Kelly’s Field Notes
Common Name: Bombardier beetle
Tribes: Brachinini, Paussini, Ozaenini, Metrini
Genus: There are over 50 genera of bombardier beetle!
Species of Note Near You (There are over 500 species of bombardier beetle!):
American bombardier beetle (Brachinus fumans) - found throughout North America and in Tabasco, Mexico.
Australian bombardier beetle (Pheropsophus verticalis) - found in eastern Australia
Common bombardier beetle (Brachinus crepitans) - found in Europe, the Middle East, central Asia, and northern Africa.
Bombardier beetles are a type of ground beetle with a very special mechanism to deter predators - ejecting a hot noxious spray from the end of their abdomens which creates a popping sound. They are found in woodland or grasslands on every continent except Antarctica. Bombardier beetles are generally pretty small, about 1.3 cm (0.5 in) in length and smaller and come in a wide variety of colors (black and orange, all black, black and yellow, orange and metallic green, etc.) and patterns (spotted, bicolor, metallic)
Bombardier beetles produce an exothermic reaction within their abdomens by shifting a hydrogen peroxide solution from a gland into a chamber lined with catalases that decomposes the hydrogen peroxide. The peroxidase enzymes oxidize hydroquinones, also secreted into that chamber from a second gland, into quinones causing the released energy to raise the liquid mixture to between 55oC and 100oC, vaporizing a fifth of the mixture. This new mixture,1,4-benzoquinone, a respiratory system and eye irritant, is then expelled with a loud popping sound through the tip of the beetle’s abdomen. They can do this about 20 times, with each ejection a series of rapid fire 70 pulses 500 pulses per second. This means no muscles are involved in the ejection of the liquid, it’s all pressure and valves! Some species can even rotate their abdomens or tuck them under their thorax to spray in front of the beetle.
The beetle’s internal organs are protected as the chamber this reaction takes place in is reinforced and as the pressure build up closes the valve to the original chamber the hydrogen peroxide was stored in. This can have some nasty issues on human skin though, burning and staining where the liquid touches.
In one study, researchers in Japan fed bombardier beetles (Pheropsophus
jessoensis) to toads (Bufo japonicus and Bufo torrenticola). The beetles used this reaction within the toads’ stomachs, causing the toads to not only vomit out the beetles, but flip their stomachs inside out and scrape out the contents. However, not all toads are created equal when it comes to lunch! Cane toads (Rhinella marina) can swallow and digest bombardier beetles despite the discomfort.
Bombardier beetles lay their eggs in moist environments. After mating she will use mud to build a small cell to lay her eggs in, the cells are attached to sticks, stones, stems, leaves or other substrates. Some species instead lay their eggs directly underground in a moist substrate. One species, Pheropsophus aequinoctialis, actively seeks out mole cricket burrows to lay her eggs in where her larvae will feed on the cricket’s eggs. In her lifetime, one female can lay between a few dozen and a few hundred eggs. The eggs take about 15 days or so to hatch, depending on species and environmental conditions. In one laboratory study they took 13.5 days on average to hatch.
Like other insects that go through complete metamorphosis, bombardier beetles have a larval stage with several instars which lasts about 10 days and a pupal stage which lasts about 20 days. Depending on the species, larvae eat a variety of things. Earlier I mentioned cricket eggs, for example. Brachinus pallidus and B. fidelis feed on aquatic beetle pupae that crawl to shore to pupate. Bombardier beetles belonging to tribe Paussini and tribe Ozaenini rely on ants to raise their larvae and pupae. The ants may even see some value in them (likely protection) as they actively transport adult beetles to new nests during migration. These species undergo their entire larval life eating one pupa. Adults are fairly omnivorous, feeding on vegetation, other insects, and fruit. As adults, they can live between two to three years or only a few weeks, it varies by species.
Caustic Hot Spray - the bombardier beetle uses a hot caustic spray to deter predators or to induce vomiting if eaten. This spray will kill ants and other smaller predators.
Flight - they are slow to take off, but they can do it!
Ant Friends - bombardier beetles of tribe Paussini and tribe Ozaenini have a relationship with ants, as the ants feed their young. It has also been hypothesized that the sounds Paussini make using their stridulating organ may be the same frequency and pulse pattern as ants, allowing them to communicate with each other. No one is really sure how these beetles can hang out with the ants, but it is likely due to chemical mimicry.
Bombardier beetles in Culture:
Irreducible complexity - Creationists argue that the bombardier beetle is an example of irreducible complexity, which means certain biological systems could not have evolved due to successive slight modifications. Evolutionary biologists have debunked this for the bombardier beetle and all other crationist claims. The parts of the bombardier beetle’s defense mechanism is made of known quantities in other beetle species. For example, quinone is a precursor to sclerotin which hardens insect exoskeletons and hydroquinone is foul smelling and stored in some beetles as a defense mechanism. Others even mix hydrogen peroxide and quinones and catalysts to create smaller chemical reactions, pushing the liquid onto the cuticle.
Starship Troopers - the sci-fi cult classic in book, comic book, movie, and television form has the bombardier bug which is a ball shaped bug that self-destructs. The tanker bugs are arachnids, but do shoot a flammable stream of corrosive liquid (from their heads not their abdomens). The plasma bug also has the ability to shoot plasma and was used as an anti-artillery and anti-aircraft defense force.
Evans, Arthur V. “Beetles of Eastern North America." Beetles of Eastern North America. Princeton University Press, 2014.
BBC’s Life hosted by Sir David Attenborough has a great scene with a bombardier beetle spraying in slow motion.
A Bombardier Beetle, Pheropsophus Aequinoctialis. entnemdept.ufl.edu/Creatures/BeneFICIAL/beetles/Pheropsophus_aequinoctialis.html.
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Moore, Wendy. Molecular phylogenetics, systematics, and natural history of the flanged bombardier beetles (Coleoptera: Adephaga: Carabidae: Paussinae). The University of Arizona, 2006.
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Rebrina, Fran, et al. "Ground beetle assemblages respond to motorway proximity through changes in functional rather than taxonomic diversity in a grassland ecosystem." Global Ecology and Conservation 38 (2022): e02255.
“Species Brachinus Fumans.” 2003-2022, Iowa State University, bugguide.net/node/view/967553.
Sugiura, Shinji, and Takuya Sato. "Successful escape of bombardier beetles from predator digestive systems." Biology Letters 14.2 (2018): 20170647.
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Wikipedia contributors. “Bombardier Beetle.” Wikipedia, en.wikipedia.org/wiki/Bombardier_beetle.