-nədə noun plural
Usage: capitalized
Etymology: New Latin, from arachn- + -ida
1. : a large class of arthropods including scorpions, spiders, mites, and related forms all lacking wings and free first antennae and having highly modified prehensile second antennae and most being also air-breathing by means of tracheae or book lungs and having head and thorax fused to form a cephalothorax that bears six pairs of appendages consisting of four pairs of walking legs, a pair of pedipalpi variously specialized in different groups, and a pair of buccal chelicerae often provided with poison glands
2. in former classifications : a more extensive group including the Arachnida together with the king crabs, tongue worms, sea spiders, water bears, and sometimes the extinct eurypterids