The ordinary meaning of nap is to doze off during the day. The secondary meaning is to be offguard. Hence the phrase Catch them napping . Flying by the nap of the earth refers to the practice of flying low to the earth (undetected by radar) to catch the enemy by surprise (or napping).
the tribunes The Tribunes were elected representatives that protected the plebeians from mistreatment. The Tribunes attended meetings of the consuls or Senate in an attempt to bring justice to the plebeians.
Tribune (noun). an officer or magistrate chosen by the people, to protect them from the oppression of the patricians, or nobles, and to defend their liberties against any attempts that might be made upon them by the senate and consuls. Tribune (noun).