Titus Livius (Livy), The History of Rome, Book 26
Frank Gardner Moore, Professor Emeritus in Columbia University, Ed.

4. meanwhile the whole effort of the war had been directed against Capua. but it was rather an intensive blockade than a series of assaults, and the slaves and commoners were unable either to endure hunger or to send messengers to Hannibal through guards so near to each other. [2] a Numidian was discovered who took a letter, declared that he would make his way out, and kept his promise. going out right through the Roman camp at night he inspired in the Capuans the hope that, while they still had some strength left, they might attempt a sally in all directions. [3] but in the many engagements they were as a rule successful in cavalry battles, while in infantry they were worsted. to be victorious, however, was by no means so cheering¹ as it was depressing to be vanquished at any point by a beleaguered and almost captured enemy. at length a method was devised, so that what was lacking to their strength might be compensated by skilful tactics. [4] out of all the legions were picked young men who by reason of strength and lightness of build were the swiftest. these were furnished with round shields of smaller size than those used by cavalry, and seven javelins apiece four feet long and having iron heads such as are on the spears of the light —armed troops.² [5] the horsemen would each of them take one of these men on to their own horses, and they trained them both to ride behind and to leap down nimbly when the signal was given. [6] when thanks to daily practice they seemed to do this with sufficient daring, they advanced into the plain which was situated between the camp and [p. 17]the city wall in the face of the Capuan cavalry in³ battle —line. [7] and when they had come within range, at a given signal the light —armed leaped to the ground. thereupon an infantry line suddenly dashes out from the cavalry at the enemy's horsemen, and while attacking they hurl one javelin after another. [8] by throwing a great number of these against horses and men in all directions, they wounded very many. but more consternation was created by the strange and the unexpected, and the cavalry charging into the frightened enemy caused them to flee with slaughter all the way to the gates. [9] thereafter the Roman side was superior in cavalry also; it was made the practice to have light —armed in the legions. [10] the originator of combining infantry with cavalry they say was Quintus Navius, a centurion, and that for so doing honour was paid him by the general.