Velites did not form their own units; a number of them were attached to each maniple of hastati, principes and triarii. They were typically used as a screening force, driving off enemy skirmishers and disrupting enemy formations with javelin fire before retiring behind the lines to allow the heavier armed hastati to attack. They were normally the ones who engaged war elephants and chariots if they were present on the field; their high mobility and ranged weaponry made them much more effective against these enemies than heavy infantry. An early Roman legion contained approximately 1,000 velites. Velites were eventually done away with after the Marian reforms.
In the legion, the 1200 velites did not form their own units; 40 of them were attached to each maniple of hastati, principes and triarii. They usually formed up at the front of the legion before battle to harass the enemy with javelin fire and to prevent the enemy doing the same before retiring behind the lines to allow the heavier infantry to attack. In a pitched battle, the velites would form up at the front of the legion and harass the enemy with javelin fire and cover the advance of the hastati, who were armed with swords, and were the first line of attack. If the hastati failed to break the enemy, they would fall back and let the principes, similarly equipped though more experienced infantry, take over. If the principes failed, they would retire behind the triarii, heavily armoured, spear armed legionaries and let them carry on.
Velites were first used at the siege of Capua in 211 BC, and were made up of citizens who would normally be too poor to join the legions but where called up due a shortage of manpower. They were trained to ride on horseback with the Equites and jump down at a given signal to fling javelins at the enemy. After the siege, they were adopted into the legions as a force of irregular light infantry for ambushing and harassing the enemy with javelins before the battle began in earnest.
With the formal military reforms of Gaius Marius in 107 BC, designed to combat a shortage of manpower due to wars against Jugurtha, the different classes of units were done away with entirely. The wealth and age requirements were scrapped. Now soldiers would join as a career, rather than as service to the city, and would all be equipped as medium infantry with the same, state purchased equipment. Auxilliae, local irregular troops would now be used to fulfill other roles such as archery, skirmishing and flanking.