Animals that eat both plants and other animals, called omnivores, occur in every major animal lineage, including insects, fish, reptiles, mammals and birds. By contrast, some lineages, such as spiders, are completely devoid of omnivores.
Many insects, including pest species such as cockroaches and ants, are omnivores that eagerly consume a variety of organic materials, regardless of whether they were produced by plants or by animals. Other insects that are omnivorous include many species of wasps, which hunt other insects and sip plant nectar. While not insects, roly-polies are small invertebrates that are somewhat similar to insects and exhibit omnivorous habits.
While most adult amphibians are only carnivorous, many feed upon plant material as larvae. For example, bullfrogs are important predators of aquatic vertebrates and invertebrates, but while they are tadpoles, they have mouths designed for scraping algae off rocks. Many lizards and turtle species are omnivorous, although all snakes and crocodiles are strict carnivores. A few large tortoises are entirely herbivorous.
Many birds and mammals are omnivorous, as they must consume a high number of calories to power their active metabolisms. Humans, bears, raccoons, opossums, dogs, wolves, robins, mockingbirds and sparrows are a few examples of omnivorous mammals and birds.