A collective vernacular name sometimes used to designate four classes of the phylum Arthropoda as coordinate with the class Insecta (Hexapoda), but no longer considered a natural group. When originally proposed almost 2 centuries ago, it was believed that any terrestrial arthropod with more than six legs was, ipso facto, a “myriapod.” It is now known that members of these four classes are as different from each other as any one is from insects, although all five classes probably derived from a remote common ancestor. Shared characters include a ventral nerve cord; open circulatory system; respiration by a system of minute tubules (trachea), instead of by blood vessels, which distribute oxygen; body composed of a head capsule and an indefinite number of similar, leg-bearing segments; a single pair of antennae; and segmented appendages. Both sexes occur, with reproduction by means of internally fertilized eggs. Insects differ by having the body divided into a thorax region, with three pairs of legs and usually wings, and an abdominal region, the segments of which lack legs.