Beetles constitute the largest order of insects. Most species secrete chemicals that can irritate the skin and eyes of humans and other animals. Beetles found in stored products cause inhalational allergies and act as intermediate hosts for helminthes that cause pathology in domestic and wild animals. Many dung-inhabiting beetles act as predators or parasitoids of pestiferous flies that breed in excrement. A few beetle species are ectoparasites or mutualistic symbionts on mammals, and a few invade the skin of mammals. The order Coleoptera is divided into four suborders. More than 300,000 species of beetles have been described, representing 30─40% of all known insects. Beetles vary in shape from elongate, flattened, or cylindrical to oval or round. Their bodies are often hardened, like the elytra. All beetles exhibit holometabolous development. Eggs are laid singly or in clusters on or in soil, living or dead plant matter, fabrics, water, and carrion and, rarely, on living animals. Beetles live within all terrestrial and freshwater habitats. Human health problems caused by beetles include skin, eye, ear, and nose irritations, respiratory allergies, and minor gastrointestinal discomfort. Beetles cause a variety of problems to domestic animals such as ingestion, mechanical transmission of disease agents, intermediate hosts for helminthic parasites, direct injury to animals by ectoparasitic species, and structural damage to poultry facilities. Some species of beetles have evolved in close association with mammals as nest dwellers or ectoparasites. Pesticides help in controlling beetles that occur in natural and cultivated vegetation.