And time was very heavy and very long. I played games with flies, with
ordinary house-flies that oozed into solitary as did the dim gray light;
and learned that they possessed a sense of play. For instance, lying on
the cell floor, I established an arbitrary and imaginary line along the
wall some three feet above the floor. When they rested on the wall above
this line they were left in peace. 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. That fly was a sullen, disgruntled creature. As
the convicts would say, it had a "grouch" against the world. He never
played with the other flies either. He was strong and healthy, too; for
I studied him long to find out. His indisposition for play was
temperamental, not physical.

Believe me, I knew all my flies. It was surprising to me the multitude
of differences I distinguished between them. Oh, each was distinctly an
individual--not merely in size and markings, strength, and speed of
flight, and in the manner and fancy of flight and play, of dodge and
dart, of wheel and swiftly repeat or wheel and reverse, of touch and go
on the danger wall, or of feint the touch and alight elsewhere within the
zone. They were likewise sharply differentiated in the minutest shades
of mentality and temperament.