Check out our buddy Thomas Bulfinch's version of the story of Io from The Age of Fable:
Juno one day perceived it suddenly grow dark, and immediately suspected that her husband had raised a cloud to hide some of his doings that would not bear the light. She brushed away the cloud, and saw her husband on the banks of a glassy river, with a beautiful heifer standing near him. Juno suspected the heifer's form concealed some fair nymph of mortal mould- as was, indeed, the case; for it was Io, the daughter of the river god Inachus, whom Jupiter had been flirting with, and, when he became aware of the approach of his wife, had changed into that form.
Juno joined her husband, and noticing the heifer praised its beauty, and asked whose it was, and of what herd. Jupiter, to stop questions, replied that it was a fresh creation from the earth. Juno asked to have it as a gift. What could Jupiter do? He was loath to give his mistress to his wife; yet how refuse so trifling a present as a simple heifer? He could not, without exciting suspicion; so he consented. The goddess was not yet relieved of her suspicions; so she delivered the heifer to Argus, to be strictly watched.
Now Argus had a hundred eyes in his head, and never went to sleep with more than two at a time, so that he kept watch of Io constantly. He suffered her to feed through the day, and at night tied her up with a vile rope round her neck. She would have stretched out her arms to implore freedom of Argus, but she had no arms to stretch out, and her voice was a bellow that frightened even herself. She saw her father and her sisters, went near them, and suffered them to pat her back, and heard them admire her beauty. Her father reached her a tuft of grass, and she licked the outstretched hand. She longed to make herself known to him and would have uttered her wish; but, alas! words were wanting. At length she bethought herself of writing, and inscribed her name- it was a short one- with her hoof on the sand. Inachus recognized it, and discovering that his daughter, whom he had long sought in vain, was hidden under this disguise, mourned over her, and, embracing her white neck, exclaimed, "Alas! my daughter, it would have been a less grief to have lost you altogether!" While he thus lamented, Argus, observing, came and drove her away, and took his seat on a high bank, from whence he could see all round in every direction.
Jupiter was troubled at beholding the sufferings of his mistress, and calling, Mercury told him to go and despatch Argus. Mercury made haste, put his winged slippers on his feet, and cap on his head, took his sleep-producing wand, and leaped down from the heavenly towers to the earth. There he laid aside his wings, and kept only his wand, with which he presented himself as a shepherd driving his flock. As he strolled on he blew upon his pipes. These were what are called the Syrinx or Pandean pipes. Argus listened with delight, for he had never seen the instrument before. "Young man," said he, "come and take a seat by me on this stone. There is no better place for your flocks to graze in than hereabouts, and here is a pleasant shade such as shepherds love." Mercury sat down, talked, and told stories till it grew late, and played upon his pipes his most soothing strains, hoping to lull the watchful eyes to sleep, but all in vain; for Argus still contrived to keep some of his eyes open though he shut the rest.
Among other stories, Mercury told him how the instrument on which he played was invented. "There was a certain nymph, whose name was Syrinx, who was much beloved by the satyrs and spirits of the wood; but she would have none of them, but was a faithful worshipper of Diana, and followed the chase. You would have thought it was Diana herself, had you seen her in her hunting dress, only that her bow was of horn and Diana's of silver. One day, as she was returning from the chase, Pan met her, told her just this, and added more of the same sort. She ran away, without stopping to hear his compliments, and he pursued till she came to the bank of the river, where be overtook her, and she had only time to call for help on her friends the water nymphs. They heard and consented. Pan threw his arms around what he supposed to be the form of the nymph and found he embraced only a tuft of reeds! As he breathed a sigh, the air sounded through the reeds, and produced a plaintive melody. The god, charmed with the novelty and with the sweetness of the music, said, 'Thus, then, at least, you shall be mine.' And he took some of the reeds, and placing them together of unequal lengths, side by side, made an instrument which he called Syrinx, in honour of the nymph." Before Mercury had finished his story he saw Argus's eyes all asleep. As his head nodded forward on his breast, Mercury with one stroke cut his neck through, and tumbled his head down the rocks. O hapless Argus! the light of your hundred eyes is quenched at once! Juno took them and put them as ornaments on the tail of her peacock, where they remain to this day.
But the vengeance of Juno was not yet satiated. She sent a gadfly to torment Io, who fled over the whole world from its pursuit. She swam through the Ionian sea, which derived its name from her, then roamed over the plains of Illyria, ascended Mount Haemus, and crossed the Thracian strait, thence named the Bosphorus (cowford), rambled on through Scythia, and the country of the Cimmerians, and arrived at last on the banks of the Nile. At length Jupiter interceded for her, and upon his promising not to pay her any more attentions Juno consented to restore her to her form. It was curious to see her gradually recover her former self. The coarse hairs fell from her body, her horns shrank up, her eyes grew narrower, her mouth shorter; hands and fingers came instead of hoofs to her forefeet; in fine there was nothing left of the heifer, except her beauty. At first she was afraid to speak, for fear she should low, but gradually she recovered her confidence and was restored to her father and sisters.