The most biodiverse groups of organisms in the forests of the Appalachians and Ozarks are fungi (especially microfungi) and invertebrate animals. The latter fact is not surprising, since more than 90% of all animals are invertebrates. Insects represent the single most diverse group of invertebrates in the forests of the Appalachians and Ozarks. Among the more ecologically important insects are pollinators (e.g., primarily butterflies, moths, bees, wasps, flies, and beetles), but insects also have many other ecological roles, including herbivory or as predators or parasites on other insects. These sometimes can play a major role in regulating outbreaks of pest insects. Insects themselves provide food for various groups of animals, including both invertebrates and vertebrates. Common non-insect invertebrates of the Appalachians and Ozarks include other arthropods such as arachnids (spiders and mites), centipedes, and millipedes as well as representatives of other phyla. The latter include earthworms and mollusks (snails and slugs). These groups and the ecological roles they play are discussed in this chapter.