Why do bees and wasps sting ?

Honeybee

Wasp 

Social bees and wasp,which live and raise their young in colonies,use their venomous stingers to defend their hive or nest against invaders.At the first sign of attack,

For example,eager armies of black hornets swarm from the nest to sting the enemy with their poison.

Predators of all sizes, from birds to bears,generally retreat from the painful onslaught.

Some solitary wasps rely on their stingers not only for defence but to provide food for their larvae.

bee larvae.

The insects' venom paralyses their prey ,usually spiders or caterpillars.

 Struggling to pull her stinger out of enemy,a honeybee rupture her abdomen and soon dies.left behind,the stinger continous to inject venom.

Poison flows from the venom sac to the stinger in the tip of the abdomen.when wasp extends it for use.

The stinger arming bees and wasp evolved from the egg-laying tube or ovipositor that their ancestors used to deposit eggs inside plants or other insects,hence only females have stingers.
In insects of the order Hymenoptera such as the horn-tail and the ichneumon flies,the ovipositore retains its original function.
For many species ,however the tube eventually came to secrete poison,which the insects used to puaralyse prey to feed their larvae.Over time bees and wasp that feed solely on pollen and nectar developed a stinger that is purely a defensive organ.

A horntail insert her ovipositor into a tree trunk to lay eggs.The larvae will feed on the wood.
An ichneumon fly drills through timber with her ovipositor to deposit eggs in the larva of another insect.
To provision the latvae in her burrow,the digger wasp paralyses a caterpillar with her stinger.
The nest dwelling long-legged wasp deters predators,for monkey with her formidable sting.
When stinging an enemy the honeybee secretes a pheromone to summon reinforcement.
How a bee sting:
two pieces with a central passage between them comprise a bee's stinger.The two halves can move independently.
First one and then the other is pushed into the wound in small steps.
Venom is injected through the central passage.In honeybees,the stinger has barbs that snag in the victim's flesh and the stinger is ripped from the bee when the insect pulls away.In most bees and wasp the stinger is not barbed and females can sting repeatedly.