The ancients, by means of the divining-rod, not only procured those things necessary for a livelihood or for luxury, but they were also able to alter the forms of things by it: as when the magicians changed the rods of the Egyptians into serpents as the writings of the Hebrews relate; and as in Homer, Minerva with a divining-rod turned the aged Ulysses suddenly into a youth, and then restored him back again into old age; Circe also changed Ulysses’ companions into beasts, but afterward gave them back again their human form: moreover by his rod, which was called Caduceus, Mercury gave sleep to watchmen and awoke slumberers. (Agricola in De Re Metallica, Basel, 1556.)
Under the ancient Greeks, the caduceus became a very sophisticated instrument in the hands of Mercury (Hermes). According to legend, Mercury, the messenger of the Gods, intervened in a fight between two serpents who then curled themselves round his wand. Hence the caduceus as a rod intertwined by two serpents, bound at the base by their tails and facing each other at the top, developed in alchemical symbolism as the polarity between the one (male, the rod) and the dual (female, the split serpents) that combine at each end. Parallel to this, the male represents Being, Fixed, Fire or Sulpher, Sex; while the female represents Becoming, Volatile, Water or Mercury, Spirit.
Visually this little wings that the caduceus sprouted in Greek times show the volatility and bisexuality of Mercury, spanning the male-female worlds that have to be sublimated in the alchemical processes toward the unity.