Minecraft wiki entry for Cave_spider_farming, associated text: Because of their size, ability to hold breath underwater, and ability to climb walls, water, fall damage is difficult to make effective on caves spiders. Also, cactus isn't very effective, because cave spiders are small enough to go in between two cacti without taking damage. That said, a cave spider can fall a maximum of 16 blocks before dying. The only ways to kill them automatically are magma blocks and iron golems. The easiest way to make a cave spider farm is to use a manual grinder.