Consumption rates and the evolution of diet-induced plasticity in the head morphology of Melanoplus femurrubrum (Orthoptera: Acrididae).

Phenotypic plasticity may be an ecologically important evolutionary response to natural selection in multiple environments. I have determined the effect of diet-induced developmental plasticity in the head size of grasshoppers (Melanoplus femurrubrum) onfeeding performance on two types of plants. Full-sib families were divided and raised on either red clover, Trifolium repens, or rye grass, Lolium perenne. In three different stages of ontogeny, grasshoppers raised on rye grass had significantly larger heads, relative to body size, than full-sibs raised on clover. A principal components analysis indicated that two to five relative head size characters covaried as a block in their plastic response to the feeding environment. Regressions of adjusted consumption rates (mg/sec) against relative head size revealed that larger head sizes, induced by the rye grass diet, enhanced consumption rates of rye grass, but not clover. Unexpectedly, a similar positive association was observed between head size and consumption rate for grasshoppers raised on clover when they were feeding on clover. These results support the inference that grasshoppers exhibit adaptive phenotypic plasticity. However, the unexpected influence of head size on consumption rates of clover indicates that the functional relationship between head morphology and feeding performance is complex and that variation in this relationship among plant environments is not sufficient to explain the evolution of diet-induced phenotypic plasticity.