Io1.0079_Velázquez

Diego Rodríguez de Silva y Velázquez (1599-1660), "Mercury and Argus" (c. 1659), oil on canvas; Madrid, Prado, no. 1175.

Enlisted by Juno to watch the suspicious heifer, Io whom Jupiter had transformed from nymph, Argus was especially equipped for vigilance. In mythological iconography, Argus has fifty pairs of eyes that never all sleep at any time. Velázquez' propensity for portraying realistic models in his paintings seems unwilling to stretch anatomical verisimilitude.
True to his Ovidian source, Velázquez depicts Mercury here in the moment before he murders the dozing cattleguard. Mercury has sung the song about the metamorphosis of Syrinx, i.e. the invention of the syrinx-pipe that lies between the god's hands. The tale, Ovid wittily jests, is so boring that it causes even the ever-vigilant Argus to sleep.
Io the bovine backdrop is apparently only there to define the iconography for the viewer.

Bibliography:
López-Rey, J. 1963. Velasquez: a catalogue raisonné of his oeuvre. London: Faber & Faber. no. 62
López Torrijos, Rosa. 1985. La mitologia en la pintura española del siglo de oro. Madrid: Catedra. pp. 309ff.
Prado Museo Nacional de Éspaña. 1985. Catalogo de las pinturas. p.730