ARACH'NIDA ( Gk. dpdxf,n, araehne, a spicier). A class of air-breathing arthropods including the mites, scorpions, spiders, and a few other less well-known groups. The typical Araehuida have the head and the thorax more or less fused into a "cephalothorax,” four pairs of legs, and no antennte, the maxillary palps functioning as The eyes are all simple, and vary in number from two to twelve. By the number and arrangement of these eyes the species of spiders are determined. The abdomen possesses no true legs, but the three abdominal spinnerets of spiders are homologous with legs. Besides the spinneret-glands in the abdominal region of spiders there are poison-glands in the last abdominal segment of scorpions, located at the base of the sting. In other forms the poison is emitted through the hollow jaws. The Arachnid:1 breathe by means of tracheic, like other insects, or by means of saeklike bodies called "lungs" that open on the under side of the abdomen: but some forms breathe by both trachea' and lungs. All the Arachnida are carniv
orous save sonic of the mites, which live on plant sap. Most of the animal-feeders prey upon other insects, and hence are the friends of agri culture. A few forms are parasitic on warm blooded vertebrates and fish, and cause or accom pany such diseases as itch and mange. The his tory of this class goes back to Palaeozoic times.
Classilication.—The Araehnida are divided into seven orders: (I) Solpugida, or wind scor pions: (2) Seorpionida, or scorpions; (3) Psem doscorpionida, book-scorpions; (4) Pedipalpida, or whip-seoridons: (5) Phalangida, or harvest men; (6) Araneida, or spiders; and (7) •earida, or mites. The following groups are believed by many to fail into the class Armimida, but their relationships are doubtful: Linguatulida, or tongue-pa•asites of the dog: Tardigrada, or water-bears; enogonida, or sea-spiders; Xiphosura, or king-crabs. See .11rrEs; SCOR PIONS; SPIDERS; HARVEST-MAN.