I'm a bit past halfway through Perdido Street Station right now, and [SPOILER] I just got to the bit where it turns out the Weaver is some sort of extra-dimensional spider. [It's not really much of a spoiler. I still have no idea what that means, in context of the setting.] Giant, terrifying spider imagery seems to be a staple of sci fi/fantasy horror, dating all the way back to Tolkien, who loved them. I'm guilty of it, myself -- there was a giant skeleton-spider monster in my first novel (still in post-production).
I think it's because of the alien nature of spiders. There's something intensely terrifying of a creature with roughly the same physiological components as us (head, body, limbs, mirror symmetry) but which nonetheless is intensely different in every appreciable way.
(For the record: I'm arachnophobic, so this post is making me twitch quite a lot. Expect me to never write about spiders again.)
Humans have the same sort of response to snakes, which I think supports my theory, and with other insects. Less with fish and birds, though. My guess would be that's because
- We interact with them less frequently
- they tend to be either fairly human shaped, or so completely not that it's hard to draw a comparison
- they tend not to be filled with poison.
And then, obviously, there are the usual irrational reasons for phobia. We're afraid because the world is scary, and some stuff just rubs us the wrong way.
Spiders also have that physics thing going for them, viz. terror. The fact that surface tension acts on their bodies more than gravity is so distant from our experience of the world that it looks, intuitively, like utter defiance of the universal laws to which we are bound. It doesn't help that they look like tiny little eldrich horrors.
So, that's what I've got for you: Spiders. So terrifying that genre writers have trouble leaving them out. Even me.
Now, I'm going to go read some comics or watch a movie or ANYTHING WITHOUT SPIDERS IN IT.