Minecraft wiki entry for Mob_farm, associated text: Alternatively, one can use a cactus to grind the spiders immediately. To do this, make the first funnel part only 7 blocks long and place one water source block instead of two. Place a sand block and a cactus on top on the same side as the source block. To place the cactus, you have to mine out the block next to it. To ensure the water flow on the next part, mine out the wall block next to the sand and add the water sources there and next to the sand, where there would usually be one.