The Hydra of Lerna (in classical Greek: Ὕδρα), in Greek mythology, was a monster, the son of Typhon and Echidna, who inhabited a swamp by the lake of Lerna in Argolid, today what would be equivalent to the east coast of the Peloponnese region.
The Hydra had a dragon's body and several serpent heads. According to legend, the Hydra's heads could regenerate themselves; some versions say that when one head was cut off, two would grow in its place, but early versions of the legend did not include this feature.
The Hydra was so poisonous that it killed men with its breath alone and ate them; if someone came near it while it was sleeping, just by smelling its trail the person already died in terrible torment.
The Hydra was defeated by Heracles (Hercules, in Roman mythology), in his second job. Initially, Heracles tried to crush the heads, but with each one he cut off two appeared in its place.
He then decided to change tactics and, so that the heads would not regenerate, he asked his nephew Iolau to burn them with a firebrand right after the cut, thus healing the wound. Only the middle head then remained, which was considered immortal. Heracles cut off and buried the last head with a huge stone. Thus, the monster was killed.
According to tradition, the monster was created by Hera to kill Heracles. When she realized that Heracles was going to kill the serpent, Hera sent him the help of a huge crab, but Heracles stepped on it and the animal became the constellation of crab (or Cancer).
Instructed by Athena, Heracles, after killing the Hydra, took the opportunity to bathe his arrows in the monster's blood, to make them poisonous. Eurystheus did not consider this work valid (Heracles was supposed to fulfill ten labors, not twelve), as the hero had help.
Heracles later died in Phrygia from the Hydra's poison: after mortally wounding the centaur Nesso with arrows poisoned in the Hydra's blood, the latter gave his blood to Dejanira, saying it was a love potion, to be used on her clothes.
Dejanira believed it, and when she felt jealous of Hiole, she used Nesso's blood to bathe the clothes of Heracles, who felt unbearable burning pains, preferring suicide on the crematory pyre.
Mythographers tell that the Hydra of Lerna and the crab were placed in the sky after Heracles killed them. Hera put the crab in the Zodiac to follow the Leo, creating the constellation of Cancer. When the sun is in the sign of Cancer, as boreal summer officially begins, the constellation of Hydra has its heads close together.