The instant they lighted on the wall below the line I tried to catch them.

I was careful never to hurt them, and, in time, they knew as precisely as did I where ran the imaginary line.

When they desired to play, they lighted below the line, and often for an hour at a time a single fly would engage in the sport.

When it grew tired, it would come to rest on the safe territory above.
Of the dozen or more flies that lived with me, there was only one who did not care for the game.

He refused steadfastly to play, and, having learned the penalty of alighting below the line, very carefully avoided the unsafe territory.