EVOLUTION OF INSECT MORPHOLOGY IN RELATION TO PLANTS

This short review points out some of the major physical problems faced by insects feeding on plants, and some of the kinds of morphological adaptations that have been noted to date. Major emphasis is given to two factors: the nature of the plant surface, and the difficulty of dealing with hard or tough food. The surface provides a great variety of terrains that require specialization for maximizing tenacity and agility, especially for small insects. It is suggested that natural enemies may provide significant selection for the relevant morphologies. The difficulty of feeding upon certain plant tissues is shown to be overcome in different ways by different herbivore groups. In the case of tough leaves for example, grasshopper mandible adaptations appear to have evolved to maximize efficiency of processing. On the other hand, in the case of caterpillars, mandible adaptations for tough food appear to be minimizing handling time with a concomitant reduction in risk of predation. Convergent evolution is shown in both grasshoppers and caterpillars for dealing with leaves of similar design and an example is given of rapid evolution of mouthpart morphology in response to differences in host characteristics. These examples are used to indicate the risks of using such characters in establishing phylogenetic relationships. Finally, it is pointed out that plant chemical qualities and plant ecological factors can influence insect morphological features.