Archaeologists Find Athenian Naval Base

[Anomalocaris]

2nd Punic war didn't deplete Rome's eligible manpower.  Rome continued to be able to field fully manned militia army for wars on multiple fronts through the 3rd Punic war.   Later social and economic factors alluded to earlier depleted the ranks of yoeman farmers, and also made those who remained unwilling to serve.  The transition happened way before Caesar, under Marius at around 110BC.  

The Roman army had always consisted of "everyday folk".  Prior to Marius, the army consisted of Yoeman farmer citizens who had some small property, and the ability to purchase and hand down through the family a set of soldier's equipments.  After Marius the army increasing consisted of the dispossessed citizens who can't pay for their own equipment and relied on the state, and their generals, for basic equipment kit.

Even during caesar's wars, not everyone could be in the legionary army.  One has to be a Roman citizen.  That usually means one had to have been either a true Roman, or from an Italian city long allied with Rome and whose citizens were granted Roman citizenship at the end of the social wars.  The phenomenon of a permenant auxiliary army that consisted of non-citizens didn't really arise until after the start of the empire.

Later Roman legionary army didn't really become open to everyone until around AD210, when serverus granted Roman citizenship to all freemen in the empire.

Well there were other battles and Cannae and Trasimene happened over a fortnight.

The key here is Citizen. One had to be a citizen to join up. Buy their own gear. Farmer or politician.

I think the Roman , say at the time of Diocletian, was quite different to those who fought under both Pliny's. One successful, the other not.

Indeed, we have biological evidence centurions at a later date came from all over the empire. People Africanus would consider barbarians. 

I do of course also agree with you. Just adding to it, if you will.

I am not sure if Africans would have considered all of them barbarians.  More likely he would have considered many of them non-Roman allies.  There were 500 years between Africanus and Diocletian, as long as between Renaissance and now.   During this time roman concept of what is Roman had under undergone expansion commensurate with changing reality appropriate to the passage of the amount of time.   Not only did Rome expand to rule the territory of almost all of rome's former allies, but also expanded the citizenship to cover all freeman in the territory it ruled.  So the core of Roman army remained an army of Roman citizens, but the citizenship expanded to all the provinces outside Italy.   The fundamental structure of the army also changed.  No longer was Roman army primarily a heavy infantry force as it had been since the beginning of the republic.  It is now primarily a heavy cavalry force backed by medium and light infantry.  Infantry force can be raised from any body of fit young men with little to lose. Cavalry forces are much harder to recruit and train from the pool of truly dispossessed who may have never ridden a horse in their lives.  So substantial use had to be made of people who had some horse rearing tradition.