Similarly, interspecific competition and predation are examples of animal co-action which influence diet breadth. Certain insects are protected from predators by resembling the plants on which they live, and in such cases selection may be expected to increase the morphological resemblance to the plants and also to favour closer association between the insects and their host plants.
Prolonged association over several generations with certain plants would enable them to feed efficiently on the plants. In some other cases, where the habitats are highly heterogeneous, animals might evolve coloration and structures matching several species of plants or some other common and general characteristic of their habitat, e.g., leaf litter, grass stems, pebbles, etc.
Thus, diet seems to be influenced by where the animals gain protection. Sometime, differences in diet can arise as secondary effects of habitat separation; species which are very similar in appearance may be either allopatrically distributed or may form a local mosaic distribution and frequently feed on different plants.
Certain properties of animals, e.g., speed of locomotion and sensory- acuity, determine their efficiency of locating rare plants. Highly mobile animals with high perceptive acuity have short search times and tend to treat their environment in a relatively coarse-grained fashion, thus behaving as specialists. Sometime there exists a correlation between body size and feeding behaviour; thus, Hansen and Ueckert (1970) found that smaller species of grasshoppers had more restricted diets than larger species.
Several instances are known of the unsuitability of plant species even to polyphagous animals, and suitability may well have a strong biochemical basis. Otte and Joern (1977) state that “In suitable grainy environments, strong competition should produce specialization along with reduced overlap, i.e., it should result in mutually exclusive specialization.
Any specialization which is accompanied by much overlap between similarly specialized species must be due to other factors.” According to Otte and Joern, the oft-repeated cases of overlapping specialization clearly indicate that competition is not the driving force behind specialization. Otte and Joern have found that desert grasshopper species have more specialized diets than grassland species of grasshoppers. In fact, most specialized species are desert or arid-zone species.
Among polyphagous species, ground inhabiting (=ground resembling) ones have more general diets than those which inhabit vegetation. Most grasshoppers are obligate specialists on dicotyledonous plants and few, if any, specialize on monocots. The number of grasshoppers inhabiting an area does not seem to affect degree of specialization or breadth of food niche. Different species have been observed to coexist and feed on the same plant species provided they differ in appearance.
According to Schultz (1988), many factors can affect the evolution of herbivore diets but the central factor is plant chemistry. He emphasizes the need for greater attention to this central factor. According to Schultz, the most important influence of plant chemistry on insect heibivory lies in mediating interactions between herbivores and other selective factors.