Bombardier beetle

Bombardier beetles are adephagan ground beetles (Carabidae) in the tribes Brachinini, Paussini, Ozaenini, or Metriini—more than 500 species altogether—which are most notable for the defense mechanism that gives them their name: when disturbed, they eject a hot, noxious chemical spray from their pygidial glands with a popping sound.

The spray is produced from a catalyzed reaction between hydroquinone and hydrogen peroxide, an aqueous solution of which is stored in the pygidial glands in the beetle's abdomen. When the solution reaches the "vestibule" (Eisner's word), catalysts facilitate the decomposition of the hydrogen peroxide and the oxidation of the hydroquinone. Heat from the reaction brings the mixture to near the boiling point of water and produces gas that drives the ejection. The damage caused can be fatal to attacking insects. Some bombardier beetles can direct the spray in a wide range of directions.

The beetle's unusual defense mechanism has been claimed by some creationists as something that could not have evolved, although this is refuted by evolutionary biologists.