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1. Seasonality is a prime selective factor promoting genetic differentiation of populations. Local adaptation in diapause response was investigated in the two geographically and altitudinally widespread dung flies Scathophaga stercoraria (Diptera: Scathophagidae) and Sepsis cynipsea (Diptera: Sepsidae).

2. Replicate sibships from three sites in Switzerland with low and high altitude dung fly populations were raised in a common laboratory environment simulating the natural decreasing photoperiod and temperature regimen before winter. From field phenologies, the critical photoperiod inducing diapause was predicted to be longer for high than for low altitude populations (12 vs. 10 h for Sc. stercoraria, and 12.25 vs. 11 h for Se. cynipsea) if they are locally adapted.

3. Contrary to expectation and to Sc. stercoraria, which diapauses in the pupal stage, Se. cynipsea diapauses in the adult stage.

4. Low but significant levels of genetic differentiation in pre-winter adult emergence were evident between low and high altitude populations of both species, but they were far from the differences predicted. Scathophaga stercoraria also showed geographical differentiation independent of altitude.

5. Sepsis cynipsea females stopped reproducing at some point before winter, but altitude did not affect the timing of adult diapause. High altitude females and females that did not initiate reproduction before winter survived the simulated winter better.

6. Both species largely used temperature rather than photoperiod as a cue inducing winter diapause, an untypical case of phenotypic plasticity. The hypothesis that Sc. stercoraria, whose generation times are much longer than those of Se. cynipsea, responds to a greater extent to temperature rather than photoperiod only was rejected.