Certain species of social wasps build open combs without any kind of protective covering, such as the Polistine wasps for instance. Stinging is an excellent defense in wasp colonies against predators and other possible dangers which present a threat to the nests and their occupants. A stinger is actually a modified egg-laying tube which is connected to a venom sac inside the insect's body. A wasp is quite capable of stinging repeatedly because it can easily withdraw its stinger from the entry point. When a wasp wants to sting, it curves its abdomen downward and punctures the victim's skin with its sharp stinger.
Muscles then drive the stinger deeper into the flesh.
Meanwhile, venom is being pumped from the venom sac, through the stinger, and into the wound. It is similar to how a hypodermic needle works. Chemicals in the venom cause the pain and irritation from the sting. The entire process from egg to adult takes several weeks to finish. Wasp colonies have a distinct caste system.