Throughout the game, especially during Mommy's chase scene, we see cobwebs stretched throughout the factory, and they seem to be so strong that we cannot penetrate it with our bodies.

Poppy, after killing Mommy Long-Legs and getting the last part of the train code, is seen covered with webs, and as we first encounter the antagonist herself, Poppy's lips are sealed with spiderwebs.

I've seen a lot of playthroughs of this game, and most people that come across these webs assume they were left behind by Mommy Long-Legs, due to her being so spider-like.

 But I have a theory about this...

First, Mommy is not really a spider. Yes, she has the abilities of a spider, like climbing on walls and ceilings, but she only has two eyes and four legs, making her seem a bit more like... A supertoy, or a hybrid toy.

Second, we already know that Mommy is not the only spider in the facility. There is also Experiment 1006, the Prototype.

MatPat seems to think that you are the Prototype, but I'm here to debunk that.

It is entirely possible that we are an experiment, but since Playtime Co. seemed to run experiments on the orphans, not the staff, I find it unlikely that we're an experiment, since we are an ex-employee.

The point is, we have never seen Mommy use spiderwebs, even when it would have been useful, like Mommy's chase scene, where she could have trapped you.

So I don't think that Mommy Long-Legs made these webs.

I think it was actually Experiment 1006.

We know very little about this monster-toy, but its hand resembles a spider very much.

So yeah... I don't think she made those webs...

Also, Daddy Long-Legs spiders aren't actually spiders and can't spin silk.