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, the beetle ejects a noxious chemical spray in a rapid burst of pulses from special glands in its abdomen. The ejection is accompanied with a popping sound. A bombardier beetle produces and stores two reactant chemical compounds, hydroquinone and hydrogen peroxide, in separate reservoirs in the rear tip of its abdomen. When threatened, the beetle contracts muscles that force the two reactants through valved tubes into a mixing chamber containing water and a mixture of catalytic enzymes. When combined, the reactants undergo a violent exothermic chemical reaction, raising the temperature to near the boiling point of water.
The corresponding 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 (flash evaporation) and is expelled through an outlet valve into the atmosphere with a loud popping sound. The flow of reactants into the reaction chamber and subsequent ejection to the atmosphere occurs cyclically at a rate of about 500 times per second and with the total pulsation period lasting for only a fraction of a second. The gland openings of some African bombardier beetles can swivel through 270° and thrust between the insect's legs, so it can be discharged in a multitude of directions with considerable accuracy.
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. The beetle typically hunts at night for other insects, but will often congregate with others of its species when not actively looking for food.
Secretory cells produce hydroquinones and hydrogen peroxide, which collect in a reservoir. The reservoir opens through a muscle-controlled valve onto a thick-walled reaction chamber. This chamber is lined with cells that secrete catalases and peroxidases. When the contents of the reservoir are forced into the reaction chamber, the catalases and peroxidases rapidly break down the hydrogen peroxide and catalyze the oxidation of the hydroquinones into p-quinones.
These reactions release free oxygen and generate enough heat to bring the mixture to the boiling point and vaporize about a fifth of it. Under pressure of the released gases, the valve is forced closed, and the chemicals are expelled explosively through openings at the tip of the abdomen. Each time it does this, it shoots about 70 times very rapidly. The damage caused can be fatal to attacking insects and small creatures and is painful to human skin.
Evolution of the defense mechanism
When a bombardier beetle is threatened by a predator or an offensive invader of any kind, at the appropriate point of approach, the bombardier beetle swings its tail end around, and hot, noxious fluid, heated to 100 °C (212 °F), is explosively released from twin combustion tubes into the face of the enemy. Various quinones are commonly produced by cells in the skin of insects to harden the dermis into a cuticle, and, owing to the foul taste they impart, may additionally act as predator deterrents. Where there are indentations in the cuticle, these vary to form little sacs that store the deterrent quinone. Where predators develop resistance to this chemical, other related chemicals such as hydroquinone develop, and in many beetles, specialised cells secrete hydroquinone from glands connected by ducts to a reservoir sac, which can be closed off by muscles to stop leakage.
While all carabid beetles have this sort of arrangement, in some cases, hydrogen peroxide, which is a common by-product of the metabolism of cells, is mixed 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, which pushes out the discharge when the insect is attacked, as in the beetle Metrius contractus, which produces a foamy discharge. In other bombardier beetles, the muscles stopping leakage have a flap forming a valve to ensure that the pressure pushes the discharge out, and muscles controlling the outlet have developed nozzles that can direct an explosive reaction to squirt the deterrent chemicals at an attacker. The exact sequence is unknown, and it is common for features with one purpose to become useful for other purposes, a process called exaptation. More detailed scenarios have been developed showing a series of small changes that could have led to this mechanism.
The beetle's explosive defensive system has become a focus for creationists. Intelligent design proponent Michael Behe contends that the system is an example of irreducible complexity and therefore could not have evolved, an argument which is rejected by scientists as logically and factually flawed.
- Piper, Ross (2007). Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals. Greenwood Press. ISBN 0-313-33922-8.
- "Bombardier Beetle". Animal Facts & Photos. Dallas Zoological Society. 2004. http://www.dallaszooed.com/animalfacts/animalfacts.php?id=100®ion=6&ci=1&li=14.
- Poetker, E. (2003). "Brachinus fumans". Animal Diversity Web. http://animaldiversity.ummz.umich.edu/site/accounts/information/Brachinus_fumans.html.
- Weber CG (Winter 1981). "The Bombardier Beetle Myth Exploded". Creation/Evolution (National Center for Science Education) 2 (1): 1–5. http://ncse.com/cej/2/1/bombardier-beetle-myth-exploded.
- Isaak, Mark (May 30 2003). "Bombardier Beetles and the Argument of Design". TalkOrigins Archive. http://www.talkorigins.org/faqs/bombardier.html.
- Stanley A. Rice (2007). Encyclopedia of Evolution. Infobase Publishing. p. 214. ISBN 978-0-8160-5515-9. http://books.google.com/books?id=YRcAVvmE6eMC&lpg=PA214&ots=o7JpJen2he&dq=Michael%20Behe%20bombardier%20beetle&pg=PA214#v=onepage&q=Michael%20Behe%20bombardier%20beetle&f=false.