a·rach·nid 
(ə-răknĭd)
(ə-răknĭd)n.
Any of various arthropods of the class Arachnida, such as spiders, scorpions, mites, and ticks, characterized by four pairs of segmented legs and a body that is divided into two regions, the cephalothorax and the abdomen.
[From New Latin Arachnida, class name, from Greek arakhnē, spider.]
a·rachni·dan (-nĭ-dən) adj. & n.