A serpent is used as a metaphor to describe the regiment when

The rushing yellow of the developing day went on behind their backs. When the sunrays at last struck full and mellowingly upon the earth, the youth saw that the landscape was strchereaked with two long, thin, black columns which disappeared on the brow of a hill in front and rearward vanished in a wood. They were like two serpents crawling from the cavern of the night.