Arachnida, a rak'ni dah, a class of air-breathing animals which include the spiders, scorpions, mites and ticks. A few live on plants, but most of them are carnivorous. As a whole, they are beneficial to agriculture, as they prey on other insects; but some parasitic forms are destructive to both plants and animals. Many have glands which secrete poisons, and the spiders have attached to their abdomens spinnerets, from which are secreted the threads of which webs are formed. They are a subdivision of the Arthropoda. See ARTHROPODA; SPIDER.