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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.