioral posture for thermoregulation called reflectance basking, in which the wings are used as solar reflectors to reflect radiation to the body. As a means of exploring the thermoregulatory significance of wing melanization patterns, I examine the relation of basking posture and wing color pattern to body temperature. A mathematical model of the reflectance process predicts certain combinations of dorsal wing melanization pattern and basking posture that maximize body temperature. Laboratory experiments and field observations show that this model correctly predicts qualitative differences in the relation of body temperature to basking posture based on differences in the extent of dorsal melanization on the wing margins, both between species and between sexes within species of Pieris. This is the first demonstration in insects that coloration of the entire wing surface can affect thermoregulation. Model and experimental results suggest that, in certain wing regions, increased melanization can reduce body temperature in Pieris; this effect of melanization is exactly the opposite of that found in other Pierid butterflies that use their wings as solar absorbers. These results are discussed in terms of the evolution of wing melanization pattern and thermoregulatory behavior in butterflies.