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1. The theory on adaptive phenotypic plasticity assumes different fitness optima in different environments. To demonstrate these fitness differences in the field or under laboratory conditions can be difficult. 2. The size of the coloured patches on the abdomen of Eristalis arbustorum varies with rearing temperature. At low temperatures small patches develop and at higher temperatures larger patches develop. Consequently, the colour patch size varies seasonally. 3. To study the relative fitness of two extreme phenotypes in abdominal coloration in the summer, a total of 591 laboratory-reared flies were released in six large population cages. As a measure of fitness, survival was estimated. 4. In all replicate cages the survival of the pale phenotypes was higher than that of the dark phenotypes, indicating a higher fitness for the pale flies under summer conditions. 5. The fat content of flies reared under identical conditions …