You breathe through your mouth and lungs. Insects breathe through their abdomen. Insects have lots of little holes on their abdomen called spiracles . These spiracles allow them to breathe, and that keeps them alive. Insects digest their food in their abdomen - just like you.
Insects also store their poison in their stomachs.
If you have ever been stung by a bee, you know that bees have a stinger . That's actually part of their abdomen. They use their stingers to defend themselves. Besides stinging, breathing and digesting, the abdomen is where insects have their reproductive organs. You can see that an abdomen is very important to the survival of an insect.