The Collaborative International Dictionary of English v.0.48:

Katydid \Ka"ty*did`\, n. (Zool.)
   A large, green, arboreal, orthopterous insect (Cyrtophyllus
   concavus) of the family Locustid[ae], common in the United
   States. The males have stridulating organs at the bases of
   the front wings. During the summer and autumn, in the
   evening, the males make a peculiar, loud, shrill sound,
   resembling the combination Katy-did, whence the name.
   [1913 Webster]

WordNet (r) 3.0 (2006):

katydid
    n 1: large green long-horned grasshopper of North America; males
         produce shrill sounds by rubbing together special organs on
         the forewings