LIFE-CYCLE REGULATION AND LIFE-HISTORY PLASTICITY IN THE SPECKLED WOOD BUTTERFLY - ARE REACTION NORMS PREDICTABLE

We investigated whether interpopulational variation in life-cycle regu lation and life-history plasticity, in response to photoperiod, is pre dictable from considerations of what would be the adaptive life cycle and life history in a given environment. The investigation war perform ed on five populations of the speckled wood butterfly, Pararge aegeria (L.) (Lepidoptera: Nymphalidae), from central and south Sweden, Engla nd, Spain and Madeira. Insects from all five populations were reared a t all daylengths from 10 h to 20 h at 17 degrees C. Larval and pupal d evelopment times were noted. Predictions were met regarding the type o f life-cycle regulation and the shape of reaction norms. Evidence for diapause (larval summer and winter diapause, pupal winter diapause) wa s found in the three northern populations (P. a. tircis) but not in th e two southern populations (P. a. aegeria). Photoperiodic thresholds f or diapause induction followed the predicted latitudinal patterns, and this was also the case regarding quantitative regulation of developme nt time (by photoperiod) among directly developing individuals. Under direct development, development time was progressively shorter in shor ter daylengths in the two Swedish populations, where this signals prog ressively later dates. This was not found in the English, Spanish and Madeiran populations where such a response is likely to be maladaptive , because one or more generations of larvae are present before summer solstice. There were also unexpected results, for which we propose pre liminary adaptive explanations.