Bombardier beetle
Bombardier beetles are 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 the tip of the abdomen with a popping sound.
The spray is produced from a reaction between two hypergolic chemical compounds, hydroquinone and hydrogen peroxide, which are stored in two reservoirs in the beetle's abdomen. When the aqueous solution of hydroquinones and hydrogen peroxide reaches the "vestibule" (Eisner's word), catalysts facilitate the decomposition of the hydrogen peroxide and the oxidation of the hydroquinone.[1] 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 is claimed by some creationists to be an example of what they call irreducible complexity,[2] though this is refuted by evolutionary biologists.[3]
Behavior[edit]
Most species of bombardier beetles are carnivorous, including the larva.[5] The beetle typically hunts at night for other insects, but will often congregate with others of its species when not actively looking for food.[6]
Anatomy[edit]
There are two large glands that open at the tip of the abdomen. Each gland is composed of a thick walled vestibule which contains a mixture of catalases and peroxidases produced by the secretory cells that line the vestibule. Both glands are also made up of a thin-walled and compressible reservoir which contains an aqueous solution of hydroquinones and hydrogen peroxide.[1]
Evolution of the defense mechanism[edit]
The unique combination of features of the bombardier beetle's defense mechanism—strongly exothermic reactions, boiling-hot fluids, and explosive release—has been claimed by creationists and proponents of intelligent design to be an example of irreducible complexity.[2]
Biologists such as the taxonomist Mark Isaak note however that step-by-step evolution of the mechanism could readily have occurred.[3][10] The full evolutionary history of the beetle's unique defense mechanism is unknown, but biologists have shown that the system could have evolved from defenses found in other beetles in incremental steps by natural selection.[11][12] Specifically, quinone chemicals are a precursor to sclerotin, a brownish substance produced by beetles and other insects to harden their exoskeleton.[13] Some beetles additionally store excess foul-smelling quinones, including hydroquinone, in small sacs below their skin as a natural deterrent against predators—all carabid beetles have this sort of arrangement. Some beetles additionally mix hydrogen peroxide, a common by-product of the metabolism of cells, with the hydroquinone; some of the catalases that exist in most cells make the process more efficient. The chemical reaction produces heat and pressure, and some beetles exploit the latter to push out the chemicals onto the skin; this is the case in the beetle Metrius contractus, which produces a foamy discharge when attacked.[14] In the bombardier beetle, the muscles that prevent leakage from the reservoir additionally developed a valve permitting more controlled discharge of the poison and an elongated abdomen to permit better control over the direction of discharge.[11][12]