IDRA, IDRE. The Hydra is the constellation the Water Snake with one hundred heads, lying in the southern hemisphere next to Corvus, The Raven (Tetrabiblos, I.9). The Hydra lived at Lerna and acted as sentinel to the border at the entry to the realm of the dead, while her brother, Cerberus, guarded the gates. Her fiery breath was her weapon, and when one head was cut off, two more sprang up in its place. In his second labor Hercules vanquished the Hydra. He killed her and dipped his arrows in her poisonous blood to make them deadly (Met IX.69, 192-193; Aeneid VI.576-577, VII.658).

The Water Snake does not appear as a constellation in Chaucer's works. It is called "the firy serpent venymus," MkT 2105, in the rehearsal of Hercules's labors. Chaucer seems to have misread Aeneid VI.288, where flammisque describes the Chimera. Lady Philosophy reminds Boethius that when one doubt is cut away, others spring up like the heads of Idra, the serpent that Hercules slew, Bo IV, Prosa 6.19-20. Hercules is celebrated for his labors, including the slaying of Idra the serpent, Bo IV, Metr 7.41-42. [Cancer: Dragoun: Ercules]

Idra is the ME variant, Idre the Italian variant, Inf IX.40. Latin initial h was not pronounced and it disappears in Italian.