Argus or argus eyed

One of Zeus' sexual escapades involved the maiden Io. In an attempt to keep Hera from discovering the truth of his dalliance, Zeus transformed Io into a cow. Hera, not easily thrown off the scent of her husband's affairs, prevailed upon Zeus to give her the cow as a present and an assurance of his good faith, after which Hera enlisted the aid of Argus, a giant with one hundred eyes, to keep a close watch over the poor girl. In English one who is ever-vigilant or watchful can be called an Argus or be described as argus-eyed.