The chelicera (named after those mouth-limbs spiders have), is a medium-sized, 12 RHD vermin, which is just really disappointing overall.
Apart from the details given above (which are bad enough already), chelicera have a mere 14 strength, lack limbs, are naturally mindless, and have the bare minimum of special abilities.
Even at its supposed speciality (grappling) the chelicera performs poorly. Sure, it grapples with its dexterity modifier, and it's got a +4 racial bonus to boot, but ultimately even something like an ogre monk is a better grappler than this thing. Blood Drain is neat, but 1d4 constitution drain matters little on something unlikely to survive the round.
Mimicry is a flavorful ability and definitely unexpected on a mindless spider, but not significantly more powerful than a Wand of Ghost Sound.
Final verdict: -0 LA. What a surprise.