View source for Bombardier beetle

{{Short description|Beetles that emit a spray when threatened}}
{{Paraphyletic group
| auto = yes
| image = Brachinus spPCCA20060328-2821B.jpg
| image_caption = ''Brachinus'' species
| parent = Carabidae
| includes =
*[[Brachinini]]
*[[Paussini]]
*[[Ozaenini]]
*[[Metriini]]
}}

'''Bombardier beetles''' are [[ground beetle]]s (Carabidae) in the [[tribe (biology)|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 [[Projectile use by living systems|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 propellant|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 "[[wikt:vestibule|vestibule]]" ([[Thomas Eisner|Eisner]]'s word), [[Catalysis|catalysts]] facilitate the decomposition of the hydrogen peroxide and the [[oxidation]] of the hydroquinone.<ref name="Anesh"/> 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 [[Creationism|creationists]] as something that could not have evolved, although this is refuted by evolutionary biologists.<ref name="Isaak2003"/><!--This is a summary of a cited section of the article below, and it has been agreed on the talk page. Do not attempt to modify it without first gaining consensus on the talk page.-->

==Habitat==
[[Image:Pheropsophus verticalis 01 Pengo.jpg|thumb|Australian bombardier beetle (''Pheropsophus verticalis'')]]
Bombardier beetles inhabit all the continents except [[Antarctica]].<ref>{{Cite web |date=2018-02-06 |title=See the Amazing Way a Beetle Survives Being Eaten |url=https://www.nationalgeographic.com/news/2018/01/beetle-survives-toad-vomit-animals/ |archive-url=https://web.archive.org/web/20200808032552/https://www.nationalgeographic.com/news/2018/01/beetle-survives-toad-vomit-animals/ |url-status=dead |archive-date=August 8, 2020 |access-date=2020-09-17 |website=National Geographic News |language=en}}</ref> 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.

==Behavior==
Most species of bombardier beetles are [[carnivorous]], including the larva.<ref>{{cite web |title=Bombardier Beetle |year=2004 |work=Animal Facts & Photos |publisher=Dallas Zoological Society |url=http://www.dallaszooed.com/animalfacts/animalfacts.php?id=100&region=6&ci=1&li=14 |access-date=2010-07-17 |archive-url=https://web.archive.org/web/20110708222640/http://www.dallaszooed.com/animalfacts/animalfacts.php?id=100&region=6&ci=1&li=14 |archive-date=2011-07-08 |url-status=dead }}</ref> The beetle typically hunts at night for other insects, but will often congregate with others of its species when not actively looking for food.<ref>{{cite web |author=Poetker, E. |title=Brachinus fumans |year=2003 |work=Animal Diversity Web |url=http://animaldiversity.ummz.umich.edu/site/accounts/information/Brachinus_fumans.html}}</ref>

==Anatomy==
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 [[catalase]]s and [[peroxidase]]s 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.<ref name=Anesh>{{cite journal |last1=Aneshansley |first1=D.J. |last2=Eisner |first2=T. |first3=J.M. |last3=Widon |first4=B. |last4=Widon |title=Biochemistry at 100°C: Explosive Secretory Discharge of Bombardier Beetles (Brachinus) |year=1969 |journal=Science |volume=165 |issue=3888 |pages=61–63 |pmid=17840686 |doi=10.1126/science.165.3888.61 |s2cid=96158109 }}</ref>

==Mechanism of defense==
{{further|Anti-predator adaptation}}
When the beetle feels threatened it opens a valve which allows the aqueous solution from the reservoir to reach the vestibule. The catalases lining the vestibule wall facilitate the decomposition of hydrogen peroxide, as in the following theoretical reaction:

:<chem>H2O2(aq) -> H2O(l) + 1/2O2(g)</chem>

The peroxidase [[enzyme]]s facilitate the oxidation of the hydroquinones into quinones (benzene-1,4-diol into [[1,4-benzoquinone]] and analogously for methyl hydroquinone), as in the following theoretical reaction:

:<chem>C6H4(OH)2(aq) -> C6H4O2(aq) + H2(g)</chem>

The known net reaction, which further accounts for the theoretical reaction of the <chem>H2(g)</chem> and <chem>1/2O2(g)</chem> products of the previous reactions, is:<ref name="Anesh" />

:<chem>C6H4(OH)2(aq) + H2O2(aq) -> C6H4O2(aq) + 2H2O(l)</chem>

[[File:Para-Benzoquinone needles.jpg|thumb|Benzoquinone]]

This reaction is very [[exothermic]], and the released [[energy]] raises the temperature of the mixture to near 100&nbsp;°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 is expelled violently through an outlet valve, with a loud popping sound. The beetles' glands store enough hydroquinone and hydrogen peroxide to allow the beetle to release its chemical spray roughly 20 times. In some cases this is enough to kill a predator.<ref>{{harvnb|Eisner|Aneshansley |1999}}</ref> The main component of the beetle spray is 1,4-benzoquinone, an irritant to the eyes and the respiratory system of vertebrates.

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<!--FOR WHAT SPECIES?-->. These pulsations are caused by repeated microexplosions which are the results of the continuous pressure on the reservoir and the oscillatory opening and closing of the valve that controls access to the reaction chamber. This pulsed mechanism is beneficial for the beetles' survival because the system uses pressure instead of muscles to eject the spray at a constant velocity, saving the beetle energy. Also, the reintroduction of new reactants into the vestibule where enzymes are stored, reduces the temperature of the chamber, thereby protecting the peroxidases and catalases from thermal denaturation.<ref>{{cite journal |last1=Dean |first1=J. |last4=Eisner |first4=T. |last2=Aneshansley |first2=D.J. |first3=H.E. |last3=Edgerton |title=Defensive Spray of the Bombardier Beetle: A Biological Pulse Jet |year=1990 |journal=Science |volume=248 |issue=4960 |pages=1219–21 |doi=10.1126/science.2349480 |pmid=2349480 |bibcode=1990Sci...248.1219D }}</ref>

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.<ref>{{cite book |last=Piper |first=Ross |author-link=Ross Piper |title=Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals |url=https://archive.org/details/extraordinaryani0000pipe |url-access=registration |publisher=[[Greenwood Press (publisher)|Greenwood Press]] |year=2007 |isbn=978-0-313-33922-6 }}</ref>

==Evolution of the defense mechanism==
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 to be an example of something that could not have evolved.<ref>{{cite book |title=[[The Blind Watchmaker]] |author=Dawkins,Richard |year=1986}}</ref>{{pagenumber|date=November 2024}}

Biologists such as the taxonomist Mark Isaak<!--The Counter-Creationism Handbook, Greenwood, 2005--> note however that step-by-step evolution of the mechanism could readily have occurred.<ref name="Isaak2003">{{cite web |last1=Isaak |first1=Mark |title=Bombardier Beetles and the Argument of Design |url=http://www.talkorigins.org/faqs/bombardier.html |website=TalkOrigins |access-date=3 August 2018 |date=30 May 2003 |orig-year=1997}}</ref><ref name="Guardian2011">{{YouTube|W96AJ0ChboU|Irreducible complexity cut down to size}} — {{cite news |last1=GrrlScientist |title=Irreducible complexity cut down to size |work=[[The Guardian]] |date=17 January 2011 |quote=This video specifically focuses on debunking the so-called "irreducible complexity" of the favourite examples used by creationists; the eye, the bombardier beetle, the venus flytrap and bacterial flagella. |url=https://www.theguardian.com/science/punctuated-equilibrium/2011/jan/10/2}}</ref> 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]].<ref name=ncse>{{cite journal |author=Weber CG |title=The Bombardier Beetle Myth Exploded |journal=Creation/Evolution |publisher=[[National Center for Science Education]] |volume=2 |issue=1 |pages=1–5 |date=Winter 1981 |url=http://ncse.com/cej/2/1/bombardier-beetle-myth-exploded}}</ref><ref name=to>{{cite web |author=Isaak, Mark |date=May 30, 2003 |title=Bombardier Beetles and the Argument of Design |work=[[TalkOrigins Archive]] |url=http://www.talkorigins.org/faqs/bombardier.html}}</ref> Specifically, [[quinone]] chemicals are a [[Precursor (chemistry)|precursor]] to [[sclerotin]], a brownish substance produced by beetles and other insects to harden their [[exoskeleton]].<ref>{{cite journal |last1 = Brunet |first1 = P.C.J. |last2 = Kent |first2 = P.W. |year = 1955 |title = Mechanism of sclerotin formation: The participation of a beta-glucoside |journal = Nature |volume = 175 |issue = 4462| pages = 819–820 |doi = 10.1038/175819a0 |pmid = 14370229 |bibcode = 1955Natur.175..819B |s2cid = 4206540 }}</ref> Some beetles additionally store excess foul-smelling quinones, including hydroquinone, in small sacs below their skin as a natural deterrent against predators—all [[ground beetles|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.<ref>{{cite journal |last1=Eisner |first1=T. |last2=Aneshansley |first2=D.J. |last3=Eisner |first3=M. |last4=Attygalle |first4=A.B. |last5=Alsop |first5=D.W. |last6=Meinwald |first6=J. |year=2000 |title=Spray mechanism of the most primitive bombardier beetle (''Metrius contractus'') |url=https://journals.biologists.com/jeb/article/203/8/1265/8530/Spray-Mechanism-of-the-Most-Primitive-Bombardier |journal=Journal of Experimental Biology |volume=203 |issue=8 |pages=1265–75 |doi=10.1242/jeb.203.8.1265 |pmid=10729276|bibcode=2000JExpB.203.1265E }}</ref> 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.<ref name=ncse /><ref name=to />

==See also==
*[[Ant nest beetle]]

==References==
{{Reflist|30em}}

== External links ==
{{Commons|Brachinus}}
{{Wikispecies|Brachinini}}
{{NIE Poster|Bombardier Beetles|year=1905}}
*{{cite journal |doi=10.1073/pnas.96.17.9705 |last1=Eisner |first1=T. |last2=Aneshansley |first2=D.J. |title=Spray aiming in the bombardier beetle: Photographic Evidence |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=96 |issue=17 |pages=9705–9 |date=August 1999 |pmid=10449758 |pmc=22274 |bibcode=1999PNAS...96.9705E |doi-access=free }}
* [http://bugguide.net/node/view/16725 Genus ''Brachinus'' — BugGuide.net]
*{{cite web |author=Hesselberg T |title=Exploding beetles suggest a new pressure relief valve and fluid discharge system |date=6 November 2007 |publisher=Life of science |url=http://www.life-of-science.net/bio--and-nanotechnology/feature/exploding-beetles-suggest-a-new-pressure-relief-valve-and-fluid-discharge-system.html}}
*{{cite web |title=Claim CB310: Bombardier beetle evolution |work=[[TalkOrigins Archive]] |url=http://www.talkorigins.org/indexcc/CB/CB310.html}}
*{{cite web |author=Isaak, Mark |date=May 30, 2003 |title=Bombardier Beetles and the Argument of Design |work=[[TalkOrigins Archive]] |url=http://www.talkorigins.org/faqs/bombardier.html}}
* {{cite news |author=BBC |year=1999 |title='Bull's-eye beetle' (''Stenaptinus insignis'')|url=http://news.bbc.co.uk/2/hi/science/nature/422599.stm |work=BBC News}}: 3 photos, 1&nbsp;micrograph, 1 b/w illustration

{{DEFAULTSORT:Bombardier Beetle}}
[[Category:Brachininae]]
[[Category:Paussinae]]
[[Category:Insect common names]]