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Bombardier beetle
Bombardier beetles are ground beetles (Carabidae) in the tribes Brachinini, Paussini, Ozaenini, or Metriini—more than 500 species altogether—that 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 all sorts of directions with considerable accuracy.
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 (Brachinus) is threatened by a predator or an offensive invader of any kind, at the appropriate point of approach the bombardier beetle swings his tail-end around and hot, noxious gases, heated to 100 °C (212 °F) , are explosively released from twin combustion tubes right into the face of his 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 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, 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.
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