There are approximately 35,000 recognized spider species worldwide. Spiders are found all over the world and are predatory, feeding largely on insects. They trap prey using silk nets or ambushing tactics. Spiders breathe through book lungs or tracheal systems and have 8 simple eyes. Their ability to spin silk from specialized glands is important for web building and sensing vibrations to catch prey. Male spiders fertilize females by depositing sperm on small webs for the females to take internally and lay fertilized eggs in silk cocoons. While spiders have venomous glands, their venom is usually harmless to humans unless it is a black widow or brown recluse spider, which can deliver severe or fatal bites when threatened.