Arach'nida (Gr. arachnē, 'a spider'), a sub-class of Tracheate Arthropoda (q.v. under ARTICULATA), including scorpions, spiders, mites, &c., and first separated by Lamarck from the Insecta of Linnaeus. The body is usually divided into cephalo-thorax and abdomen, the latter destitute of appendages, the former possessing six pairs, of which the posterior four pairs are walking limbs, thus furnishing a ready means of distinction from insects, which have three pairs only. The two anterior pairs, known as chelicerae and pedipalpi, are of various forms, the former usually chelate or sub-chelate; the latter chelate, ambulatory, or antenniform. Respiration is effected by means of tracheal tubes, or by pouches—the so-called respiratory sacs.
Those arachnida with segmented abdomen are termed Arthrogastra, and include five families, of which Scorpio, Thelyphonus, Chelifer, Galeodes, and Phalangium are types. These show a distinct gradation to true spiders or Araneina, which are easily recognised by their unsegmented abdomen, usually furnished with spinning glands, opening by four to six posterior papillæ, and by their sub- chelate chelicerae and ambulatory pedipalpi. The mites and ticks (Acarina) have the unsegmented abdomen continuous with the thorax, and the chelicerae and pedipalpi are modified into a sucking or piercing apparatus. The Linguatulida (Pentastomum), the Tardigrada, and the Pycnogonida, have usually been reckoned as highly modified arachnides, somewhat akin to the Acarina; the most recent anatomists, however, tend to remove them from the arachnida altogether. On the other hand, it has lately been clearly shown that the Silurian Eurypterida, and the ancient, but still persistent Limulus (see KING-CRAB), must be reckoned rather as arachnides than as crustaceans, and thus the two great divisions of the Arthropoda, the Tracheata and the Branchiata, are shown to have diverged in palæozoic times. See ACARINA, MITE, SCORPION, SPIDER, and TICK; also Huxley's Anatomy of Invertebrated Animals, Balfour's Comparative Embryology, and Cambridge's monograph in Encyclopædia Britannica (9th ed.).