According to the most accepted legend, the constellation is related to the deeds of Heracles - Hercules for the Latin - i.e., the most popular hero of Greek-Roman mythology. Its name probably derives from Sarkinoς - in greek "crab" - that lived in the swamp of Lerna, where Heracles fought against the terrible Hydra. Zeus, in fact, fell in love with Alcmene, wife of Amphitryon, king of Tire, and made love to her while her husband was on his way back from war. To protract their intercourse, he ordered the sun not rise for three days, and throughout that time he told Alcmene his supposed adventures of war. When Amphitryon returned, he was disappointed to be greeted with less transport than he might have expected; nevertheless he made love to his wife who gave birth to two sons: Heracles by Zeus, and Iphicles by Amphitryon.
Hera, furious at this umpteenth affair of her divine husband, subjected the infant to any sort of abuse. A revenge of hers, in his adulthood, consisted in the assignment of twelve labors which, if accomplished successfully, would have gained him immortality from Zeus. One of the labours required the killing of the Hydra of Lerna, i.e., a snake with very many heads - according to some authors no less than a hundred - that had the property to regrow immediately after amputation. During the fight, Hera sent a huge crab to the aid of the Hydra, which in fact bit the hero to the heel, but was crushed. Grateful all the same, Hera transformed the crab into a constellation.
The constellation, devoid of bright stars, is not very visible. It extends over 500 sq. degrees and borders on the Lynx to the north, on the Lion to the east, on the Hydra to the south and on the Twins in the west. It is visible in October, before dawn.
A different version of the myth is connected to the constellation Hydra.