The details of the struggle are explicit in the Bibliotheca (2.5. 2): realizing that he could not defeat the Hydra in this way, Hercules called on his nephew Iolaus for help. His nephew then came upon the idea (possibly inspired by Athena) of using a firebrand to scorch the neck stumps after each decapitation. Hercules cut off each head and Iolaus cauterized the open stumps. Seeing that Hercules was winning the struggle, Hera sent a giant crab to distract him. He crushed it under his mighty foot. He cut off the Hydra 's one mortal head with a golden sword given to him by Athena. Hercules placed it under a great rock on the sacred way between Lerna and Elaius (Kerenyi 1959: 144), and dipped his arrows in the Hydra 's poisonous blood, and so his second task was complete. The alternative version of this myth is that after cutting off one head he then dipped his sword in it and used its venom to burn each head so it could n't grow back. Hera, upset that Hercules had slain the beast she raised to kill him, placed it in the dark blue vault of the sky as the constellation Hydra. She then turned the crab into the constellation Cancer.
who aided heracles in his second labor by cauterizing
Iolaus Iolaus