They have four pairs of legs, no antennae nor wings, a pair of mandibles, and one pair of maxillae or palpi. The head is usually consolidated with the thorax. The respiration is either by trancheae or by pulmonary sacs, or by both. The class includes three principal orders: araneina, or spiders; arthrogastra, including scorpions, etc.; and Acarina, or mites and ticks.