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Insects are animals with three body segments (the word insect means "segmented"): the head, the abdomen, and the thorax. Insects have, typically, three pairs of legs located on the abdomen; however, there are exceptions, such as brush-footed butterflies. Insects (class Insecta) are just one class of arthropods, a phylum which includes Crustacea (crabs, lobsters, etc.), Arachnida (spiders, mites, etc.), and Myriapoda (Centipedes and Millipedes). Insects, being related, undergo similar life cycles. They begin as eggs, which usually vary widely in shape and size. Then insects undergo a period of growth, which may include metamorphosis. Insects such as silverfish do not undergo metamorphosis at all— they grow directly from a diminutive size to their full size. Some Insects, such as true bugs, undergo incomplete metamorphosis. When they hatch from eggs they are called nymphs. These nymphs will eventually grow into their adult form. Insects such as butterflies and moths undergo complete metamorphosis. When they hatch from eggs, they are called larvae. Once they have grown through several periods called instars, they become pupae. Eventually, an adult or imago will emerge from the pupa. Insects are very successful organisms. They exist in every type of terrestrial habitat, and are able to adapt easily to an environment. Insects also vary widely in shape, size, colour, etc. There are currently well over half-a-million named species of insect, and estimates place the total number of species over two million.