" ZEUS dear, I haven’t had a gift from you for ages," she wheedled at the breakfast table.
"Anything you desire, dearest," he smirked, "It’s yours."
"Well now, there’s a lovely white heifer over on a yonder hill that would go so well with my prize herd..."
She saw ZEUS grow pale and his eyes cloud over. "I.. er, I er... Oh, yes of course darling."
They nearly got away with it, but before ZEUS could get IO’s halter off, HERA sent a gadfly to sting the poor young cow to the point of madness. Bellowing with pain, she ran off with the fly in never-ending pursuit.
IO ran in every which direction to dodge her tormentor — over the hills and far away, up and down, in and over. She even passed PROMETHEUS chained to his rock on Mount Caucasus, who managed to shout as much encouragement as he could in the circumstances.
There came a time when she reached Egypt. As she and the gadfly paused for breath, ZEUS managed to return her to her original comely form, and she discovered she was pregnant.
We have no idea what happened to the gadfly.