When Zeus seduced Alcmene by taking the form of her husband Amphityron, the great hero Hercules was born. In anger at yet another of Zeus’ indiscretions, Hera, his wife, cast a fit of madness over Hercules, which caused him to murder all of his children. To make retribution for his crime, Hercules was given twelve tasks to undertake. The second of these was the slaying of the Hydra, a monstrous sea-serpent. Whenever one of its nine heads was cut off, two more would take its place. To further frustrate the efforts of Hercules, Hera sent the Crab to nib at his heels, but the hero crushed it underfoot and with the help of his friend Ioalus managed to overcome his adversary.

In gratitude for its loyalty and self-sacrifice, Hera placed the Crab into the heavens where it eternally remains.

Walter Stevens-Hofer: Myth of the Zodiac