What figure of speech is "You are a thorn in my side."

Read the excerpt from part 3 of the call of the wild. but it was in giving the law and making his mates live up to it, that buck excelled. dave and sol-leks did not mind the change in leadership. it was none of their business. their business was to toil, and toil mightily, in the traces. so long as that were not interfered with, they did not care what happened. billee, the good-natured, could lead for all they cared, so long as he kept order. the rest of the team, however, had grown unruly during the last days of spitz, and their surprise was great now that buck proceeded to lick them into shape. what theme is most supported by the ideas in this excerpt? good leaders are strong and intelligent. loyalty can be a very powerful force. only the strong survive in the wilderness. we are all ruled by our natural instincts.