Bombardier beetle
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 Brachinus species
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Wikispecies has information related to: Brachinini |
Bombardier Beetles are ground beetles (Carabidae) in the tribes Brachinini, Paussini, Ozaenini, or Metriini—more than 500 species altogether—that 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 beetles 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 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 all sorts of directions with considerable accuracy.[1]
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[edit] Defense mechanism
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 gasses 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.
[edit] Evolution of the defense mechanism
The defense mechanism uses features which are common in other beetles and have evolved as variations have been subject to natural selection. Various quinones are commonly produced by cells in the skin of insects to harden their skin into a cuticle, and as they taste bad to predators many insects secrete this to deter predators. Where there are indentations in the cuticle, these can vary to form little sacs which store the deterrent quinone. Where predators develop resistance to this chemical, other related chemicals such as hydroquinone can develop, and in many beetles specialised cells secreting 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 can become mixed in with the hydroquinone, and some of the catalases which exist in most cells can make the process more efficient. The chemical reaction can produce heat and pressure which pushes out the discharge when the insect is attacked, as in the beetle Metrius contractus which produces a foamy discharge, while a flap forms a valve to ensure that the pressure pushes the discharge out. In other bombardier beetles, muscles controlling the outlet have developed nozzles that can direct an increasingly explosive reaction to squirt the deterrent chemicals at an attacker. More detailed scenarios have been developed showing a series of small changes that could have led to this mechanism.[2][3]
[edit] Creationist dispute
It has been claimed by some creationists such as Duane Gish that the various components needed to make the system work could not have evolved, as the components appeared to him to provide no benefit in themselves and therefore the entire system would have to be created at once. His presentation mistakenly stated that the hydrogen peroxide and hydroquinone would explode on contact, and that the beetle adds an inhibitor to keep this from happening, when in practice catalysts are used to cause the reaction to start. Gish blamed a problem of translation leading him to misunderstand his German source, but continued to use his description in debates.[2] A similar argument is set out in the children's book Bomby the Bombardier Beetle (ages 4–8), published by the Institute for Creation Research in June 1984, and still on sale as of July 2007[update].[4][3][5] Its arguments have been described as riddled with errors.[6]
In his 1996 book Darwin's Black Box the intelligent design creationist Michael Behe reviewed the same argument as "irreducible complexity" which he claimed to have found in various organisms, but he agreed that Darwinian natural selection may possibly have been responsible for the formation of the bombardier beetle defensive system.[5] Another creationist analysis published by Answers in Genesis accepts much of the scientific view, but contends that complexity suggests an origin by design.[5]
[edit] References
- ^ Piper, Ross (2007), Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals, Greenwood Press.
- ^ a b The Bombadier Beetle Myth Exploded – National Center for Science Education (Creation/Evolution Issue 03, Volume 2, Number 1 - Winter 1981)
- ^ a b Bombardier Beetles and the Argument of Design by Mark Isaak, 1997, updated May 30, 2003, TalkOrigins Archive, retrieved 2007-07-09
- ^ Rue, Hazel (June 1984), Bomby the Bombardier Beetle, Institute for Creation Research, ISBN 0-932766-13-7
- ^ a b c Armitage, Mark H.; Mullisen, Luke (April 2003), Preliminary observations of the pygidial gland of the Bombardier Beetle, Brachinus sp., Answers in Genesis, http://www.answersingenesis.org/tj/v17/i1/beetle.asp, retrieved on 2007-07-09
- ^ Angier, Natalie (February 25, 1985), Drafting the Bombardier Beetle ^, Time Magazine, http://www.time.com/time/magazine/article/0,9171,961206-1,00.html, retrieved on 2007-07-09
[edit] External links
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Wikimedia Commons has media related to: Brachinus |
- Proceedings of the National Academy of Sciences article: "Spray aiming in the bombardier beetle: Photographic evidence"
- Genus Brachinus--BugGuide.net
- Journal of Experimental Biology "Spray mechanism of the most primitive bombardier beetle (Metrius contractus)" T Eisner, DJ Aneshansley, M Eisner, AB Attygalle, DW Alsop and J Meinwald, Journal of Experimental Biology, Vol 203, Issue 8 1265-1275, Copyright © 2000 by Company of Biologists
- Overview feature about recent research on the fluid discharge mechanism in Bombardier beetles
- Claim CB310: Bombardier beetle evolution from TalkOrigins Archive
- Bombardier Beetles and the Argument of Design from TalkOrigins Archive
