Minecraft wiki entry for Mob_grinder, associated text: This cactus choke kills spiders, solving potential spider size/jump/climbing issues. It is a simple design, relying on the spider's 2×2 size. The grinder stops all spiders at a bottleneck, where the water-conveyor narrows from 2 blocks wide to 1 block. A cactus is placed on the side, on top of a sand block, where spiders run into it. This grinder works well, but the cactus can destroy some of the loot.