The study of latitudinal body size clines can illuminate processes of local adaptation, but there is a need for an increased understanding of the relative roles of genetic variation, environmental effectstions or this reason, we combined an investigation of a museum collection of the common blue butterfly Polyommatus icarus (Rottemburg) (Lycaenidae: Polyommatini) from Sweden with a common-garden experiment in the laboratory, using strains reared from individuals collected from three different latitudes. Sizes of the field-collected butterflies tended to smoothly decrease northwards in a latitudinal cline, but suddenly increase at the latitude where the life cycle changes from two to one generations per year, hence allowing more time for this single generation. Further north, the size of the field-collected butterflies again decreased with latitude (with the exception of the northernmost collection sites). This is in accordance with the “converse Bergmann” pattern and with the “saw-tooth model” suggesting that insect size is shaped by season length and number of generations along latitudinal transects. In contrast, under laboratory conditions with a constant long day-length there was a different pattern, with the butterflies pupating at a higher mass when individuals originated from southern populations under time stress to achieve a second generation. This is indirect evidence for field patterns being shaped by end-of-season cues cutting development short, and also suggests counter-gradient variation, as butterflies from the time-stressed populations over-compensated for decreasing larval development time by increasing their growth rates, thus obtaining higher mass. Hence, we found support for both adaptive phenotypic plasticity and local genetic adaptation, with gene-environment interactions explaining the observed field patterns.

In the Lepidoptera, sex-linked genes have been found to be of importance for species differences in, for example, hostplant preference, and have been implicated in ecological speciation. Variation within species is typically not sexlinked. However, in the comma butterfly Polygonia c-album (Nymphalidae) an X-linked gene has been found to play a major role in determining differences in host-plant use between two well separated populations. For this reason, we studied the role of sex-linked genes for host-plant preference within a single Swedish population of this species. Three generations of females with known pedigrees were studied in the laboratory, and they were given a choice between Urtica dioica and Salix caprea in flight cages. We found strong variation among females and significant genetic variance for host-plant preference, but no evidence for major importance of sex linkage of host-plant preference on this local scale. To what extent the observed genetic variation was due to additive genes and/or effects of major genes was not clear from the maximum likelihood analysis. In a follow-up study we sampled females over a larger area. We found strong variation among females, but not among localities, suggesting an open population structure with strong gene flow. From the combined stock, a selection experiment was performed over 2 years and six generations. The selection lines diverged after the first generation of selection and remained separate, but did not diverge further, suggesting a low degree of narrow-sense heritability and that the genetic differences may be effects of major genes. We discuss these results in relation to the possible role of genetics in the radiation of the Lepidoptera and other phytophagous insects.

Comma butterflies (Nymphalidae: Polygonia c‐album L.) from one Belgian site and three Spanish sites were crossed with butterflies from a Swedish population in order to investigate inheritance of female host plant choice, egg mass and larval growth rate. We found three different modes of inheritance for the three investigated traits. In line with earlier results from crosses between Swedish and English populations, the results regarding female oviposition preference (choice between Urtica dioica and Salix caprea) showed X‐linked inheritance to be of importance for the variation between Sweden and the other sites. Egg mass and growth rate did not show any sex‐linked inheritance. Egg mass differences between populations seem to be controlled mainly by additive autosomal genes, as hybrids showed intermediate values. The growth rates of both hybrid types following reciprocal crossings were similar to each other but consistently higher than for the two source populations, suggesting a nonadditive mode of inheritance which is not sex‐linked. The different modes of inheritance for host plant preference vs. important life history traits are likely to result in hybrids with unfit combinations of traits. This type of potential reproductive barrier based on multiple ecologically important traits deserves more attention, as it should be a common situation for instance in the early stages of population divergence in host plant usage, facilitating ecological speciation.

What is the role of time-constraints in determining geographical variation in the resource use of organisms? One hypothesis concerning phytophagous insects predicts a local narrowing of host plant range at localities where a short development time is important (because an additional generation per season is only just possible), with increased specialization on host plants permitting fast development. To test this hypothesis, populations of the polyphagous comma butterfly (Nymphalidae: Polygonia c-album) from five European areas (localities in Norway, Sweden, England, Belgium and Spain) were sampled and the preferences of laboratory-reared female butterflies were investigated, by a choice test between Salix caprea and the fastest host Urtica dioica. The results suggest that females of both of two northern univoltine populations (time-stressed from Norway and time-relaxed from Sweden) accept the slow host S. caprea to a higher degree than females of more southern populations with partial additional generations (time-stressed). We thus found partial support for the tested hypothesis, but also conflicting results that cast doubt on its broad generality. Moreover, a split-brood investigation on Swedish stock demonstrated that larval performance is similar on S. caprea and U. dioica early in the summer, but that later in the season S. caprea is a much inferior host. This is reflected by a seasonal trend towards specialization on U. dioica and also provides a simpler explanation than the time-constraints theory for avoidance of S. caprea (and other woody hosts) in areas with two or more generations of insects per year, illustrating the importance of plant phenology as a constraint on resource use in phytophagous insects. Absolute and relative larval performance on the two hosts varied little among populations across Europe, but lower survival on S. caprea in the populations most specialized on U. dioica and related plants may be indicative of performance trade-offs.

We studied host plant preference of the common blue butterfly, Polyommatus icarus, and larval performance on two different host plants, Oxytropis campestris and Lotus corniculatus. The study species is a small lycaenid butterfly believed to be relatively sedentary. The study populations originated from two different and widely separated geographical areas. In one area both hosts are naturally occurring, with O. campestris being most abundant at the study sites, in the other area only one of the host plants, L. corniculatus, is present. There was no difference in oviposition preference or larval performance between populations from the two different areas. Hence, P. icarus from sites dominated by O. campestris has not evolved a higher preference for or better performance on this host plant. More surprisingly, P. icarus from the area were O. campestris is completely absent has retained not only good larval performance on this host plant but also high female preference for it. This conservatism at a large geographical scale is seen even though there seems to be genetic variation present in both populations, at least for preference but perhaps also for performance. We suggest that such lack of variation in resource utilization between populations may be evidence for weak selection against ''preferences'' for plants that are rare or absent. A combination of other constraining factors may also contribute to some degree, especially stepping-stone gene flow between populations.