ABSTRACT
This chapter presents the essential aspects of insects that distinguish them from other animals and that contribute to their survival and importance in humans' lives. Insects have survived all five major extinction events that eliminated many other plant and animal groups, and they continue to show remarkable ability to adapt to ongoing anthropogenic changes. Insects are characterized by three body regions and three pairs of legs, whereas arachnids have two body regions and four pairs of legs. Many arachnids, chilopods and insects are venomous, with fangs, stingers or “urticating” hairs or spines that can be dangerous to humans and other animals. Mayflies are unique among insect orders in having two fully-winged terrestrial adult stages, a subimago which molts into a sexually mature imago. Many species are important predators of aphids and other small insects and provide important biological control services for agricultural pests.
