a.) for the Limes troops we know for sure that Romans were phased out and replaced by local Numeri.
b.) Romans military diploma
http://www.romancoins.info/MilitaryDiploma-3a.html
the bronze plate which is kind of contract. we find northern names for troops serving in Spain and Africa on this "documents", giving the soldier official leave from the service as legionares. other nations are known, too many and from times which few relocations of whole legions, than it could be coincidence. supposing large distance between recruiting and place of service
c.)
for the time -250/-100 a long period the original 2 legions of Rome were extended to 4 and we talk about 18.000 - 20.000 men with, as far the historians can figure it out, few fluctuations. if the legions were lost (2nd Punic war for instance) they [color="#FFFF00"]levied complete new legions instead[/color], you find sources of not yet levied legions of somewhat over 4.000 men when Hannibal was on a stroll in the park
Delbrück calculates a amount of potential recruits of 10-15.000 a year, drafting only 1.000 - 2.000 yearly as replacements. service term 16 to 20+ years
d.)
like the Usipetes i.e. had to deploy a whole COHORT on the order of Emp. Domitian, Roman-Chattian War
Eadem aestate cohors Usiporum per Germanias conscripta
Aurelian levied:
ala VIII Vandilorum
ala I Quadorum
ala I Francorum
cohors IX Alamanorum
cohors XI Chamavorum
cohors VII Francorum
many of this men were integrated in legions partially while the original troops were eventually filled with local replacements. as later in the empire, as more frequent the AUX were merged with legions cohorts.
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and so on.
additionally many LEGIONS troops were replaced by allieds, while the legions refilled legions, as he said, by sending detachments to create forces elsewhere
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also for AUX
i.e.
Cohors II Gallorum veterana equitata existed for far over 200 years, the son of an AUX member became citizen (earliest situation)
non concluding list of AUX with durations often far longer than service term
http://en.wikipedia.org/wiki/List_of_Roman_auxiliary_regiments
see:
According to Holder, during the Julio-Claudian era (14-68), a regiment's ethnic identity was preserved to some extent, with evidence of continued recruitment from the original people. By the time of Hadrian, however, a regiment's name, in most cases, probably represented the ethnic origin of few, if any, of its members. This is because in the Flavian era (69-96), as a matter of deliberate policy, most regiments were deployed in provinces far from their original home and drew the majority of their recruits from local natives and the rest from all parts of the empire. In most cases, therefore, a regiment's name had become an identification tag devoid of ethnic significance. A regiment deployed long-term in the same province would thus, over time, acquire the ethnic character of its host population.
There are exceptions to this rule:
A minority of regiments remained stationed in their original home province, e.g., cohors I Delmatarum mill eq, still attested in Dalmatia in 130.
Regiments founded a relatively short period before 130, e.g., the cohors I Aelia Dacorum stationed in Britain in 130 would probably still have contained mostly Dacian recruits at this time, as it had been established by Hadrian only about a decade earlier.
Some specialised regiments (e.g., Syrian archers) and the elite Batavi show some evidence of continued preferential recruitment from their original province.
Holder, Paul: Auxiliary Deployment in the Reign of Hadrian (2003)
Spaul, John: COHORS 2 (2000)
Spaul, John: ALA (1994)
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you find even evidence that legions cohorts coming from ad hoc assemblies of men or recruited during winter quarters / in local castrums were NOT granted citizenship under some emperors
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PS:
sorry i dont have the time at the moment to fit it in a conclusive text.
if i am messing up legion/AUX names, it comes from not checked copy and paste from different databases, not having my original sources at hand
...not paid by AGEOD.
however, prone to throw them into disarray.
PS:
‘Everything is very simple in War, but the simplest thing is difficult. These difficulties accumulate and produce a friction which no man can imagine exactly who has not seen War . . . in War, through the influence of an infinity of petty circumstances, which cannot properly be described on paper, things disappoint us, and we fall short of the mark.‘
Clausewitz