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.
That sentiment is reflected in Nast's cartoon, where Columbia, the feminine symbol of the United States, shields the dejected Chinese man against a gang of thugs, whom she emphatically reminds that America means fair play for all men.. This Harper's Weekly cartoon by Thomas Nast defends Chinese immigrants against the fierce prejudice and discrimination which they faced in late-nineteenth-century America.