The effects of mandible morphology and photosynthetic pathway on selective herbivory in grasshoppers

In the present study the following hypotheses were tested: 1) Selective herbivory is influenced by the photosynthetic pathway of plants, and 2) selective herbivory is influenced by the mandible morphology of herbivores. To test these hypotheses, grasshoppers with different mandibular patterns were offered C₃ and C₄ plants in controlled feeding experiments.Mandible morphology was found to be significantly associated with energy assimilation and feeding frequency. Grasshoppers with herbivorous mandibles had greater assimilation efficiencies and fed significantly less often than grasshoppers with forbivorous mandibles. These results indicate grasshoppers (with comparable caloric requirements) feed with a frequency determined, in part, by the functional morphology of their mandibles.Photosynthetic pathway also influenced the foraging behavior of grasshoppers tested. As predicted by the C₄ avoidance hypothesis (Caswell et al. 1973), C₄ plants were fed upon significantly less often than C₃ plants. These results suggest grasshoppers may discriminate between C₃ and C₄ plants, but apparently not on the basis of available energy.Recent studies of insect herbivory, previously interpreted within the framework of C₄ plant avoidance, are reviewed. The results of these studies may be critically dependent on incomplete considerations of mandible morphology and photosynthetic pathway.