I've read countless times that the triarii were used after the other units were broken in a battle because they were the most heavily armed and most veteran units in the legion and thus the most likely to route the enemy troops. What confuses me, however, is why they would not be deployed on the front lines in an attempt to break the enemy immediately, rather than used only in dire situations.
Being heavily armored, I would think that they would take less losses than their hastaati and principe counterparts, so why "sacrifice" those men?
Bonus question: I have also read that they were the "oldest and wealthiest" in the army. What would the average triarii's age be and how would their wealth compare to, say, a member of the hastaati?
Thank you!
I'll go ahead and help you out a bit here - the manipular Roman legions are a fascinating topic, and I almost wish more questions were asked about them! even if we honestly don't know nearly as much about them as we do about later legions. But let's get to the question.
First off, let's look at who the triarii were. A triarius was generally an older man - 40-60 would be a good spitball number - who had already spent quite a few years fighting. They were equipped differently than the other troops in the legion; while the hastati and the principes used a sword as their primary weapon, the triarii used spears, in an echo of the tradition of phalanx warfare. They were generally wealthier because they were older and had had more time to invest in their armour, and the average triarius usually had enough to invest in a shirt of mail at the least, if not also greaves, a good helmet, etc. In a battle, the triarii would be kept behind the lines, usually kneeling, until all other options were exhausted.
This wasn't only an exercise in Roman discipline (Not fighting while your friends and relatives were possibly dying), but it was also pretty strategically sound. For one thing, the sight of a fresh formation just waiting to attack at the opportune moment was unnerving, but that formation also gave the hastati and the principes a bastion to regroup if things went horribly wrong. Being able to regroup a routing force and have the oldest veterans talk to them was an invaluable asset.
So why wouldn't they just be on the front lines? Well, first off, the Roman ideal of winning glory in battle cannot be underestimated, and the young men certainly thirsted after it - they (and their families) demanded that they have the honour of serving on the front. Practically, this served an equal purpose: it shaped those young, untried men into hardened soldiers. If only the veterans got to see combat, Rome's armies would be completely untried within a couple of generations. And so, the young men who were eager for battle and for glory fought in Rome's front ranks.
They weren't really being sacrificed, either. One note about ancient battles is that casualties for the winning side were generally relatively low (Unless you were Pyrrhus invading Italy). For some examples of that, I've picked a few famous battles off the top of my head- and these were some of the hardest-fought battles in the ancient world: Pydna, Pharsalus, Zama, Cannae. Wikipedia's not bad for a quick eyeball of the numbers (sometimes), and if you check those numbers out, you'll note that casualties on the winning side really weren't so bad. Battle was more psychological than anything else, and the first side to run was the side that got slaughtered.
The hastati were deployed to essentially soak the enemy's vigour. When the hastati began to flag (And it took a skilled commander to pick the opportune moment, not too soon and not too late), the principes were committed as a fresh force to bolster the hastati in numbers, morale, and skill. The principes were abooooout 10 years older than the hastati, and they not only had the skill and veterancy, but they also could afford pretty good armour as well. Pair all of that with the fact that these were men in their prime (mid 20s-mid 30s) and you have the best soldiers in the Roman manipular legions - while the triarii had similar armour, their age made a difference. The principes were committed into the heat of the battle, when their impact mattered the most, which usually was the blow that won the battle for Rome.
Oh right! And regarding the wealth of a triarius compared to a hastatus? Think teenager vs. grandpa. Grandpa has had a lot more time to build up a lot more money :)