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 - Spirit  4/3 
/ Stone Spirit can't be blocked by creatures with flying.

  Nonflying-evasion:

  Treetop Scout  {G}  Creature - Elf  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.

Some parts © 2009 Advameg, Inc.