Normally, the fang is retracted ins

Normally, the fang is retracted inside the basal segment. When the spider catches its prey, it swings the fangs out into the animal's body. The fangs work something like hypodermic needles. They have a small hole in the tip and a hollow duct inside. The duct leads to the venom gland, either inside the basal segment or farther back in the cephalothorax. When the spider pierces its prey with the fang, it squeezes out the venom, injecting the animal with enough neurotoxin to paralyze or kill. This makes it safe for the spider to feed on its prey, without the risk of a struggle.In the mygalomorph spider suborder, which includes the various tarantulas, the chelicerae are positioned so that the fangs swing forward into the prey, like an axe. In the dominant araneomorph suborder, the chelicerae swing in toward each other, like a pincer. For the mygalomorph system to work effectively, the prey has to be on ground or another solid surface -- the spider has to sandwich the prey between something else and the fangs. The araneomorph system works whether or not the prey is on solid ground -- the chelicerae simply push against each other.