I am building a new mob trap and I want to be sure no spiders will spawn in it. Now I have heard that the spawn behavior of spiders will change slightly in the 1.3 update so I want to be prepared for that.
On https://minecraft.wiki/w/Spider I read this:
spawn: Light level of 7 or less, 2x2x2 space on solid blocks. Top half of space may be occupied by transparent blocks (glass, etc.).
So if I understand this correctly all that has changed is that the spiders will spawn on a upside-down half slab. But they still can not spawn on a normal half slab, right?
Secondly I am questioning the spawning dimensions. The first trap I build, and I still use sometimes, I build using this tutorial:
I ended up with spawning pads looking like this:
Now I never saw spiders in my collection area, but I I look closely I can surely see some 2x2x3 area's one each spawning pad. How come spiders did not spawn there?
In my new situation I have different spawning pads. They are 11 blocks wide, and 8 blocks deep.
I guess if I don't want any spiders, I should make the pads like this:
0 = normal block
x = half-slab