The spiders is supposely not able to climb sandstone, but can climb fences, making them pretty useless. And spiders shouldn't be able to climb vertical on glass, make no sense for them to do so.
AH! I thought it was a skele and spider spawner. Damn, that sucks! I can only imagine all my heart ache when I do find one but, OHHHH the goodies that will be harvested!