Summon monster : web spinner

[Timy]

I was considering the summoning angle for my cleric ans I was looking for the neat summon possibilities.

And coming across the monstrous spider, I asked myself : does a web disapear once the spinner disappear ?

The idea I have for my campaign where we are defending a fortress against an army of hobgobelin (coming via tunnels in the mouain) is to put some (a lot ?) of web in the tunnels in order to greatly impede the hobbo invasion.

Does it work and if it works what are the arguments I could give to my DM :D

[Segev]

The rule you're looking for is here: http://www.d20srd.org/srd/magicOverview/spellDescriptions.htm#summoning

In short, spell effects created by summoned creatures end when the summoning effect does.

[Ellowryn]

But what if they cast a spell/use a spell-like ability that has a duration of instantaneous like Wall of Stone? The effect of the spell has already ended, but does the wall disappear with the summoned creature?

[Aleolus]

Or what if it isn't a spell or spell effect? Spiders don't use magic to make webs, they produce it biologically

[Necroticplague]

But what if they cast a spell/use a spell-like ability that has a duration of instantaneous like Wall of Stone? The effect of the spell has already ended, but does the wall disappear with the summoned creature?
Nope, for reasons you already listed. Instantaneous spells expire the moment they are cast.

I was considering the summoning angle for my cleric ans I was looking for the neat summon possibilities.

And coming across the monstrous spider, I asked myself : does a web disapear once the spinner disappear ?

Nope, only spells disappear.

[Inevitability]

Summoned objects do not disappear, I believe. This should allow for the webs sticking around.