This is the Methiokos mosaic. The Zeugma site explains that the man Methiokos loved the girl Parthenope, who loved him too, but had made a vow to keep her virginity. She fled to Italy, devoting herself to the winegod Dionysos. However, Aphrodite could not accept she refused physical love, and transformed her into the demon Siren (woman's head, bird's body).