Like the initial number of heads, the monster's capacity to regenerate lost heads varies with time and author. The first mention of this ability of the Hydra occurs with Euripides, where the monster grew back a pair of heads for each one severed by Heracles. In the "Euthydemus" of Plato, Socrates likens Euthydemus and his brother Dionysidorus to a Hydra of a sophistical nature who grows two arguments for every one refuted. Palaephatus, Ovid, and Diodorus Siculus concur with Euripides, while Servius has the Hydra grow back three heads each time; the "Suda" does not give a number. Depictions of the monster dating to c. 500 BC show it with a double tail as well as multiple heads, suggesting the same regenerative ability at work, but no literary accounts have this feature.