An instar is a developmental stage of arthropods, such as insects, between each molt. Because of the external chitinous skeleton, arthropods must shed the exoskeleton in order to grow or assume a new form. For some species the instar merely means the size of the individual increases, but in others a complete change in structure or metamorphosis occurs at some point, such as the familiar change from larva to butterfly in Order Lepidoptera. In other cases, the new instar is altered in the proportions between body regions or number of segments. The number of instars depends on the species and its length of life.