Alexander's army consisted of about fifty thousand men, with the phalanx in the center. This army moved along down the eastern bank of the Tigris, the scouts pressing forward as far as possible in every direction in front of the main army, in order to get intelligence of the foe. It is in this way that two great armies _feel_ after each other, as it were, like insects creeping over the ground, exploring the way before them with their _antennæ_. At length, after three days' advance, the scouts came in with intelligence of the enemy. Alexander pressed forward with a detachment of his army to meet them. They proved to be, however, not the main body of Darius's army, but only a single corps of a thousand men, in advance of the rest. They retreated as Alexander approached. He, however, succeeded in capturing some horsemen, who gave the information that Darius had assembled his vast forces on the plain of Arbela, and was waiting there in readiness to give his advancing enemy battle.