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The evolution of body size is a dominant feature of animal evolution. However, little is known about how the underlying developmental mechanisms that determine size change as body size evolves. Here we report on a case of body size evolution in the tobacco hornworm Manduca sexta that occurred over a period of nearly 30 years. We take advantage of an extensive series of physiological studies performed in the early 1970s that established the parameters that regulate body size in this species and compare their values with those of modern individuals that are descendants of the same colony. We show that three of the five processes that determine adult body size changed during this period, while two remained constant. Changes in these three developmental processes completely account for the observed evolutionary change in body size.