The reaction norms in Drosophila melanogaster of thorax length, wing length and cell size were determined for 28 isofemale lines from three populations to investigate the role of cell size in determining the response of body size to temperature during the preimaginal stages. Both overall level and plasticity of the reaction norms of thorax length and wing length are highly correlated, leading to a relatively constant wing-thorax ratio between lines. Genetic differences in overall level of wing size reaction norms are mainly caused by differences in cell number. The response of wing size to temperature consists of changes in cell size and, to a lesser extent, cell number. The cellular basis of genetic differences in plasticity shows a transition point at an intermediate level. In steeper reaction norms, genetic differences in plasticity result from differences in the plasticity of cell size, whereas less steep reaction norms only differ in the plasticity of cell number. A significant partial correlation between wing length plasticity and cell size plasticity, correcting for thorax length plasticity, indicates a role of cell size in determining the wing-thorax ratio.