Phylogeny

Where do I fit in?

There is much controversy over how the Bombardier beetle actually fits into a phylogenetic tree. Most often, the beetle is included in the carabid subfamily Brachini, a group that includes over 19,000 species. However, in some analyses, bombardier beetles are actually classified as either  Harpalinae or Austral Psydrines; a sister groups to Brachini. (Ober 2003)

As the tree becomes more specific, the controversy continues. In one Western Hemisphere bombardier beetles species, the Ozaenini, motochondiral and nuclear DNA sequencing predicted its extremely close relationship between several other  clades including the Ozaena, Tropopsis, and Pachyteles groups. (Moore 2008)

Chemical Defense History

The history of the Bombardier beetle begins in the early Cretaceous period soldier beetle (Coleoptera: Cantharidae). Recently, scientists have found evidence of gland reservoir structures along with tarces of chemicals in the fossil record of this soldier beetle making it the earliest know ancestor of beetle chemical defense. (Poinar, Marshall et al. 2007)

Although scientists have yet to discover exactly when they began producing chemicals and storing them for defence, they estimate that chemical defensive mechanisms have evolved at least 30 times in beetles and that this evolution is likely due to injesting and processing the differing chemicals found in their natural habitats.(Poinar, Marshall et al. 2007)

A few examples of the results of these evolution can be found in the list below:

  • Rove beetles (Stenus Latreille and Dianous Leach, see left) are aquatic beetles which, when disturbed, release chemicals that react with water to propel the beetle away from danger. (Lang, Seifert et al. 2012)
  • When attacked by predators Staphylinid beetles emit a number of chemicals that react in the air and harden on the predator's mouth preventing it from feeding. (Dettner 1993)
  • Chrysomelid beetles secrete a series of chemicals when attacked which act like glue sticking the beetle to any surface making it impossible for a predator to remove it. (Pasteels, Duffey et al. 1990)

Creationism?

It should be noted that the bombardier beetle is at the head of the creationism/darwinism debate. Because the beetle’s defensive technique is so unique and so complex, creationists have argued that the only way in which this organism could have evolved is from God. Although it is difficult to at first see how this beetle could have developed over an evolutionary timescale ethologist Richard Dawkins provides a counterexample disproving this creationist notion. As Dawkins hypothesizes, the beetle’s defensive chemicals have become so effective due to a gradual increase in the hydrogen peroxide concentrations stored in its gland. Over time, the once weak solution of hydrogen peroxide proved to be an adaptive advantage for the beetles and thus natural selection favored the selection of beetles who could store increasingly more concentrated hydrogen peroxide solutions.