The rules on the spider ability (the ability to block creatures with
   flying, without actually having flying) have changed over the course
   of the game.  First they were mandatory, then they could have been
   considered optional, then they were mandatory again but people often
   read them as optional, then another wording change was used, and
   finally the whole thing was replaced with a new ability, reach.  So
   now, the situation is as follows:

   The spider ability itself:

   Giant Spider  {3}{G}  Creature - Spider  2/4
/ Reach (This creature can block creatures with flying.)

   Flying-evasion:

   Stone Spirit  {4}{R}  Creature - Elemental Spirit  4/3
/ Stone Spirit can't be blocked by creatures with flying.

   Nonflying-evasion:

   Treetop Scout  {G}  Creature - Elf Scout  1/1
/ Treetop Scout can't be blocked except by creatures with flying.

   The Giant Spider can block a creature with flying, and it can block
   the Stone Spirit, but it can't block the Treetop Scout.