Bombardier beetle

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Bombardier beetle
Brachinus spPCCA20060328-2821B.jpg
Brachinus species
Scientific classification
Kingdom: Animalia
Phylum: Arthropoda
Class: Insecta
Order: Coleoptera
Family: Carabidae


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 their abdomen, with a popping sound.

The spray is produced by a reaction between two chemical compounds, hydroquinone and hydrogen peroxide, stored in separate reservoirs in the beetle's abdomen and mixed when needed in a third chamber with water and catalytic enzymes. 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 and small creatures and is painful to human skin.[citation needed] Some bombardier beetles can direct the spray over a wide range of directions. The ratio has not yet been clearly established between how much of this 'near boiling point' heat is caused by the thermochemical reaction itself and how much comes from the Boyle's law principle that gas or liquid released under pressure produces extra heat.


Australian Bombardier Beetle (Pheropsophus verticalis)

Bombardier beetles inhabit most of the continents, with the exception of Antarctica. They typically live in woodlands or grasslands in the temperate zones but can be found in other environments if there are moist places to lay their eggs.


Most species of bombardier beetles are carnivorous, including the larva.[1] The beetle typically hunts at night for other insects, but will often congregate with others of its species when not actively looking for food.[2]

Defense mechanism[edit]

The two reactant chemical compounds, hydroquinones and hydrogen peroxide, are secreted by specialized glands and are stored in separate reservoirs in the rear tip of its abdomen. When threatened, the beetle contracts muscles that open the valves of these reservoirs and force the two reactants into a thick-walled mixing chamber lined with cells that produce enzymes including catalases and peroxidases.

In the mixing chamber the enzymes rapidly break down the hydrogen peroxide, releasing free oxygen and catalyzing the oxidation of the hydroquinones into p-quinones.[citation needed] The reaction is very exothermic, and the released energy raises the temperature of the mixture to near 100 °C, vaporizing about a fifth of it. The resultant pressure buildup forces the entrance valves from the reactant storage chambers to close, thus protecting the beetle's internal organs. The boiling, foul-smelling liquid partially becomes a gas by flash evaporation and is expelled explosively through an outlet valve, with a loud popping sound.

The flow of reactants into the reaction chamber and subsequent ejection occur in a series of about 70 pulses, at a rate of about 500 pulses per second. The whole sequence of events takes only a fraction of a second.

Typically the beetle turns its body so as to direct the jet towards whatever triggered the response. The gland openings of some African bombardier beetles can swivel through 270° and thrust between the insect's legs, discharging the fluid in a wide range of directions with considerable accuracy.[3]

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—have been used by creationists and proponents of intelligent design as examples of irreducible complexity that could not have been produced by evolution.[4] However, while the true evolutionary path is still unknown, biologists have shown that the system could have theoretically evolved from defenses found in other beetles in incremental steps by natural selection.[5][6]

Specifically, quinone chemicals are a precursor to sclerotin, a brownish substance produced by beetles and other insects to harden their exoskeleton.[7] 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, in with the hydroquinone, and some of the catalases that exist in most cells makes 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.[8] 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.[5][6]

See also[edit]


  1. ^ "Bombardier Beetle". Animal Facts & Photos. Dallas Zoological Society. 2004. 
  2. ^ Poetker, E. (2003). "Brachinus fumans". Animal Diversity Web. 
  3. ^ Piper, Ross (2007). Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals. Greenwood Press. ISBN 0-313-33922-8. 
  4. ^ Stanley A. Rice (2007). Encyclopedia of Evolution. Infobase Publishing. p. 214. ISBN 978-0-8160-5515-9. 
  5. ^ a b Weber CG (Winter 1981). "The Bombardier Beetle Myth Exploded". Creation/Evolution (National Center for Science Education) 2 (1): 1–5. 
  6. ^ a b Isaak, Mark (May 30, 2003). "Bombardier Beetles and the Argument of Design". TalkOrigins Archive. 
  7. ^ P. C. J. Brunet and P. W. Kent (1955), Mechanism of sclerotin formation: The participation of a beta-glucoside. Nature, volume 175, pages 819-820. doi:10.1038/175819a0
  8. ^ T. Eisner T, D. J. Aneshansley, M. Eisner, A. B. Attygalle, D. W. Alsop, J. Meinwald (2000) Spray mechanism of the most primitive bombardier beetle (Metrius contractus). Journal of Experimental Biology, volume 203, issue 8, pages 1265–1275. Eisner, T.; Aneshansley, D. J.; Eisner, M.; Attygalle, A. B.; Alsop, D. W.; Meinwald, J. (2000). "Spray mechanism of the most primitive bombardier beetle (Metrius contractus)". The Journal of experimental biology 203 (Pt 8): 1265–1275. PMID 10729276.  edit
  9. ^ a b c d R. W. G. Hingston (1927), The liquid-squirting habit of oriental grasshoppers. Transactions of the Entomological Society of London, part I, volume 75, pages 65-69. doi:10.1111/j.1365-2311.1927.tb00060.x

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