katydid n.

popular name for a large, green locust-like insect, 1784, American English, imitative of the stridulous sound the male makes when it vibrates its wings. In the eastern U.S., a familiar sound of a summer night; the sound itself was more accurately transcribed in 1751 as catedidist.

[T]heir noise is loud and incessant, one perpetually and regularly answering the other in notes exactly similar to the words Katy did, or Katy Katy did, repeated by one, and another immediately bawls out Katy didn't, or Katy Katy didn't. In this loud clamour they continue without ceasing until the fall of the leaf, when they totally disappear. [J.F.D. Smyth, "A Tour in the United States of America," 1784]