LotR -- Book 4 - Chapter 10 - The Choices of Master Samwise

[Kuruharan]

'horn', 'the arches of her legs', the 'splaying legs'

Yeah, but how else would one describe a spider? I mean, legs are their most prominent feature (the awful, wretched things).

Why would both Tolkien and Lewis choose to depict a primeval force of evil as female?

Why not? They were equal opportunity villain creators. Besides, the White Witch was not the only baddie in the Narnian world. Tash was there. I wouldn't exactly say the White Witch lost her place to him.