Who was the priestess of Hera at Argos who was loved by Zeus?

Io was a mortal woman who was favored in the sight of Zeus. He then turned Io into a heifer, to hide her from his wife. However, Hera was no fool and had set spies upon Io. After Io's transformation, Hera afflicted her with a gadfly (irritating biting fly) to torment her, as punishment.