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Arthropods are invertebrates such as insects, spiders and other arachnids, and crustaceans that comprise the phylum Arthropoda. The phylum Arthropoda includes three major classes—the Insecta, Arachnida, and Crustacea.

Arthropods are characterized by their external skeleton, or exoskeleton, made mostly of chitin, a complex, rigid carbohydrate usually covered by a waxy, waterproof cuticle. This integument is important in reducing water loss in terrestrial habitats, in providing protection, and in providing a rigid skeleton against which muscles can work in order to develop motion of the animal or in its body parts. The exoskeleton is segmented, which allows for easy movement of the body, and there are numerous paired, segmented appendages, such as legs, antennae, and external mouth parts. Periodically, the entire rigid exoskeleton is shed, the temporarily soft animal swells in size, and its new, larger exoskeleton hardens.

Most arthropods have compound eyes, each with numerous lenses capable of forming complex, composite images. Arthropods have various mechanisms for the exchange of respiratory gases which, depending on the group, include gills, chambered structures known as book lungs, tracheal tubes, and various moist areas of the body surface.

Most arthropods exhibit sexual dimorphism, in that the male animals look distinctly different from the females, at least in the appearance of their external genitalia. Arthropods have internal fertilization, and they lay eggs. Arthropods have a complex life cycle. This generally involves eggs, a juvenile larval stage, and the adult form, with complex metamorphosis occurring during the transitions between these stages. In some insects, there is an additional stage between the larva and the adult, known as a pupa.

Arthropods are extremely diverse in species richness. Approximately 874,000 living species of arthropods have been named, comprising more than 80% of all named species of animals. However, some estimates predict large numbers of species of arthropods that have not yet been described and named by biologists. Most of these unnamed species are small beetles and other inconspicuous arthropods, and most of these occur in old-growth tropical rainforests, a biome that has not yet been well explored and studied by taxonomists and ecologists.

Species of arthropods utilize an enormous variety of Earth's habitats. Most species of crustaceans are aquatic, although a few, such as woodlice and land crabs, occur in moist habitats on land. The spiders, mites, scorpions, and other arachnids are almost entirely terrestrial animals, as are the extremely diverse insects.

Some species of arthropods are very important to humans. A few species are important as vectors in the transmission of microbial diseases, such as malaria, yellow fever, encephalitis, plague, Chagas disease, and Lyme disease. Some arthropods are venomous, and can hurt or kill people by single or more multiple stinging, for example, scorpions, some spiders, and bees and wasps. Some arthropods are a highly nutritious source of food for people, as is the case of lobsters, crayfish, shrimp, many species of crabs, and some insects.

However, the most critical importance of arthropods relates to the extremely diverse and beneficial ecological functions that they carry out. Arthropods play an important role in nutrient cycling and other aspects of ecological food webs. Earth's ecosystems would be in a great deal of trouble if there was any substantial decline in the myriad species of arthropods in the biosphere.

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