Ethnic makeup of legionaries

[Megas Methuselah]

Also, paradoxically, the auxilia had become more prestigious than the legions by that time.

That's interesting. Can you enlighten me?

[Michiel de Ruyter]

That's interesting. Can you enlighten me?

Well,

I have to slightly modify my statement. Going by the Notitua Dignitatum (a late 4th century list oof offices and military commands), the legiones palatinae (the legions in the central field armies) are formally still the most more prestigeous (followed by the auxilia palatinae and then the regional field armies) by law and formalities. Which probably has more to do with tradition and conservatism than anything else

Yet judging by numbers involved in the palatinae field-armies (that is more or less what could be considered the central reserve), and usually commanded by either the emperor or the central magister militiae the auxilia are in reality the more important and most valued. There are 65 auxilia in the West vs only 12 legions. On a similar note, when asking for reinforcements from the Caesar Julian the emperor Constantius he primarily asks for auxiliae, not legions.

What the reason was/is? Probably fighting prowess, in combination with demographic and economic problems within the empire. Even when taking the literary topoi in consideration, it seems that in general the best troops were those recruited among the Germanic peoples (Goths, Alamanni, Franks), and those fighting them. As an illustration, it seems that at Mursa (a battle in one of the civil wars) the western army was outnumbered almost 2:1 by the eastern army but alledgedly inflicted far more casualties on its opponent (IIRC something like 50% more). I am not completely certain, but in the depictions of Constantine storming the Milvian bridge on his arch, it is the auxilia which are shown as his crack troops.
Another issue might be in recruiting and enlistment conditions (i.e. stipulations where the soldiers would serve), with many Roman citizen recruited units more and more being tied to their base/local area. Another is that probably in the legions there were additional tasks not forced upon the auxilia. IIRC there are stipulations known that specifically [B]ban[B] Roman citizens from enlisting in the auxilia.