A creature with an Arachnid body type is similar to a quadruped, but it does not possess a neck or chest. Despite presumably having eight eyes, only the "left eye" and "right eye" can be targeted. Arachnids cannot be killed by criticals made to the back, although they will take damage as normal.
Arachnids bleed normally, but they cannot take additional critical damage from a level 3 wound in any location. If a player attempts to target a body part that has a level 3 wound, the Arachnid will take raw hitpoint damage instead.
Most Arachnids have remarkable perception regardless of level, making it somewhat difficult to hide in their presence. This is easily counteracted by blinding the Arachnid in one eye, or by inflicting severe head damage to the Arachnid.
Note that removing a "right arm" or "left arm" will not bring the Arachnid to the ground. The arachnid seems to be a biped, for purposes of bringing the creature to the ground. The arachnid will not be able to stand back up without a leg.