When a honey bee stings a mammal, its barbed stinger lodges in the skin, and the honeybee cannot remove it. Instead, it leaves the double lancet behind, along with part of its digestive tract, muscles and nerves. This abdominal rupture is what kills the bee.
But there's an advantage for the bees in this. Even after you swat the bee away, a cluster of nerve cells coordinates the muscles of the stinger left behind. The barbed shafts rub back and forth, digging deeper into your skin. Muscular valves pump toxins from an attached venom sac, and deliver it to the wound for several minutes after the bee is gone.