Ik solver & weight painting

Hi everyone,
today i was creating an armature of a spider, but, after adding an ik solver, every time i moved the final bone, all the leg went crazy! I solved this problem by enabling the auto ik button, but i still don’t know why mine ik solver didn’t work!!
I faced another problem while assigning the vertices to the bones.
Thanks to Envelopes the vertices are automatically assigned to the relative bones, but in some cases some vertices are assigned while they mustn’t! In addition, weight paint some time works, other times doesn’t work!
Can anyone explain where i’m wrong?
http://img134.imageshack.us/img134/9525/armature.jpg

thanks everyone!! :o:o
http://profile.imageshack.us/user/makers_f/images/detail/#134/armature.jpg

Turn off Auto IK and make sure you have the chain length set.
Do not use envelopes.
Weight painting should work as long as the name of the vertex group is exactly the same as the bone (it is case-sensetive)