In Greek mythology, Io (pronounced /ˈaɪ.oʊ/ or /ˈiː.oʊ/, in Ancient Greek Ἰώ [iːɔ́ː]) was a priestess of Hera in Argos, a nymph who was seduced by Zeus, who changed her into a heifer to escape detection. Her mistress Hera set ever-watchful Argus Panoptes to guard her, but Hermes was sent to distract the guardian and slay him. Heifer Io was loosed to roam the world, stung by a maddening gadfly sent by Hera, and wandered to Egypt, thus placing her descendant Belus in Egypt; his sons Cadmus and Danaus would thus "return" to mainland Greece.
Io's father is generally given as Inachus, a river god credited with inaugurating the worship of Hera in the countryside round Argos, thus establishing her as an autochthonous spirit of the Argolid and thus by her nature the nymph of a spring, a naiad.
The myth is told most anecdotally by Ovid, in Metamorphoses. According to Ovid, one day, Zeus noticed the maiden and lusted after her. As Io tells her own story in Aeschylus' Prometheus Bound, she rejected his whispered nighttime advances until the oracles caused her own father to drive her out into the fields of Lerna. There, Zeus covered her with clouds to hide her from the eyes of his jealous wife, Hera, who nonetheless came to investigate. In a vain attempt to hide his crimes, Zeus turned himself into a white cloud and transformed Io into a beautiful white heifer. Hera was not fooled. She demanded the heifer as a present.
Hera tethered Io to the olive-tree in the temenos of her cult-site, the Heraion, and placed her in the charge of many-eyed Argus Panoptes to keep her separated from Zeus. Zeus commanded Hermes to kill Argus; Ovid added the detail that he lulled all hundred eyes to sleep. Hera then forced Io to wander the earth without rest, plagued by a gadfly (Οίστρος or oestrus: see etymology of "estrus" ) to sting her into madness. 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.
Prometheus had been chained on Mt. Caucasus by Zeus for teaching Man how to make fire and tricking him into accepting the worse part of a sacrifice while the mortals kept the better part (meat); every day, a giant eagle fed on Prometheus' liver. Despite his agony, he comforted Io with the information that she would be restored to human form and become the ancestress of the greatest of all heroes, Heracles. 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, Danaos, eventually returned to Greece with his fifty daughters (the Danaids), as recalled in Aeschylus' play The Suppliants.
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