Pathfinder 1E Giant Spiders and Webs

[Grogg of the North]

The PCs wander into a chamber in a cavern. The chamber is occupied by a Large spider. To prevent its dinner's escape, the spider throws a web at the entrance to the chamber, effectively blocking it up. My questions to you are:

1) Would this even work with a monstrous spider's web throwing ability? A large spider can ensnare a medium creature. The entrance to the spider's lair is five feet wide.
2) Would this be out of line for a mindless vermin?

I feel like this would be a fun way to mix up the encounter. Maybe even separating the party from each other. But I don't want to be out of line and playing the monster much smarter than it is.

 

[Voadam]

Here is the text of the web ability:

Web (Ex)​

Creatures with the web ability can use webs to support themselves and up to one additional creature of the same size. In addition, such creatures can throw a web up to eight times per day. This is similar to an attack with a net but has a maximum range of 50 feet, with a range increment of 10 feet, and is effective against targets up to one size category larger than the web spinner. An entangled creature can escape with a successful Escape Artist check or burst the web with a Strength check. Both are standard actions with a DC equal to 10 + 1/2 the creature’s HD + the creature’s Con modifier. Attempts to burst a web by those caught in it take a –4 penalty.

Web spinners can create sheets of sticky webbing up to three times their size. They usually position these sheets to snare flying creatures but can also try to trap prey on the ground. Approaching creatures must succeed on a DC 20 Perception check to notice a web; otherwise they stumble into it and become trapped as though by a successful web attack. Attempts to escape or burst the webbing gain a +5 bonus if the trapped creature has something to walk on or grab while pulling free. Each 5-foot-square section of web has a number of hit points equal to the Hit Dice of the creature that created it and DR 5/—.

A creature can move across its own web at its climb speed and can pinpoint the location of any creature touching its web.

Format: web (+8 ranged, DC 16, 5 hp); Location: Special Attacks.

 

[Voadam]

I do not find it unreasonable to have a web throwing (technically mindless) giant spider throw its projectile web to block off a corridor choke point to prevent prey from escaping.

It might not be that effective a barrier compared to stringing up the sheets though.

 

[Tonguez]

Trap door spiders are clever enough to build ‘doors’ to cover their lair entrance - I’d justify the giant spider as doing the same thing

 

[DrunkonDuty]

YEah sure, why not? Maybe even make it a trapdoor that can snap shut after prey wanders in. That is, after the first PC enters. It won't be a very strong barrier, especially not to fire, but it will raise the stakes a little.