The Republican Period
The Roman army comprised different types of forces - the armoured infantry were Roman citizens. They were supported by auxiliaries who were recruited from Latin allies to provide the specialised support troops - light infantry, archers, cavalry. The Roman farmer made a good heavy infantryman, but these other necessary arms were needed to provide a balanced force, and it was easier to call out allies or hire them in already trained than to try to train their own. So the auxiliary became an essential and integral part of the Roman army.
After the Second Punic War, the supply of farmers was reduced and progressively too few to meet the demands of the expanding empire, even using the infantry of the Italian allies, and with the threat of the Germanic invasion in the late 2nd Century BCE and initial heavy loss of traditional infantry, the infantry recruitment was expanded to the non-propertied class of Romans and Italians. And as the demand for auxiliaries also increased proportionately, the auxiliaries were recruited from further afield and from much more specialised providers than Italy could provide, eg slingers from the Balearic Islands, archers from Crete, and Spanish, Gallic, Numidian, German cavalry.
Imperial Period Auxilia
Roman auxiliaries were soldiers of the Roman army.They weren't citizens .
At one time, the auxilia were thought to be inferior to the Legions, second class soldiers, but this is far from the truth. The auxilia were very often in the thick of the fighting during the expansion of the Empire and when there was no battle, the auxilia manned the outposts of the Empire, and policed the provinces, keeping law and order, and defending the borders. After serving for 25 years, with good conduct, the average soldier received his Roman Citizenship, which would make life easier for him, his family and his descendants. It is highly likely that many Legionaries were sons of auxiliaries, for example.
The auxilia also provided almost all of the cavalry and (argueably) the archers for the Roman army. There were three types of Auxiliary unit, these being a Cohors, (infantry), Ala (cavalry), and Cohors equitatus, which was a mixed unit of infantry and cavalry. A typical Cohors WA a heavy infantry unit and would be lead by a Roman officer, and its number would most likely include Legionaries serving as instructors, sub-unit commanders, or on the promotion ladder, (going both up and down!), and like a Legion, a Cohors included its own artillerymen, engineers, and pioneers.
Apart from not being Citizens, the auxilia were equipped slightly differently than the Legions. They were normally equipped with oval shields, thrusting spears, and a sword and dagger. Apart from this, the same body armour was available to the auxilia. So far, three different types have been discovered, these being 1: cuirasses made from tens of thousands of interlocking iron rings (Hamata), 2: cuirasses made from thousands of overlapping iron or bronze scales (Squamata), and 3: cuirasses made from overlapping horizontal iron hoops (Segmentata). A padded garment called a Subarmalis would be worn under armour. Helmets made from either bronze or iron were also worn.
Soldiers had to pay for their own equipment, so the re-use of equipment is almost a certainty. Helmets have been found to have three or four names inscribed in them, for example.
