Although the primordial Cyclopes of the "Theogony" were presumably immortal (as were their brothers the Titans), the sixth-century BC Hesiodic "Catalogue of Women", has them being killed by Apollo. Later sources tell us why: Apollo's son Asclepius had been killed by Zeus' thunderbolt, and Apollo killed the Cyclopes, the makers of the thunderbolt, in revenge. According to a scholiast on Euripides' "Alcestis", the fifth-century BC mythographer Pherecydes supplied the same motive, but said that Apollo, rather than killing the Cyclopes, killed their "sons" (one of whom he named Aortes) instead. No other source mentions any offspring of the Cyclopes. A Pindar fragment suggests that Zeus himself killed the Cyclopes to prevent them from making thunderbolts for anyone else.