Divergence in larval responses to food plants between temperate and tropical populations of the black swallowtail butterfly

.1 Larvae of the black swallowtail butterfly, Papilio polyxenes Fabr., from New York and Costa Rica were reared under the same environmental conditions on Daucus carota L. and Spananthe paniculata Jacq. (Umbelliferae), which are the host plants most commonly used by the two populations, respectively. Feeding rates, growth rates and feeding efficiencies of the fifth instar larvae were measured by standard techniques. 2 Larvae from each population grew faster on the host‐plant species from their region of origin than did larvae from the other population, though differences on S.paniculata were not significant. On D.carota the faster growth by New York larvae was attributable to greater efficiency. On S.paniculata , by contrast, the somewhat faster growth by Costa Rican larvae seemed to have resulted chiefly from faster feeding. The two populations have thus come to differ in metabolic and, probably, behavioural responses to host plants. 3 S.paniculata was a superior host plant for larvae of both populations, even though the total energy, nitrogen and water content of its foliage was similar to that of D.carota.4 These results are consistent with the hypothesis that insects in the two populations have diverged in adaptation to their host plants following curtailment of gene flow between them as the species colonized Central America from the north.