Prevent spider spawning in a Mob trap (Minecraft 1.3)

Question

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?

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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?

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Question:

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

0x0x0x0x0x0
00000000000
0x0x0x0x0x0
00000000000
0x0x0x0x0x0
00000000000
0x0x0x0x0x0
00000000000

Answer 1

Spiders are spawned in the centre of a block, and because they are larger than a single block, part of them overlaps the other eight blocks surrounding it.

This means that when a spider is placed to be spawned they must not have any solid blocks around them in a 3x3 area in.

An area like this will prevent spiders from spawning.

ooooooooooo
ooooooooooo
ooxooxooxoo
ooooooooooo
ooooooooooo
ooxooxooxoo
ooooooooooo
ooooooooooo
ooxooxooxoo
ooooooooooo
ooooooooooo

o = air
x = solid block (half-slab)