TAXONOMY Kingdom Animalia Phylum Arthropoda Class Insecta Order Coleoptera Family Adelphaga Subfamily Paussinae Tribe Brachini Genus Brachinus Common Name Bombardier Beetle
As Poetker (2003) describes, the bombardier beetle, like all members of the insect order Coleoptera, has two elytra (sheaths) over its wings, although the wings themselves are considered vestigial in the American species and rather useless for flying. To compensate for this inability to escape by flying away from predators, the beetle possesses a rather interesting apparatus for defending itself against predators, which will be elaborated on later. All of the other characteristics of insects in general (six legs; two antennae; body segmented into head, thorax, and abdomen; etc.) are present (Isaak, 1997).
The only genus, Brachinus (described in 1983), 4-15 mm in length, contains about 40 species which occur over much of North America. All have a narrow head and prothorax; legs are reddish yellow; and elytra bluish or black (White, 1983).
As Poetker (2003) describes, any place will do for a ground beetle to lay its eggs, so long as it's out of the way of most predators but not too far away from a good food source. Small underground tunnels or cracks in rotting wood are viable places, as are the decomposing remains of other living things (which quite often serve as the food source). The egg hatches into the larval stage, which begins taking in nourishment from the food source and occasionally molting. After it sheds its skin for the last time, it metamorphoses into a pupa, the stage at which the juvenile looks most like the adult which it will eventually become. At the end of the pupal stage, the pupa sheds its skin and a new adult bombardier beetle emerges. Ground beetles tend to live for several weeks, during which they have ample opportunity to mate and pass on their genes (Shetlar, 1988).
Larvae of these species are parasitic (White, 1983).
As Poetker (2003) describes, bombardiers can inhabit a fairly wide variety of environments (including: desert, dune, savanna, grassland, chaparral, and forest) as long as there is sufficient moisture to allow for good places to lay their eggs. Bombardier beetles of all types generally live in temperate zone woodlands or grasslands (Isaak, 1997; Salleh, 1999).
They're quick on their feet, but not quick to fly. It gives off odors when disturbed and discharges a visible cloud from its rear that feels hot (Eisner, 2003).
As Poetker (2003) describes: a member of the family Carabidae, more commonly known as the ground beetles, the bombardier beetle quite naturally shares some of the habits of its family, and like most other ground beetles, tends to come out at night to prey on smaller insects. Unlike most other ground beetles, however, the bombardier is rather gregarious, so when not wandering around looking for food (thus usually during the day) it will congregate with others of its kind in dark, damp places such as hollow logs (Eisner, 2000; Shetlar, 1988).
Poetker (2003) states, "Brachinus, like nearly all the species in the family Carabidae, is a predator, and eats a number of other insects that are agricultural pests" (as cited in Shetlar, 1988).
Poetker (2003) states, “Bombardier beetles can be found on most continents around the world” (as cited in Isaak 1997; Salleh 1999).
The Beetle will only fire only when directly contacted.
The spray’s temperature is very close to 100 degrees Celsius.
It can spray more than 20 times before it has exhausted its secretory reserves. Benzoquinones are what compose the expelled substance. It was discovered though that the beetle does not store the benzoquinones as such in glands, but instead stores chemical precursors called hydroquinones. It stores the hydroquinones in a reservoir together with hydrogen peroxide; they don’t interact in this mixture, but if appropriate enzymes are added, the two substances react (explosively). The enzymes are composed of two kinds of chemicals: catalases and peroxidases. The moment reservoir fluid enters the reaction chamber, the catalases promote the breakdown of hydrogen peroxide into oxygen and water, while the peroxidases promote the oxidation of the hydroquinones to benzoquinones (Eisner, 2003).
As Poetker (2003) describes, added to this, the beetle can rotate the end of its abdomen 270 degrees in any direction, which allows for an impressive "firing range." In effect, the beetle can spray in whatever direction the predator comes from, a decided advantage (Dawkins 1985; Eisner 2000; Salleh 1999).
There have been several predation tests in which tree frogs were offered bombardier beetles. The end result was the frogs’ aversion to the beetles.
When one dissects a bombardier beetle, the glands are easily exposed. They are an identical pair of organs, lying side by side in the posterior half of the abdomen. Each consists of two confluent chambers: an inner large, compressible reservoir, enveloped by muscles, and a smaller, rigid reaction chamber, interposed between the reservoir and the outer opening of the gland.
The source of this secretion is a lobed mass of tissue connected to the reservoir by a highly coiled duct, longer than the beetle itself (Eisner, 2003).
Bombardier beetles have not always been called bombardiers. In the proceedings of the Royal Swedish Academy of Sciences of 1750, there is noted a Schussfliege (shooting fly) encountered by the student author, Daniel Rolander, under a rock. Rolander noted that the animal gave off both sound and smoke. He concluded, “So is nature full of wonder in its diversity” (Eisner, 2003).
On 1 October 1999, the United States issued a stamp featuring a picture of the bombardier beetle (Eisner, 2003).
Dawkins, R. 1985. The blind watchmaker. W.W. Norton, London.
Eisner, T. 2000. A shot in the dark. (On-line). Accessed 12 October 2000 at http://www.thebookery.com/Bookpress/article.cfm/10.4/177.Eisner7615.html.
Eisner, Thomas. 2003. For love of insects. Harvard University Press, Cambridge, MA.
Eisner T., D.J. Aneshansley, M. Eisner, A.B. Attygalle, D.W. Alsop, and J. Meinwald. 2000. Spray mechanism of the most primitive bombardier beetle (Metrius contractus). Journal of Experimental Biology, 203. Retrieved 2 November 2007 from http://jeb.biologists.org/cgi/reprint/203/8/1265.pdf.
Isaak, M. 1997. Bombardier beetles and the argument of design. (On-line) Accessed 12 October 2000 at http://www.talkorigins.org/faqs/bombardier.html
Poetker, E. 2003. Brachinus fumans. (On-line) Animal Diversity Web. Accessed 2 November 2007 at http://animaldiversity.ummz.umich.edu/site/accounts/information/Brachinus_fumans.html.
Salleh, A. 1999. "No place to hide". (On-line) Accessed 12 October 2000 at http://www.abc.net.au/science/news/stories/s44387.htm.
Shetlar, D. 1988. Beetle. World Book Encyclopedia, Inc., USA.
White, R. E. 1983. A field guide to the beetles of North America. Houghton Mifflin Company, Boston.
This webpage was created by Isaac Balderas and Linda Martinez in partial fulfillment of Entomology, 2007, University of Texas at El Paso. The views in this webpage are based on our research and may not reflect the views of the University of Texas system.
Photo by Israel Del Toro.