alternation between senior civilian and military posts fell into disuse in the 2nd and 3rd centuries, as the Italian hereditary aristocracy was progressively replaced in the senior echelons of the army by the primipilares (former chief centurions). In the 3rd century, only 10% of auxiliary prefects whose origins are known were Italian equestrians, compared to the majority in the previous two centuries. At the same time, equestrians increasingly replaced the senatorial order in the top commands. Septimius Severus (193–211) placed equestrian primipilares in command of the three new legions he raised and Gallienus (260–268) did the same for all the other legions, giving them the title praefectus pro legato ("prefect acting as legate"). The rise of the primipilares may have provided the army with more professional leadership, but it increased military rebellions by ambitious generals. The 3rd century saw numerous coup d'état and civil wars. Few 3rd-century emperors enjoyed long reigns or died of natural causes. Emperors responded to the increased insecurity with a steady build-up of the forces at their immediate disposal. These became known as the comitatus ("escort", from which derives the English word "committee"). To the Praetorian Guard's 10,000 men, Septimius Severus added the legion II Parthica. Based at Albano Laziale near Rome, it was the first legion to be stationed in Italy since Augustus. He doubled the size of the imperial escort cavalry, the equites singulares Augusti, to 2,000 by drawing select detachments from alae on the borders. His comitatus thus numbered some 17,000 men, equivalent to 31 infantry cohortes and 11 alae of cavalry. The trend for the emperor to gather round his person ever greater forces reached its peak in the 4th century under Constantine I the Great (306–337) whose comitatus may have reached 100,000 men, perhaps a quarter of the army's total effective strength. The rule of Gallienus saw the appointment of a senior officer, with the title of dux (plural form: duces, the origin of the medieval noble rank of duke), to command all the comitatus cavalry. This force included equites promoti (cavalry contingents detached from the legions), plus Illyrian light cavalry (equites Dalmatarum) and allied barbarian cavalry (equites foederati). Under Constantine I, the head of the comitatus cavalry was given the title of magister equitum ("master of horse"), which in Republican times had been held by the deputy to a Roman dictator. But neither title implies the existence of an independent