Minecraft wiki entry for Spider_Eye, associated text: Spiders and cave spiders have a 1⁄3 chance of dropping a spider eye when killed by a player or tamed wolf, but not when killed by environmental damage (such as falling). The maximum amount of spider eyes is increased by 1 per level of Looting. The chance of a spider not dropping any spider eyes can be found using the formula 2 / (Looting Level + 3). For example, Looting III gives a 1⁄3 chance of not dropping any spider eyes. All other amounts have an equal chance of occurring.