Body size and the relative proportions of body parts are the key characteristics by which we recognize different species of animals. The mechanisms that control the sizes of the body and its organs and appendages, are among the great unsolved problems in developmental biology. The control of size requires a mechanism that somehow assesses size, and a mechanism that stops growth when a characteristic size is reached. We have studied the control of growth and the regulation of size in the tobacco hornworm, Manduca sexta. This work has uncovered rather unexpected mechanisms by which these animals assess their body size, and by which they control the proper scaling of wing size to and body size.