I think you have to hit those invisble textures (or whatever) just right to take the viper of the back ledge... did you know that if you take that viper up to a wall sideways and then keep hitting your jump button you _will_ climb the wall? :)  Doesn't work all the time at that back ledge because there's a bit of a lip to it but you can go over the top of the structure pretty quick.
I'd read you could &quot;steer&quot; your spiders using the avril but didn't know you could use that to move them somewhere and leave them... nice to have videos of this stuff :)