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.
Sharks' eyes are protected when attacking prey. Also unlike humans, shark eyelids serve to protect the eye when attacking prey. Some sharks have a nictating membrane, or a clear membrane that covers and protects the eye when a shark bites its prey. Sharks like the Great White, lacking a nictating membrane, roll their pupils back in their heads for protection when feeding.