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.^[1]

Habitat

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.

Behavior

Most species of bombardier beetles are carnivorous, including the larva.^[2] The beetle typically hunts at night for other insects, but will often congregate with others of its species when not actively looking for food.^[3]

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 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 their skin into a cuticle, and as they taste bad to predators, many insects secrete them to deter predators. 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.^[4][5]

Creationist debate

Duane Gish and some other creationists claim that the various components needed to make the system work could not have evolved, because they believe the components provide no benefit in themselves and therefore the entire system would have to have been created at once. Others such as intelligent design proponent Michael Behe and Answers in Genesis, accept most of the scientific view but contend that "complexity" suggests an origin by design.^[6][7] Contrary to the creationist views, all necessary intermediate stages have been found in extant beetles within or closely related to the bombardier beetle family, with each intermediate giving an advantage to the organism.^[5] Richard Dawkins has addressed this argument in his book The Blind Watchmaker.

This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (February 2011)

Ground beetles are a large, cosmopolitan family of beetles, Carabidae, with more than 40,000 species worldwide, approximately 2,000 of which are found in North America and 2,700 in Europe.^[2]

Description and ecology

Although there is some variation in their body shape and coloring, most are shiny black or metallic and have ridged wing covers (elytra). The elytra are fused in some species, particularly large Carabinae, rendering the beetles unable to fly. The genus Mormolyce is known as violin beetles due to their peculiarly shaped elytra. All carabids except the quite primitive flanged bombardier beetles (Paussinae) have a groove on their foreleg tibiae bearing a comb of hairs. This is used for cleaning their antennae.^[3]

A Brachinus sp. typical bombardier beetle (Brachininae: Brachinini) from North Carolina

Defensive secretions

Typical for the ancient beetle suborder Adephaga to which they belong, they have paired pygidial glands in the lower back of the abdomen. These are well developed in ground beetles, and produce noxious or even caustic secretions used to deter would-be predators. In some, commonly known as bombardier beetles, these secretions are mixed with volatile compounds and ejected by a small combustion, producing a loud popping sound and a cloud of hot and acrid gas which can injure small mammals like shrews, and is liable to kill invertebrate predators outright. To humans, getting "bombed" by a bombardier beetle is a decidedly unpleasant experience. The "bombing" ability has evolved independently twice as it seems – in the flanged bombardier beetles (Paussinae) which are among the most ancient ground beetles, as well as in the typical bombardier beetles (Brachininae) which are part of a more "modern" lineage. The Anthiini, meanwhile, can mechanically squirt their defensive secretions for considerable distances and are able to aim with a startling degree of accuracy; in Afrikaans they are known as oogpisters ("eye-pissers"). In one of the very few known cases of a vertebrate mimicking an arthropod, juvenile Heliobolus lugubris lizards are colored similar to the aposematic oogpister beetles, and move in a way that makes them look surprisingly similar to the insects at a casual glance.^[4]

It is sometimes suggested that Charles Darwin found himself on the receiving end of a bombardier beetle's defences on a collecting trip in 1828,^[5] but this is based on a misreading of his autobiography;^{[6][not in citation given]} a bombardier beetles' "bombing" is already triggered by picking it up, and Darwin had been carrying the beetle in question in his closed hand for some time already before he ran afoul of its secretions. He discussed this incident and another such case in a letter to Leonard Jenyns as follows:

"A Cychrus rostratus once squirted into my eye & gave me extreme pain; & I must tell you what happened to me on the banks of the Cam in my early entomological days; under a piece of bark I found two carabi (I forget which) & caught one in each hand, when lo & behold I saw a sacred Panagæus crux major; I could not bear to give up either of my Carabi, & to lose Panagæus was out of the question, so that in despair I gently seized one of the carabi between my teeth, when to my unspeakable disgust & pain the little inconsiderate beast squirted his acid down my throat & I lost both Carabi & Panagæus!"^[7]

A Lophyra sp. tiger beetle from Tanzania

Ecology

Common habitats are under the bark of trees, under logs, or among rocks or sand by the edge of ponds and rivers. Most species are carnivorous and actively hunt for any invertebrate prey they can overpower. Some will run swiftly to catch their prey; tiger beetles (Cicindelinae) can sustain speeds of 8 km/h (5 mph) – in relation to their body length they are among the very fastest land animals on Earth. Unlike most Carabidae which are nocturnal, the tiger beetles are active diurnal hunters and often brightly coloured; they have large eyes and hunt by sight. Ground beetles of the species Promecognathus laevissimus are specialised predators of the cyanide millipede Harpaphe haydeniana, countering the hydrogen cyanide which makes these millipedes poisonous to most carnivores.

Relationship with humans

As predators of invertebrates, including many pests, most ground beetles are considered beneficial organisms. The caterpillar hunters (Calosoma) are famous for their habit of devouring insect larvae and pupae in quantity, eagerly feeding on tussock moth (Lymantriidae) caterpillars, processionary caterpillars (Thaumetopoeidae) and woolly worms (Arctiidae), which due to their urticating hairs are avoided by most insectivores. Large numbers of the Forest Caterpillar Hunter (C. sycophanta), native to Europe, were shipped to New England for biological control of the Gypsy Moth (Lymantria dispar) as early as 1905.

A few species are nuisance pests. Zabrus is one of the few herbivorous ground beetle genera, and on rare occasions Zabrus tenebrioides for example occurs abundantly enough to cause some damage to grain crops. Large species, usually Carabinae, can become a nuisance if present in numbers, particularly during outdoor activities such as camping; they will void their defensive secretions when threatened, and if they hide among provisions this can despoil food. Since ground beetles are generally reluctant or even unable to fly, it is usually easy to block their potential routes of entry mechanically or with a topical insecticide.

A crucifix ground beetle (Panagaeus cruxmajor) got Charles Darwin into trouble in 1828.

Especially in the 19th century and to a lesser extent today, their large size and conspicuous coloration as well as the odd morphology of some (e.g. the Lebiini) made many ground beetles a popular object of collection and study for professional and amateur coleopterologists. High prices were paid for rare and exotic specimens, and in the early to mid-19th century there was a veritable "beetle craze" in England. As mentioned above, Charles Darwin was an ardent collector of beetles when he was about twenty years old, to the extent that he'd rather scour the countryside for rare specimens with William Darwin Fox, John Stevens Henslow and Henry Thompson than to study theology as his father wanted him to do. In his autobiography he fondly recalled his experiences with Licinus and Panagaeus, and wrote:

"No poet ever felt more delight at seeing his first poem published than I did at seeing in Stephen's Illustrations of British Insects the magic words, 'captured by C. Darwin, Esq.'"^[6]

Evolution and systematics

This article is outdated. Please update this article to reflect recent events or newly available information. Please see the talk page for more information. (October 2011)

The Adephaga are documented since the end of the Permian, about 250 million years ago. Ground beetles evolved in the latter Triassic, having separated from their closest relatives by 200 million years ago. The family diversified throughout the Jurassic, and the more advanced lineages, such as the Harpalinae, underwent a vigorous radiation starting in the Cretaceous. The closest living relatives of the ground beetles are the false ground beetles (Trachypachidae) and the wrinkled bark beetles (Rhysodidae). They are sometimes even included in the Carabidae as subfamilies or as tribes incertae sedis, but more preferably they are united with the ground beetles in the superfamily Caraboidea.

Much research has been done on elucidating the phylogeny of the ground beetles and adjusting systematics and taxonomy accordingly. While there is no completely firm consensus, a few points are generally accepted: As it seems, the ground beetles consist of a number of more basal lineages and the extremely diverse Harpalinae which contain over half the described species and into which several formerly independent families had to be subsumed.^[8]

Subfamilies and selected genera

The taxonomy used here is based on the Catalogue of Palaearctic Coleoptera^[9] and the Carabidae of the World Database.^[10] Other classifications, while generally agreeing with the division into a basal radiation of more primitive lineages and the more advanced group informally called "Carabidae Conjunctae",^[11] differ in details. For example, the system used by the Tree of Life Web Project makes little use of subfamilies, listing most tribes as incertae sedis as to subfamily.^[12] Fauna Europaea on the other hand splits rather than lumps the Harpalinae, restricting them to what in the system uses here is the tribe Harpalini.^[13]

All the approaches mentioned above are legitimate as they agree with the phylogeny as far as it has been resolved.^{[citation needed]} The inclusive Harpalinae presented here are used for two reasons, one scientific and one practical – first, the majority of authors presently uses this system, following the Catalogue of Palaearctic Coleoptera. Second, the MediaWiki markup cannot at present adequately represent the relationships of the ground beetle subgroups in detail if the restricted view of the Harpalinae is chosen.^{[citation needed]}

Basal ground beetles

Carabus lateralis (Carabinae: Carabini)

Loricera pilicornis (Loricerinae: Loricerini)

Notiophilus substriatus (Nebriinae: Notiophilini)

Clivina fossor (Scaritinae: Clivinini)

Amblytelus (Amblytelinae: Amblytelini)

Carabinae Latreille, 1802 – including Agoninae and Callistinae

• Altagonum
• Aplothorax (monotypic genus)
• Calosoma - including Callisthenes
• Carabus
• Cychrus
• Homothes
• Fortagonum
• Laemostenus
• Notagonum

Cicindelinae – tiger beetles (roughly 2,100 species; sometimes included in Carabidae)

Cicindinae

Elaphrinae Latreille, 1802

• Blethisa
• Diacheila Motschulsky, 1844
• Elaphrus

Hiletinae

Loricerinae Bonelli, 1810

• Loricera

Migadopinae

Nebriinae (includes Notiophilinae, often included in Carabinae)

• Leistus
• Nebria
• Notiophilus
• Pelophila Dejean, 1821

Nototylinae

Omophroninae Bonelli, 1810 – round sand beetles

• Omophron

Paussinae – ant nest beetles, flanged bombardier beetles

Promecognathinae

Scaritinae Bonelli, 1810 – pedunculate ground beetles

• Clivina
• Dyschirius Bonelli, 1810
• Scarites

Siagoninae Bonelli, 1810

Carabidae Conjunctae

Broscus cephalotes (Broscinae: Broscini)

Dixus sphaerocephalus (Harpalinae: Harpalini)

Mormolyce phyllodes (Harpalinae: Lebiini)

Bembidion quadrimaculatum (Trechinae: Bembidiini)

Amblytelinae Sloane, 1898^{[Note 1]}

• Amblytelus

Apotominae

• Apotomus Illiger, 1807

Brachininae Bonelli, 1810 – typical bombardier beetles

• Aptinus (tentatively placed here)
• Brachinus
• Mastax Fischer von Waldheim, 1828

Broscinae Hope, 1838

Harpalinae – including Chlaeniinae, Cyclosominae, Dryptinae, Lebiinae, Licininae, Mormolycinae, Odacanthinae, Oodinae, Panagaeinae, Perigoninae, Platyninae, Pseudomorphinae, Pterostichinae, Zabrinae (over 20,000 species)

Melaeninae

Psydrinae

• Mecyclothorax

Trechinae Bonelli, 1810 – including Bembidiinae, Patrobinae

Tribes incertae sedis

• Amarotypini – Harpalinae?
• Gehringiini – Trechinae or a distinct subfamily
• Metiini – Harpalinae?

Notes

References

Further reading

	Insects portal
	Arthropods portal

• E. Csiki (1946) (in German). Die Käferfauna des Karpaten-Beckens [The beetle fauna of the Carparthian basin]. Budapest. pp. 71–546. 
• K. Kult (1947) (in Czech). Klíč k určování brouků čeledi Carabidae Československé republiky [Key to the beetles of family Carabidae of the Czech Republic]. Prague. 
• C. H. Lindroth (1942) (in Swedish). Coleoptera, Carabidae. Svensk Insectenfauna, Vol. 9. Stockholm. pp. 1–260. 
• Edmund Reitter (1908–1917) (in German). Die Käfer des Deutschen Reiches [The beetles of the German Empire]. Stuttgart: K. G. Lutz.