COLD TOLERANCE OF THE PUPAE IN RELATION TO THE DISTRIBUTION OF SWALLOWTAIL BUTTERFLIES

A steep decline in the diversity of swallowtail butterfly species at h igh latitudes could be due to limited cold tolerance of overwintering pupae. If this is so, species with unusually northerly distributions s hould be unusually cold tolerant. We compared the northerly distribute d Papilio canadensis with its southern relative, P. glaucus. Pupae wer e exposed for 2-5 months to four acclimatization treatments: outdoors in Alaska, outdoors in Michigan constant 5-degrees-C. Field temperatur es encountered by pupae in Alaska were lower than in Michigan. The sup ercooling point of P. glaucus pupae was unaffected by acclimatization (mean +/- SE = -23.5 +/- 0.52-degrees-C). The supercooling point of P. canadensis pupae did not differ from that of P. glaucus pupae, except following acclimatization in Alaska, when it dropped to -27.0 +/- 0.5 5-degrees-C. Survival of pupae in Michigan was high for all population s (70-90%); in Alaska, survival of P. canadensis was just as high, but survival of P. glaucus dropped to 14%. Freezing was usually fatal in both species, but death was not immediate. No pupae survived 6 weeks a t -25-degrees-C. Trehalose was the most conspicuous metabolite reveale d by nuclear magnetic resonance spectroscopy of live pupae and hemolym ph. Labelled glucose was metabolized differently by the two species, w hich may underly the difference in acclimation potential and cold tole rance. The results support the hypothesis that winter temperatures lim it swallowtail distributions.