Newly emerged adult dragonflies are physiologically immature in a number of ways, including the mass, ultrastructure and biochemistry of their flight muscles. In Libellula pulchella dragonflies, adult maturation of flight muscle is accompanied by striking changes in thermal physiology. Vertical force production during fixed flight attempts in newly emerged adults (tenerals) shows a broad plateau of near-peak performance, first attained at cool thoracic temperatures (typically 28­34 °C) and maintained up to thoracic temperatures of 40­45 °C [mean optimal thoracic temperature (OTT)=34.6 °C; mean upper lethal temperature (ULT)=45.3 °C]. In contrast, fully mature adults show narrow thermal sensitivity curves, wherein peak performance is approached only within a few degrees of the thermal optimum, which invariably occurs at hot thoracic temperatures (38­50 °C; mean OTT=43.5 °C; mean ULT=48.6 °C). These changes in the shape and position of thermal performance curves are compared with predictions from hypotheses for the evolution of thermal sensitivity.