founder–emperor Augustus (ruled 30 BC – 14 AD), the legions, c. 5,000-strong all-heavy infantry formations recruited from Roman citizens only, were transformed from a mixed conscript and volunteer corps serving an average of 10 years, to all-volunteer units of long-term professionals serving a standard 25-year term (conscription was only decreed in emergencies). In the later 1st century, the size of a legion's First Cohort was doubled, increasing legionary personnel to c. 5,500. Alongside the legions, Augustus established the auxilia, a regular corps of similar numbers to the legions, recruited from the peregrini (non-citizen inhabitants of the empire – about 90% of the empire's population in the 1st century). As well as comprising large numbers of extra heavy infantry equipped in a similar manner to legionaries, the auxilia provided virtually all the army's cavalry (heavy and light), light infantry, archers and other specialists. The auxilia were organised in c. 500-strong units called cohortes (all-infantry), alae (all-cavalry) and cohortes equitatae (infantry with a cavalry contingent attached). Around 80 AD, a minority of auxiliary regiments were doubled in size. Until about 68 AD, the auxilia were recruited by a mix of conscription and voluntary enlistment. After that time, the auxilia became largely a volunteer corps, with conscription resorted to only in emergencies. Auxiliaries were required to serve a minimum of 25 years, although many served for longer periods. On completion of their minimum term, auxiliaries were awarded Roman citizenship, which carried important legal, fiscal and social advantages. Alongside the regular forces, the army of the Principate employed allied native units (called numeri) from outside the empire on a mercenary basis. These were led by their own aristocrats and equipped in traditional fashion. Numbers fluctuated according to circumstances and are largely unknown. As all-citizen formations, and symbolic guarantors of the dominance of the Italian hegemony, legions enjoyed greater social prestige than the auxilia. This was reflected in better pay and benefits. In addition, legionaries were equipped with more expensive and protective armour than auxiliaries. However, in 212, the emperor Caracalla granted Roman citizenship to all the empire's inhabitants. At this point, the distinction between legions and auxilia became moot, the latter becoming all-citizen units also. The change was reflected in the disappearance, during the 3rd century, of legionaries' special equipment, and the progressive break-up of legions into cohort-sized units like the auxilia. By the end of Augustus' reign, the imperial army numbered some 250,000