Bombardier beetle

Bombardier beetles are ground beetles in the family Carabidae. There are more than 500 species which are notable for their defence mechanism. Most of them are found in rainforests.
When disturbed, the beetle squirts a poisonous chemical spray from special glands in its abdomen. The ejection makes a popping sound.
Mechanism.
A bombardier beetle produces and stores (separately) two chemical compounds which react with each other: hydroquinone and hydrogen peroxide. 
When threatened, the beetle contracts muscles that force the two reactants through tubes into a mixing chamber containing water and enzymes. The reactants undergo a violent chemical reaction which gives off heat. This raises the temperature to near the boiling point of water.
The beetle swivels the tip of its abdomen around, and shoots a spray of boiling hot, poisonous chemicals at its attacker.