Source : Webster's Revised Unabridged Dictionary (1913)

Katydid \Ka"ty*did`\, n. (Zo["o]l.)
   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.

Source : WordNet®

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