The Fly (1958)

In this original version, a scientist (David Hedison) is experimenting with his latest creation, a matter transporter. With varying degrees of success, he eventually decides to use himself as a human subject, with tragic consequences. During the transferance, a fly is accidentally let into the machine and his atoms become merged with the fly. He ends up with the fly's head and one of its arms. The fly winds up with the scientist's head and one of his arms.