Idas belonged to the royal house of Messenia. His father was Aphareus, son of Perieres and Gorgophone, daughter of Perseus. His mother was Arene, daughter of Oebalus and Gorgophone, making the marriage a union within the same extended family. A third brother, Pisus or Peisus, appears in some genealogies, but the operative pair is Idas and Lynceus. Some traditions even make Poseidon the father of Idas, although Aphareus remains the standard parent. The brothers grew up in Arene, a city Aphareus named after his wife and ruled beside his brother Leucippus.

Idas and Lynceus form a natural heroic pair. Idas supplies strength and aggression, while later traditions give Lynceus extraordinary sight, even the ability to see through obstacles or over great distances. The Apharetidae can therefore be set against their cousins Castor and Pollux as another famous brother-pair. Modern interpretations sometimes read the feud through regional Messenian and Spartan rivalry, but that political reading is an interpretation of the myth rather than an explicit ancient explanation.

Idas and Lynceus joined the Argonauts. Apollonius of Rhodes lists Idas first among the Messenian contingent, proud of his strength and paired inseparably with Lynceus. During the voyage Idas displayed both his courage and his dangerous irreverence. When the seer Idmon predicted that the expedition would succeed despite his own death, Idas mocked prophecy and boasted that he trusted in his spear rather than in omens. After drinking, he threatened the prophet and nearly brought disorder upon the crew until Polydeuces, the Dioscurus and skilled boxer, calmed the quarrel. The scene shows Idas as the Argonaut most willing to challenge authority, divine signs and social order at once.

His strength was real. At Cyzicus, where the Argonauts fought the Doliones in a night-time misunderstanding, Idas killed Promeus in the battle. His impulsiveness was also dangerous, but the voyage could not do without him. The myth keeps him close to Polydeuces, the future rival who would one day fight him to the death.

Idas' most famous personal adventure was his contest with Apollo for Marpessa. Marpessa, daughter of the Aetolian king Evenus and sometimes said to be loved by Apollo herself, was guarded by her father against suitors. Evenus challenged each suitor to a chariot race and killed the losers, fixing their skulls to his palace as a warning. Idas did not submit to the race. Poseidon gave him a winged chariot, and Idas seized Marpessa and drove away faster than any earthly pursuit. Evenus chased him to the River Lycormas, sacrificed his horses in despair and drowned himself. The river thereafter bore his name, the Evenus.

Apollo also desired Marpessa. In one tradition the god carried her away, and Idas pursued him with bow drawn. In another Apollo simply confronted Idas over the woman he had abducted. Either way, Idas aimed his weapon at an Olympian. Zeus separated the combatants before the contest could be decided and granted Marpessa the choice. She chose Idas. Her reason, preserved by Homer through Phoenix's speech to Achilles, was that she feared the immortal Apollo would abandon her when she grew old. A mortal husband would age beside her. The choice made Idas the winner of a contest no mortal should have survived and gave the pair a daughter, Cleopatra, also called Alcyone, who later married Meleager.

Homer's mention is brief but powerful. In the story of Meleager's anger, Phoenix recalls that Marpessa bore Cleopatra to Idas, the strongest of men then on earth. Cleopatra married Meleager, and her mother's history of contested beauty and divine rivalry prefigures the domestic tragedy of the Calydonian War. Idas' marriage thus entered one of the Iliad's most important embedded narratives.

The feud with the Dioscuri belonged to the same kinship network. Castor and Pollux were sons of Leda and either Tyndareus or Zeus. Their father Tyndareus was the brother of Aphareus, making the four cousins. In one tradition the Dioscuri carried off Phoebe and Hilaeira, the daughters of Leucippus, who were betrothed to Idas and Lynceus. The abduction of brides gave the feud a sexual and dynastic cause. In another and possibly older tradition, the immediate cause was cattle.

Pindar gives the spare epic version. Idas was angered over cattle and stabbed Castor with his bronze spear. Lynceus looked down from Mount Taygetus and saw the Dioscuri seated inside a hollow oak. The sight belonged to his supernatural vision: the tree did not hide Castor from him. Idas and Lynceus rushed down, Idas struck through the oak and killed the mortal twin. Zeus then launched his consuming bolt against Idas, and Pollux pursued and killed Lynceus. The brothers burned together on a single pyre. Pollux asked Zeus to share his immortality with Castor, and the twins thereafter divided their days between Olympus and Therapne.

Apollodorus supplies the cattle story in comic detail. Idas, Lynceus, Castor and Pollux together stole a herd from Arcadia. Idas was chosen to divide the spoils. He slaughtered a bull, quartered it and proposed that the one who ate first should take half the herd, the one who ate second the other half. Before the Dioscuri understood the arrangement, Idas devoured his own portion and then Lynceus' portion as well. He claimed the entire herd and drove it to Messenia. The Dioscuri later raided the cattle back, taking additional plunder, and hid in a hollow oak to ambush their cousins.

Lynceus saw them from Taygetus. Idas drove his spear through the oak and killed Castor. Pollux pursued. Near the tomb of Aphareus, Idas and Lynceus tore up the grave-column, the carved monument to Hades, and hurled it at Pollux. The immortal son of Zeus was not stopped. He killed Lynceus, and Zeus struck Idas with a thunderbolt. Some accounts say the two brothers burned together. Others end Idas alone beneath the lightning.

Theocritus gives a duel-focused variant. In Idyll Twenty-Two, Castor and Lynceus confront one another after the abduction of the Leucippides. Their spears break, their swords fail and the fight becomes hand to hand. Castor severs the fingers of Lynceus' shield hand, pursues him to the tomb of Aphareus and kills him. Idas then tries to crush Castor with the tombstone, but Zeus hurls his bolt. The essential ending remains constant: Lynceus dies, Idas dies by divine lightning, Castor is mortally wounded, and only Pollux survives.

The battle transformed the Dioscuri. Pollux, immortal because Zeus was his father, begged Zeus to let him share his deathlessness with Castor. Zeus granted the request, and the twins thereafter alternated between Olympus and the grave at Therapne. Homer already gives them this divided fate in the Odyssey, honoured like gods and living one day among the immortals, one day beneath the earth. Pindar makes the choice the emotional climax of the story. Idas' spear created the crisis that produced the Dioscuri's immortal brotherhood.

Because Idas and Lynceus are Messenian heroes while Castor and Pollux are strongly associated with Sparta, modern scholars have sometimes read the feud as carrying memories of rivalry between Messenia and Sparta. The ancient sources themselves do not explain the battle as an allegory of historical conquest, and different poets assign blame differently. The regional interpretation is therefore useful context, not a fact contained within the myth.

Ancient monuments preserved the brothers. Pausanias says the grave of Idas and Lynceus was shown at Sparta but expresses his own belief that they were buried in Messenia. The Chest of Cypselos at Olympia depicted Marpessa following Idas from the temple of Apollo, preserving an early visual version of the contest with the god. The Sicyonian Treasury at Delphi carried a metope showing the Dioscuri, Idas and Lynceus with cattle, linking the feud to its economic cause. Polygnotus painted the abduction of the Leucippides, and vase painters returned to Marpessa, the Argonautic voyage and the cattle raid.

Idas' character is defined by excess. He eats too fast, boasts too loudly, threatens prophets, abducts women, cheats cousins and raises his weapon against a god. Yet the same excess makes him compelling. Greek myth often punishes mortals who forget the boundary between human and divine, but Idas does not forget it. He challenges it. His strength approaches the heroic ideal; his refusal to recognise limits destroys him.

His marriage to Marpessa provides the counterweight. In that story his boldness wins a genuinely reciprocal love. Marpessa chooses him not because he is divine but because he will grow old with her. The mortal marriage gives Idas his most human moment and links him to Meleager, whose wife Cleopatra inherits the same family. His daughter's name, meaning glory of the father or famous ancestry, extends Idas' glory beyond his own death.

The name Idas also belongs to a minor Trojan warrior killed by Achilles in the Iliad, son of Alastor. That Idas is not the Messenian hero and should not be confused with him. The Apharetid Idas is defined by his brother, his wife, his feud and his thunderbolt death. The Trojan Idas appears only to fall.

Idas endures because his story contains two opposed myths in one life. In the Marpessa episode he is the successful mortal whose love defeats divine desire. In the Dioscuri feud he is the overreacher whose strength dies beneath Zeus' lightning. He is both husband and cattle thief, both Argonaut and prophet-mocker, both brother and avenger. The two halves cannot be separated without losing the figure.

His final image is compact and powerful: Lynceus sees through the oak, Idas strikes Castor, the tomb-stone flies at Pollux, Zeus answers with thunder. The gods protect their son, the cousins burn and the Spartan twins become the constellation Gemini. Idas' spear thus creates both the tragedy of Castor and the apotheosis of the Dioscuri. He is the necessary enemy whose destruction turns two Spartan brothers into gods.