Survival of colour-polymorphic Eristalis arbustorum hoverflies in semi-field conditions

1. The theory on adaptive phenotypic plasticity assumes different fitness o ptima in different environments. To demonstrate these fitness differences i n 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 col oration in the summer, a total of 591 laboratory-reared flies were released in six large population cages. As a measure of fitness, survival was estim ated. 4. In all replicate cages the survival of the pale phenotypes was higher th an that of the dark phenotypes, indicating a higher fitness for the pale fl ies under summer conditions. 5, The fat content of flies reared under identical conditions to those rele ased in the cages was measured. Pale flies had approximate to 0.7 mg more f at than dark flies (+/- 25% extra fat). However, it is argued that this dif ference in fat reserves could not have been the only cause of the observed difference in survival. 6, The results are discussed in relation to mimicry and thermoregulation.