The extremities of animals tend to grow in concert with the development of the body and end up proportionally sized for that body. But an insect's body size is set by temperature and food supply in the larval stage, well before the wings have begun forming. Biologists Fred Nijhout and Laura Grunert measured  growth in the tobacco hornworm, Manduca sexta, and found that proportional growth of the wings is controlled by hormones which are, in turn, regulated by the moth's central nervous system. They found they could control wing size with precise hormone injections, and are still studying how the nervous system senses the body's size in the first place.