bobm wrote:The Hellenistic world more clearly identifies three categories of foot;
phalanx
peltast (then theurophorae)
skirmishers
The Romans don't fit into the system; Legionaries are heavier than peltasts but lighter than phalanx. Auxilliaries are "jack of all trades", equipped cheaper than the Legions and not trained to hurl pila then get stuck in with sword but otherwise still as "heavy".
I don't agree with that description of Romans.
Early on, legionaries were members of the Roman state or "Allies" obliged to serve for a normally short time, i.e. conscripts trained to fight in various roles (which would include velites as well as the other graades with probably more armour and normally a bigger shield than phalangites). Auxilia were other nationality troops, e.g. mercenaries, using native methods and organisation, which could be heavy or light.
Later on (from Marius), legionaries became full-time professionals and heavily armed and protected.
Later still, Auxilia were part of the organic Roman army structure, although it seems that some units of particular ethnic origin probably retained specialist capabilities from their pre-Roman life. It also seems that legionary troops did a lot of engineering and "life support" projects. I get the impression (from my fairly superficial knowledge) that legions were engineers first, but also very good in a simple line up and fight situation, auxilia were more the professional infantry/rangers/special forces.
Very late, it is not clear what the difference was, except that some Auxilia units seem to have been 500 men instead of the supposedly 1000 man legion, but some were 1000. However, it is clear that at least some Auxilia were capable of holding their own as front line units in battle. Possibly Auxilia was just a name in the same way that we now have "dragoons", "hussars" and "lancers", not indicating battlefield role at all.