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.
The three original tribes known as the Ramnes or Ramnenses, Tities or Titienses, and the Luceres, were each headed by a tribune, who represented each tribe in civil, religious, and military matters. Subsequently, each of the Servian tribes was also represented by a tribune.