The Legionaries of the Republic
They were skirmishers, light-armed soldiers who preceded the other ranks of the legion.
They were equipped with two or more short and light javelins they threw to the enemy, in the attempt to disrupt his formation before the attack of the legionaries. Their distinctive feature was a wolf skin they wore on their heads. They were from the poorest class of Rome's society and were the youngest as well.
They were the first of the legion's three lines. Equipped with pilum, Montefortino-type helmet, oval shield and spanish-type sword, they owe their name to the hastae (spears) they carried in origin. They were the first to be engaged in the fight, after having thrown their pila (heavy javelins) . They were from the lower classes of census and were amongst the youngest citizens, thus fit for first line fight.
Principes were the second line of the army, behind the hastati and before the Triarii. They carried a pilum, like the Hastati, and were equipped with a full panoply, with a c lorica hamata (chainmail) or a cardiophilax (brsastpalte), depending on the level of census.
They were the third and last line of the armi, thus their name (tria=three) and they had defensive pourposes; if the attack of the Hastati and Principi had failed, they would have taken up a defensive formation protecting the re-organization of the retreating lines. They were veterans, equipped with a full panoply and a lond hasta (spear) they used to withstand the impact of the enemy.
The Manipular Army was based on census, that is on roman citizens' wealth. Basically the Legions were the expression of that period's roman society in arms, which was divided into five "classes" of census. Every roman citizen had to provide for his weaponry and military equipment at his own expenses: the wealthiest made up the mounted and heavy infantry units, down to the poorest in the lower classes, who fought as light soldiers.