A legion at this time comprised perhaps 5000 men, organised into maniples (units of 120 men, all of a single class or troop type.)
Velites
The youngest, poorest troops were the velites, equipped with a buckler (small shield), gladius (short sword) and several pila (javelins). Their task was to screen the main body of the legion and shower the enemy with javelins, disrupting enemy infantry or preventing enemy light troops from doing the same to the Roman force. Velites wore distinctive headgear fashioned from wolf skin.
Hastati
Hastati were also young, relatively poor men. They occupied the front line of battle and wore a small bronze cuirass, greaves (if they could afford them), bronze helmets and a scutum ( large oval legionary shield.) They originally carried thrusting spears ( hasta) but these were eventually replaced by two pila and a gladius.
Principes
Behind the hastati came the principes. Principes were older and more experienced and their equipment, although basically similar to the hastati, was generally of better quality as principes could afford better weapons and armour.
Triarii
The triarii were the final line of defence in an early republican legion and occupied the rear. Their equipment was the same as the hastati and principes but they carried a long thrusting spear instead of pila. Triarii were veterans and their position at the rear encouraged and steadied the younger troops in front.
Battle tactics
The basic order of battle was for the velites to harass an oncoming foe with javelins before retiring through the lines and letting the enemy spend themselves on the ranks of the hastati. If the hastati failed to break the enemy, they would retire to the rear and let the principes commence their assault. If that failed, the triarii would join the fray, either in a last ditch defensive action or to fight a rearguard action to cover the armies retreat.
This battle order was characterised by its fluidity in an age when most armies still relied heavily on phalanx-style tactics inherited from the age of Alexander and his successors. Historians debate as to whether the roman system was responsible for the decline in phalanx tactics - showers of javelins weakened and encumbered a phalanx and roman legionaries were trained swordsmen. Most phalangites carried relatively small round shields and in close quarters the legionaries large scutum and expert use of the gladius generally ensured that they got the better of their opponents.
However, it has also been observed that most phalangites were of an inferior quality to the soldiers of Alexanders day and the decline in troop quality and training would undoubtedly have played a large role in the undermining of the phalanx. Despite this, the issue was often in doubt, as Rome suffered many defeats or, at the very least, costly and dearly bought victories at the hands of phalanx-based armies.
Warfare in the Classical World An Illustrated Encyclopedia of Weapons, Warriors and Warfare in the Ancient Civilisations of Greece and Rome
John Warry
University of Oklahoma Press, 1995
Structural History of the Roman Military - Wikipedia, the free encyclopedia
