From The Art and Popular Culture Encyclopedia
In Greek mythology, Io (pronounced "EE oh" though "EYE oh," is also acceptable) was the daughter of Inachus, a river god. One day Zeus noticed her and she quickly became one of his many lovers. 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.
Once Io was given to Hera, she placed Io in the charge of Argus (the many-eyed monster) to keep her separated from Zeus. Zeus then commanded Hermes to kill Argus, which he did by lulling all one-hundred eyes to sleep. Hera sent a gadfly to sting Io as she wandered the earth, eventually running into Prometheus, after crossing the path between Propontis and Black Sea, which thus took the name Bosporus (meaning ox passage). Prometheus, at that time chained on Mt. Caucasus, told her that she would eventually be restored to human form and become the ancestress of a great hero (Heracles). Io escaped across the Ionian Sea to Egypt, where she was transformed back into human form by Zeus. In Egypt Io gave birth to Epaphus.
The term Io fly is derived from the gadfly Hera sent to torment Io into fleeing to Egypt after Argus was slain.