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Argus Panoptes was a giant with a hundred eyes. He was also the nymph Io's brother. He was thus a very effective watchman, as only a few of the eyes would sleep at a time; there were always eyes still awake. Argus was Hera's servant. His great service to the Olympic pantheon was to slay the chthonic serpent-legged monster Echidna as she slept in her cave. Hera's last task for Argus was to guard a white heifer from Zeus. She charged him to "Tether this cow safely to an olive-tree at Nemea". Hera knew that the heifer was in reality Io, one of the many nymphs Zeus was coupling with to establish a new order.
To free Io, Zeus had Argus slain by Hermes. Hermes, disguised as a shepherd, first put all of Argus's eyes asleep with boring stories. To commemorate her faithful watchman, Hera had the hundred eyes of Argus preserved forever, in a peacock's tail.