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.
A tribune was someone in the government of ancient Rome who looked out for ordinary people, in other words, an elected representative. There are many types of tribunes, but the earliest dates from ancient Rome. In Rome, a tribune was an elected representative of the common people.