Both development and evolution under chronic malnutrition lead to reduced adult size in Drosophila. We studied the contribution of changes in size vs. number of epidermal cells to plastic and evolutionary reduction of wing size in response to poor larval food. We used flies from six populations selected for tolerance to larval malnutrition and from six unselected control populations, raised either under standard conditions or under larval malnutrition. In the control populations, phenotypic plasticity of wing size was mediated by both cell size and cell number. In contrast, evolutionary change in wing size, which was only observed as a correlated response expressed on standard food, was mediated entirely by reduction in cell number. Plasticity of cell number had been lost in the selected populations, and cell number did not differ between the sexes despite males having smaller wings. Results of this and other experimental evolution studies are consistent with the hypothesis that alleles which increase body size through prolonged growth affect wing size mostly via cell number, whereas alleles which increase size through higher growth rate do so via cell size.