LIFE-HISTORY PLASTICITY IN THE SATYRINE BUTTERFLY LASIOMMATA-PETROPOLITANA - INVESTIGATING AN ADAPTIVE REACTION NORM

This study addresses the general hypothesis that insects living in sea sonal environments should shorten development times at progressively l ater dates in the growth season, and that insects living outside equat orial areas should use daylength as a cue to determine the date. Diapa use strategies and reaction norms relating the duration of larval deve lopment to daylength was investigated in a French population of the bu tterfly, Lasiommata petropolitana. The results are compared with those of an earlier study of the species in Sweden. Because of the diapausi ng strategy and phenology of the population, it was expected that an a daptive reaction norm relating larval time to daylength should have a positive slope, i.e. relatively shorter daylengths induce faster growt h and development. This prediction was supported, and the reaction nor m was qualitatively similar to the one found in Swedish populations. I n the French population it was, however, shifted to a range of shorter photoperiods which corresponds to the regime of shorter daylengths in southern Europe. Shorter larval development times and high growth rat es were associated with a reduction in pupal size, suggesting a trade off between time and size at pupation. There was no evidence of a trad e off between growth rate and starvation endurance. The results sugges ts that the daylength-dependent decision of what growth trajectory an individual larva will follow, is not made continuously but rather at o ne or a few occasions during larval development. It is clear that larv ae of L. petropolitana make developmental decisions in relation to the daylength they experience during larval growth. The result is a react ion norm that agrees closely to what is predicted by some life history models, suggesting that it is an adaptation for optimising life histo ry traits in a seasonal environment.