Before the gladius did its close work, two pila were already in the air. The Roman approach to spears and javelins was characteristically systematic: pick a small number of designs, refine them ruthlessly, manufacture them in volume, and use them in choreographed sequences that opponents could rarely match.
Pilum
The pilum was the heavy javelin every Roman legionary carried into battle — usually two of them — and it was the weapon that opened virtually every Roman fight from the late Republic through the early Empire. Roughly 6.5 feet long, it had a long, narrow iron shank tipped with a small pyramidal point, mounted on a heavy wooden shaft. The whole thing weighed two to five pounds.
Its genius was in what happened when it hit something. The point was hardened to punch through shields and armor; the long iron shank behind it was deliberately left soft. When the pilum struck an enemy shield, the point would drive through, and then the soft shank would bend under the weight of the trailing wooden shaft. The shield was now carrying a heavy, awkwardly-angled spear that could not be quickly removed. Most opponents had to drop the shield. The Romans, advancing seconds behind the volley, found their enemies suddenly unshielded.
Marius is credited with a refinement that made the pilum essentially unreusable: of the two pins fixing the iron shank to the wooden shaft, one was iron and one was wood. On impact, the wooden pin sheared, the iron shank twisted sideways, and the pilum could not be picked up and thrown back. It was a small detail. It was also typical of how the Romans thought about weapons.
Hasta
The hasta was the older Roman spear — a thrusting weapon, not a throwing one. Six to seven feet long with an iron head, it was used in close formation by the early Republican army, in a style inherited from Greek and Etruscan fighting traditions. The hasta predates the pilum by centuries and gave its name to the hastati, the front rank of the Republican legion (though by Marius's reforms the hastati had switched to the pilum).
Where the hasta survived longest was with the triarii, the third line of veteran reservists in the Republican legion. The triarii kept their hastae and fought in a tighter, more phalanx-like formation, held in reserve for emergencies. The Latin phrase ad triarios redisse — "it has come to the triarii" — was Roman shorthand for a desperate situation, the kind of fight where the front lines had failed and the old men in the back row had to settle it with spears.
The hasta survived as a ceremonial weapon long after it had been retired from front-line service. In late Republican and Imperial Rome, a hasta was carried by certain magistrates as a symbol of office, presented to soldiers as an award (the hasta pura), and used in religious rituals. The thrusting spear had been the weapon of the citizen-soldier, and even when the army left it behind, the state kept it as a symbol.