are ground beetles
) in the tribes Brachinini
, or Metriini
—more than 500 species
altogether—that are most notable for the defense mechanism that gives them their name: They can fire a mixture of chemicals from special glands
in their abdomen.
Bombardier beetles store two separate chemicals hydroquinone and hydrogen peroxide. When they are threatened, the two chemicals are squirted through two tubes, where they are mixed along with small amounts of catalytic enzymes and undergo a violent exothermic chemical reaction. The boiling, foul-smelling liquid partially becomes a gas (flash evaporation) and is expelled with a loud popping sound.
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 gasses 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
The defense mechanism uses features which are common in other beetles and have evolved
as variations have been subject to natural selection
. 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 this to deter predators. Where there are indentations in the cuticle, these can vary to form little sacs which store the deterrent quinone. Where predators develop resistance to this chemical, other related chemicals such as hydroquinone
can develop, and in many beetles specialised cells secreting hydroquinone form 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 can become mixed in with the hydroquinone, and some of the catalases
which exist in most cells can make the process more efficient. The chemical reaction can produce warmth and pressure which pushes out the discharge when the insect is attacked, as in the beetle Metrius contractus
which produces a foamy discharge, while a flap forms a valve to ensure that the pressure pushes the discharge out. In other bombardier beetles, muscles controlling the outlet have developed nozzles that can direct an increasingly explosive reaction to squirt the deterrent chemicals at an attacker. More detailed scenarios have been developed showing a series of small changes that could have led to this mechanism.
It has been claimed by some creationists
such as Duane Gish
that the various components needed to make the system work could not have evolved, as the components appeared to him to provide no benefit in themselves and therefore the entire system would have to be created at once. His presentation mistakenly stated that the hydrogen peroxide and hydroquinone would explode on contact, and that the beetle adds an inhibitor to keep this from happening, when in practice catalysts are used to cause the reaction to start. Gish blamed a problem of translation leading him to misunderstand his German source, but continued to use his description in debates.
A similar argument is set out in the children's book Bomby the Bombardier Beetle
(ages 4–8), published by the Institute for Creation Research
in June 1984, and still on sale as of July 2007. Its arguments have been described as riddled with errors.
In his 1996 book Darwin's Black Box the intelligent design creationist Michael Behe reviewed the same argument as "irreducible complexity" which he claimed to have found in various organisms, but he agreed that Darwinian evolution may possibly have been responsible for the formation of the Bombardier Beetle defensive system, as well as creationism. Another creationist analysis published by Answers in Genesis accepts much of the scientific view, but contends that complexity could suggest an origin by design.