This constellation represents the Lernaean Hydra, slain by Hercules as his second labor. The Hydra was a multi-headed monster--according to Diodorus (first century B.C.), it had a hundred heads; Simonides (sixth century B.C.) said it had fifty. The most common opinion, however, seems to be that it had nine. What made the Hydra so difficult was the fact that, whenever one of its heads was chopped off, two would grow in its place. Hercules managed to get around this rather major obstacle by having his nephew, Iolaus, cauterize each stump with a hot iron as soon as Hercules could chop off a head. The hero then buried the monster's immortal head beneath a rock. The task was made somewhat more difficult by Juno, who sent a crab to nip at the feet of Hercules while he battled the Hydra.

Hydra is a long and wandering constellation, stretching almost from Canis Minor to Libra. It lies south of Cancer, Leo, and Virgo, and is best seen in the northern hemisphere during the months of February through May.