The legend of Cancer is related to a relatively minor chapter in Greek Mythology and is associated with the one of the legendary Twelve Labors of Heracles (known to the Romans as Hercules). As his Second Labor, Heracles was sent to do battle with the fearsome, nine-headed serpent monster known as the Hydra which lived in the marshes of Lerna. During the fight, all the animals favored the hero but a giant Crab sent by the Goddess Hera (Queen of Olympus and the one who had dispatched Heracles to perform his twelve tasks, hoping he would not survive them) emerged from a cave and took a claw-hold upon the toe of the hero, thereby drawing his attention away from the Hydra. Due to this distraction, Heracles very nearly lost the battle but, after crushing the life from the unfortunate Crab, returned to the fight and eventually defeated the serpent. Impressed by its loyalty and courage, Hera (who frequently encouraged Heracles' failure in his appointed labors) placed the image of the Crab in the night sky. Since the Hydra had been a favorite of Hera, it was equally honored upon its death and also placed in the heavens, not far from the location of Cancer.
Among the stars of Cancer is a small grouping known as the Asses, which appear on the back of the Crab's shell. According to legend, Hera had driven the God Dionysus insane and he roamed through Egypt and Syria in the midst of his madness. He attempted to recover his sanity by visiting Dodona in order to ask the Oracle of
Zeus what he should do. As he came to a large swamp, he encountered two Asses, captured them and rode them over the swamp, thereby avoiding geting wet. Upon reaching the other side, Dionysus realized that he was cured and rewarded the Asses by placing them in the heavens. Another explanation for the Asses grouping of stars in the constellation of Cancer states that during the war between the Olympians and the Titans, Dionysus, Hephaestus and the Satyrs rode into battle upon Asses. Their braying and the confusion they caused made the Giants flee and for these reasons, they were given a place in the heavens on the Western side of Cancer.
In Malaysian myth, Cancer was the "First and Only Crab" which existed long ago. This huge crustacean lived in a deep hole in the sea and was so large that her comings and going caused the ocean tides.
希腊神话里的一段故事正强调了巨蟹座特有的攻击性。 这先得从赫五力说起赫五力是宙斯与凡人生的儿子，天后希拉三番两次要置他於死：他也是希腊最伟大的英雄，世间最壮的人。世上没有他办不到的事，连神明们都是靠着他的协助才征服了巨人人族，当然，赫五力也为神明立下许多汗马功劳。有一天他来到了麦西尼王国，正准备接受英雄式的欢迎，国王却因受到希拉的指使，给他出一道难题杀掉住在沼泽区的九头蛇，这事很难办，因为每砍掉一个头便会马上生出无数个头。 赫五力想到一个办法用火烧焦蛇头，就这样轻易解决了八个蛇头。眼看只剩最后一个了，希拉在天上气得怒火中烧，"难道这次又失败了？"她不甘心啊！於是从海里叫来一支巨大的螃蟹要阻碍赫五力，巨蟹伸出了强有力的双蟹夹住赫五力的脚，但是谁都知道，赫五力是世间最壮的人啊！这支巨蟹最后仍死於他的蛮力之下。 希拉又失败了，但对巨蟹不顾一切的牺牲，却感到心有戚戚，为了感佩巨蟹的忠於使命，即使没有成功，希拉仍将它放置在天上，也就成了巨蟹座。