The reason why Antares, far down below the ecliptic, was chosen to be in our zodiac (basically following the ecliptic) is readily explained - Scorpio once marked the entrance to the Underworld. As to Spica the explanation presumably is that once a long time ago in high summer Mother Earth was so important as to deserve to be personified and at that time Virgo was created. Otherwise our zodiac has no female constellations. The female Earth corresponds to 'down' (È) in contrast to the male Sky which is 'above' (Ç), and therefore the Mother Earth constellation was chosen with a 'latitude' down in the south.

Hydra has a central position (which means it is important) and its front part lies below Cancer and Leo, which is one explaination for why it was not chosen to be in our zodiac - there cannot be more than one 'house' for Sun at a given time. Furthermore, the constellation was too long, and for us a water snake seems to be out of place in high summer - people in cities do not recognize that 'water' is the complementary reflection of 'fire':

... The rays drink up the little waters of the earth, the shallow pools, making them rise, and then descend again in rain.' Then, leaving aside the question of water, he summed up his argument: 'To draw up and then return what one had drawn - that is the life of the world.'

Neither is Hydra depicted in the round Dendera zodiac. But in the Babylonian zodiac it was placed between the ecliptic and the Milky Way - like a midstation hanging in the gap between water and sky:

The Serpent has no positive allusions for those who have read the Bible:

(Illuminated parchment from Spain ca AD 950)