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An aquilifer was a senior signifer bearing the eagle standard of a Roman legion. The name derives from the type of standard, aquila meaning "eagle" (which was the universal type used since 104 BC), and fers, related to the Latin word for bringing or carrying. Before that time, the wolf, boar, bull and horse were also used. The eagle standard was the most important possession of the legion and its loss was a terrible disgrace.
The aquila emblem generally had up-raised wings surrounded by a laurel wreath. It was mounted on a narrow trapezoidal base and mounted on a pole that was held aloft.
The aquilifer's position was accordingly one of enormous prestige, and he was ranked immediately below the centurions and above the optiones, receiving twice the pay of an ordinary legionary (Brunt, 1950). Unlike other standard bearers (such as signifers), the aquilifer probably did not wear an animal skin, instead going bareheaded (no contemporary depiction of an aquilifer shows him with a headdress or helmet). He is depicted as carrying a small circular shield called a parma that could be strapped on if his hands were already full. (Allen, 1908)
Examples of aquilifers in Commentarii de Bello Gallico
The idea that great disgrace would fall upon a legion should its eagle fall into enemy hands or the symbolism of its passage being barred by enemies places aquilifers in a key position-- that of maintaining a legion's honor.
An aquilifer plays an important role in the landing of Roman soldiers in Britannia, as accounted by Julius Caesar. In De Bello Gallico IV.25, Britons put up stiff resistance against the Roman landing party, therefore the legionaries delay to avoid engaging with the enemy. To spur on the troops, the aquilifer shouts out, so that all the soldiers can hear him, that despite the common lack of initiative, he would have fulfilled his office for both Julius Caesar his general and for the public (pūblicae) thing (reī) (i.e. for the republic); he then immediately jumps from the ship and makes his way to the shore with the eagle so that others may be inspired to follow him. Unfortunately for the Romans, chaos ensues as soldiers from different divisions group themselves to the closest standard rather than their assigned one, disturbing battle formation greatly.
In V. 37, most of the Roman command and a large portion of the army is massacred in an ambush from the traitor Ambiorix (leader of the Eburones who promised the Romans safe passage through his lands). As the other centurions of the first ranks are falling in their retreat Lucius Petrosidius the aquilifer bravely hurls the eagle within the ramparts of the Roman redoubt, out of reach from the Britons, just before he himself goes down fighting. 
- The Eagle (2011 film) - The protagonist seeks to restore the eagle of the 9th legion from Celtic hands.
"Pay and Superannuation in the Roman Army," by P.A. Brunt; Papers of the British School at Rome, Vol. 18, (1950), pp. 50–71.
"The Advancement of Officers in the Roman Army," by George H. Allen; Supplementary Papers of the American School of Classical Studies in Rome, Vol. 2, (1908), pp. 1–25.