Spiders do not track players through transparent blocks.
Linked issues
duplicates 1
Comments 5
1.8 added that they did not track through Opaque blocks. I'm talking about Transparent blocks, as the title says.
How long has it been like this? I just tested it in 1.7.10 and they have the same behavior as in 1.8. Not sure if this is just WAI or an actual bug.
Intended, since mobs (including Spiders) cannot see through transparent blocks except water and lava.
Was added in 1.8, so works as intended.