Natural selection? By means of suicide

Among bees, it's mostly honeybees whose workers are kamikaze stingers.And honeybees may have an additional impetus toward evolving this feature: their large hives (~10,000 individuals, with lots of honey and poilen and grubs in their honeycombs). This makes their hives a much more attractive target than those of many other social bees, making it necessary to administer extra-potent stings to their hives' predators. Furthermore, the large population of workers means that individual workers are more expendable than they would be in a small hive.By comparison, for a small hive, workers surviving the stings they administer is a reasonable tradeoff for inflicting less potent stings -- and such stings would be OK for the relatively small predators that their hives attract. Thus, small-hive bees (and wasps) have smooth stingers.Furthermore, queen honeybees have to survive the stings they administer in order to reproduce; they continue to have smooth stingers.There is even some evidence of venom optimization; worker-bee venom is twice as lethal to mice as queen-bee venom. Queens only sting rival queens, while workers sting hive predators, which are often vertebrates.

When they emerge from their hive cells, queen honeybees try to sting all the other queens that they find -- even those still in their hive cells. This is difficult to explain in "good of the species" fashion, but it is a straightforward consequence of the "selfish gene" concept -- a queen who stings all her rivals gets an entire hive for herself.