The Lernaean Hydra (Ancient Greek: Λερναῖα ὕδρα) was a serpentine water-monster of Greek and Roman mythology whose lair was the lake of Lerna in the Argolid, a site considered an entrance to the Underworld. It possessed many heads — the exact number varied by source — together with poisonous breath and blood so virulent that even its scent was deadly; in later versions, two heads grew back for every one cut off. Heracles slew it as the second of his Twelve Labours, aided by his nephew Iolaus, who cauterized the severed neck-stumps. In ancient Greece the phrase "cutting off a hydra" became proverbial for tasks that are hopeless or endlessly futile.
Adapted from Wikipedia, "Lernaean Hydra" · CC BY-SA 4.0
I.Development of the Myth
The oldest Hydra narrative appears in Hesiod's Theogony, with the earliest artistic depictions on bronze fibulae dating to about 700 BCE. Both sources present the core myth: a multi-headed serpent slain by Heracles and Iolaus.
The number of heads varied across sources. Early fibulae showed six heads; Alcaeus (c. 600 BCE) fixed it at nine; Simonides increased it to fifty; Euripides, Virgil and others gave no specific number. Heraclitus the Paradoxographer rationalized the myth by suggesting it was a single-headed snake accompanied by its offspring.
The regenerative ability first appears in Euripides' accounts, where two heads grew back for each one severed. Plato's Euthydemus uses the Hydra as a metaphor for sophistic arguments. Palaephatus, Ovid and Diodorus Siculus agreed with Euripides' two-for-one regeneration; Servius described three heads regrowing; the Suda gave no number. Depictions around 500 BCE show a double tail as well as multiple heads, though no literary source confirms this feature for the tails.
The Hydra had parallels in ancient Near Eastern mythology, particularly in Sumerian, Babylonian and Assyrian traditions. The god Ninurta slew multiple monsters including a seven-headed serpent and Bashmu, whose constellation the Greeks later associated with Hydra.
II.Second Labor of Heracles
Eurystheus, king of Tiryns, commanded Heracles to slay the Hydra — a beast that Hera had raised specifically to kill him. On reaching the swamp near Lake Lerna, Heracles covered his mouth and nose to protect against the poisonous fumes. He shot flaming arrows into the Hydra's lair at the spring of Amymone, a deep cave from which it emerged only to terrorize neighbouring villages.
When Heracles attempted to decapitate the monster, each severed head regenerated into one or two new heads. Realizing that direct combat failed, he summoned his nephew Iolaus, who devised a strategy of using a firebrand to scorch the neck-stumps after each decapitation. Heracles cut off each head while Iolaus cauterized the wounds.
During the struggle, Hera sent a giant crab to distract Heracles; he crushed it under his foot. The one immortal head was severed with a golden sword given to Heracles by Athena. This head — still alive and writhing — was placed beneath a rock on the sacred way between Lerna and Elaius. Heracles then dipped his arrows in the Hydra's poisonous blood, completing his second task.
An alternate version states that Heracles dipped his sword in the severed head's neck and used the venom to burn the remaining heads, preventing them from regrowing. A furious Hera placed the Hydra in the sky as the constellation Hydra and transformed the crab into Cancer.
Heracles later used these poisoned arrows against various enemies — the Stymphalian Birds, the giant Geryon, the dragon Ladon, and the centaur Nessus. The centaur's tainted blood, applied to the Tunic of Nessus, eventually caused Heracles' own death. Strabo and Pausanias report that the river Anigrus in Elis allegedly reeked from the Hydra's poison washed from these arrows.
III.Constellation
Greek and Roman writers described how Hera placed both the Hydra and the crab as constellations in the night sky after Heracles' victory. When the sun enters Cancer, the constellation Hydra's head lies nearby.
Both constellations derive from earlier Babylonian signs: Bashmu, "The Venomous Snake," and Alluttu, "The Crayfish."