For almost a century, biologists have used trait scaling relationships (bi-variate scatter-plots of trait size versus body size) to characterize phenotypic variation within populations, and to compare animal shape across populations or species. Scaling relationships are a popular metric because they have long been thought to reflect underlying patterns of trait growth and development. However, the physiological mechanisms generating animal scaling are not well understood, and it is not yet clear how scaling relationships evolve. Here we review recent advances in developmental biology, genetics, and physiology as they pertain to the control of growth of adult body parts in insects. We summarize four mechanisms known to influence either the rate or the duration of cell proliferation within developing structures, and suggest how mutations in these mechanisms could affect the relative sizes of adult body parts. By reviewing what is known about these four processes, and illustrating how they may contribute to patterns of trait scaling, we reveal genetic mechanisms likely to be involved in the evolution of insect form.