In order to exact her revenge, Hera sent a gadfly to sting Io continuously, driving her to wander the world without rest. Io eventually crossed the path between the Propontis and the Black Sea, which thus acquired the name Bosporus (meaning "ox passage"), where she met Prometheus, who had been chained on Mt. Caucasus by Zeus. Prometheus comforted Io with the information that she would be restored to human form and become the ancestress of the greatest of all heroes, Heracles (Hercules). Io escaped across the Ionian Sea to Egypt, where she was restored to human form by Zeus. There, she gave birth to Zeus's son Epaphus, and a daughter as well, Keroessa. She later married Egyptian king Telegonus. Their grandson, Danaus, eventually returned to Greece with his fifty daughters (the Danaids), as recalled in Aeschylus' play "The Suppliants".