From Ovid's "Metamorphoses", Book I:

... For now the girl had run;
Through Lerna's meadows and the forest lands
Of high Lyrceus she sped until the god
Drew down a veil of darkness to conceal
The world and stayed her flight and ravished her.

Juno meanwhile observed the land of Argos
And wondered that the floating clouds had wrought
In the bright day the darkness of the night.
These were no river mists! No clouds like these
The humid earth exhaled! She looked around
To find her husband; well she knew his tricks,
So often had caught him in his escapades;
And searched the sky in vain. "If I'm not wrong"
She thought, "I'm being wronged"; and gliding down
From heavens hight she lighted on the earth
And bade the clouds disperse. Jove had for-sensed
His spouse's visit and transformed poor Io
Into a sleek white heifer (lovely still
Altough a cow). Juno, against her will
Admired the creature and asked who she was,
And whence she came and to what herd belonged,
Pretending not to know the truth. He lied -
"The earth has brought her forth" - so to deflect
Questions about her birth. Then Juno begged
The heifer as a gift...