Black-figure painting on an amphora dating to 540-530 B.C., now in the
Staatliche Antikensammlungen, Munich, Germany. The figure at the centre
represents Io transformed into a cow. Daughter of the king of Argos
and priestess of the goddess Hera, Io was loved by Zeus and consequently
turned into a heifer by the anger of the goddess to whom she had consecrated
herself. Hera put the maiden under the custody of Argus, a one-hundred-eyes
giant, who was killed by Hermes, under Zeus’s order, thus permitting
the union of the girl with the god. The implacable Hera haunted Io with
the stings of a horsefly, compelling the girl, in cow’s shape,
to wander on the earth pursued by the insect, until she reached Egypt,
where she was able to acquire her human aspect again. Here the maiden
gave birth to a child, Epaphus. This was the name with which the Greeks
called the Egyptian god Apis, represented in the shape of a bull, while
Io herself was identified with the Egyptian goddess Isis, frequently
represented with bovine horns on her head.
It is possible to hypothesize that this myth was a kind of prototype
of a ritual performance which took place in the sanctuary of Hera in
Argos, a goddess who was frequently called with the epithet of Boópis,
“with cow eyes” (e.g. Iliad, I, 551). Here, one
or more selected maidens spent an initiatory seclusion period within
the temple, before marriageable age. During this time, the girls were
assimilated to white cows, animals sacred to the goddess, and showed
an abnormal, “foolish” behaviour, produced by the contact
with the deity. The period of the passage was regarded as a dangerous
period, represented in the myth by the “wrath of the goddess”
motif (Dowden 1989).
[Source: http://commons.wikimedia.org/wiki/File:Hermes_Io_Argos_Staatliche_Antikensammlungen_585.jpg]