The simulation seems to be going OK, I guess, except that every so often the spider gets catapulted into the air. 
Edit: Solving a four-chain Inverse Kinematics (IK) problem is proving tricky, so I turned to Box2D, an open source 2D physics simulation library. It can’t do IK, but I don’t need it to. On the plus side, it’s allowed me to add rocks and other physics obstacles.
mycardboarddreams 
Wait… so exploding spiders means something’s going WRONG??? What kind of weirdo AI are you designing? ;)