http://youtu.be/zqAVj0uzQ00?t=1m52s

That video shows the following: Spiders attack on a large distance and poison the player. This happens when there are spiders crowded, e. g. in the XP farm in the video above. Sometimes it also happens with single spiders but I have no proof for that.

It does not make sense that spiders attack through blocks and on a large distance and I don't think that this is intentional, so please fix it. Spiders don't kill their enemies from afar, they have to be next to it to inject their poison - they are not like skeleton archers but more like zombies in that regard.

Have a nice day!