The Fly(1958)

André Delambre (David Hedison) has his head and right hand transposed with that of a fly. He walks around with a blanket over his head and scrawls on a blackboard to communicate, much to the dismay of his wife. When his wife pulls the blanket off to find a huge fly’s head with multi-faceted eyes, this is too much for everyone and Andre is crushed in a mechanical press.
A police inspector interviews Francois Delambre (Vincent Price), Andre’s brother and is skeptical about the entire affair, until the two find a fly with a human head caught in a spider’s web. Francois crushes the fly with a rock.
(It’s never explained how both the human with the fly head AND the fly with the human head BOTH appear to have full human intelligence.)