The Greeks took their myths into the heavens by identifying the constellations with their gods and heroes, and in this case, the crab is associated with the famous Greek hero, Hercules, and the story of the second of his Twelve Labors.

Zodiac Origins

Traditionally Cancer is the Zodiacal constellation between Leo and Gemini and the 4th sign of the Zodiac. The sign of The Crab, with its hard outer shell and soft interior, seems an appropriate metaphor for the hard 'outer shell' and the sensitive inner life of the classic Cancerian personality.

The Greeks took their myths into the heavens by identifying the constellations with their gods and heroes, and in this case, the crab is associated with the famous Greek hero, Hercules, and the story of the second of his Twelve Labors. To atone for the killing of his children, he was sent to slaughter the Hydra, a nine-headed monster. But each time he severed a head, two more grew in its place, until his servant, Iolaus, cauterized the stumps with firebrands. The goddess Hera, who had a grudge against Hercules, sent a giant crab to attack him while he fought the Hydra, but he crushed its shell and finally overcame the Hydra.