The development and evolution of exaggerated morphologies in insects

We discuss a framework for studying the evolution of morphology in insects, based on the concepts of "phenotypic plasticity" and "reaction norms." We illustrate this approach with the evolution of some of the most extreme mor phologies in insects: exaggerated, sexually selected male ornaments and wea pons, and elaborate social insect soldier castes. Most of these traits scal e with body size, and these scaling relationships are often nonlinear. We a rgue that scaling relationships are best viewed as reaction norms, and that the evolution of exaggerated morphological traits results from genetic cha nges in the slope and/or shape of these scaling relationships. After review ing literature on sexually selected and caste-specific structures, we sugge st two possible routes to the evolution of exaggerated trait dimensions: (a ) the evolution of steeper scaling relationship slopes and (b) the evolutio n of sigmoid or discontinuous scaling relationship shapes. We discuss evolu tionary implications of these two routes to exaggeration and suggest why so many of the most exaggerated insect structures scale nonlinearly with body size. Finally, we review literature on insect development to provide a com prehensive picture of how scaling relationships arise and to suggest how th ey may be modified through evolution.