One Inch Square

The Ambush Spider is a level 2 lurker with the ability to become hidden if it has cover or concealment (i.e. it doesn't require superior cover/total concealment).  Strangely, its only while it is hidden that its attack delivers the additional ongoing poison damage and effect associated with it.  I'm not sure how one would explain this "in-world":  Does the spider have shy venom sacs that can only produce when no one is watching?

The Spiderling is a level 4 skirmisher minion with a mechanic that makes it imperative to kill most of them as quickly as possible.  When a spiderling hits with its attack, the target gains vulnerability to poison.  If the target already had vulnerability (i.e. from a previous attack), the vulnerability increases.  What starts off as a typical dinky minion can soon turn into a threat capable of inflicting massive damage.

In a sidebar, the section on spiders also discusses Spider Webs.  MM3 describes a terrain called Web Sheets which has all the hallmarks of a simple trap (perception DC, attack and effects, and countermeasures) but for whatever reason they stop short of making it an actual trap.  In my implementation, I chose to make the trap automatically hit when a character failed perception and walked into it.  In their version, it makes an attack versus reflex (like most traps).  I probably should have done something similar in my implementation, but I wanted the drama of the players having to deal with being stuck in a web while a giant spider descended upon them.  They also chose to make it simply immobilize a character (can't move) whereas I reasoned that a web meant to trap a meal would be more likely to restrain the individual (can't move, -2 to attacks, grant combat advantage).