Insects differ from arachnids in that they typically have a distinct head, thorax, and abdomen. Most insects have a pair of wings which originate from the thorax, but these may be lost in some groups (the siphonaptera or phthiraptera) or reduced to a single pair, the diptera (although in the diptera, there are halteres that represent wings developed for specialized functions). Some groups such as the lice, undergo simple metamorphosis, i.e., the larval stages that come out of the egg look very similar to the adult except for the lacking of sexual characters. Other groups such as the flies and the fleas, have a worm-like larva that leaves the egg. This larva will undergo complete metamorphosis in a cocoon or pupal case to ultimately develop into an adult that looks very little like its worm-like larva.