The Fly (1986)

When scientist Seth Brundle completes his teleportation device, he decides to test its abilities on himself. Unbeknownst to him, a housefly slips in during the process, leading to a merger of man and insect. Initially, Brundle appears to have undergone a successful teleportation, but the fly's cells begin to take over his body. As he becomes increasingly fly-like, Brundle's girlfriend is horrified as the person she once loved deteriorates into a monster.