The Collaborative International Dictionary of English v.0.48:

Arachnid \A*rach"nid\, n.
   An arachnidan. --Huxley.
   [1913 Webster]

WordNet (r) 3.0 (2006):

arachnid
    n 1: air-breathing arthropods characterized by simple eyes and
         four pairs of legs [syn: arachnid, arachnoid]