an extreme type of specialized feeding in animals, in which the diet consists of a single food. It is common predominantly among groups with a large number of species, particularly among insects, some worms, crustaceans, and mollusks; it is very rare in vertebrates. Monophagy is common among herbivorous species, for example, the small form of the weevil Calandra granaria feeds only on wheat grains, while the large form feeds only on corn; monophagy is also characteristic of Phylloxera and other aphids.
Monophagy is common among animal parasites. For example, the nematode Cystoopsis acipenseris lives only in the sterlet, and the fluke Anchylodiscus siluri is found only in the sheatfish. Monophagy also characterizes a number of isopodan Malacostraca and bloodsucking parasites (ticks and fleas).