were never grouped into units of similar size to the legions. The infantry were formed in cohorts and the cavalry into similarly sized alae
. Each cohort or ala was independent, with its own commander, an equestrian usually holding the rank of prefect. A number of auxiliary units were attached to a legion, and prolonged service together raised the efficiency of such forces, but there was no standard complement of auxilia
permanently supporting every legion. The smaller size of auxiliary units made it much easier to shift them from one area or province to another. The mixed cohorts (cohortes equitatae
), which included both foot and horse in a ratio of about four to one, were especially suited to garrison and local policing activities. The auxilia
provided a more flexible and cheaper supplement to legionary numbers. They also supplied the army with some troop types in which the legions were especially deficient, in particular supplying large numbers of very good cavalry. Auxiliary infantry also included units of archers and slingers, but the traditional view that that auxiliary foot were lighter equipped and fought in looser order than the legions is mistaken. The typical auxiliary infantryman wore scale or bronze scale or mail armor of similar weight to the legionary cuirass and a bronze helmet, carried a flat, oval shield and was armed with a gladius and a javelin or spear. This is not the equipment of a nimble skirmisher. There may have been a few cohorts with lighter equipment who fought as skirmishers, but we have no direct evidence for this. The vast majority of auxiliary cohorts fought in close order in a way not markedly different from legionaries.