Bombardier beetle
Bombardier beetles belong to the family Carabidae (ground beetles). More than 500 species are described world wide. Shown here is a bombardier beetle from the genus Brachisinus. Photo taken by Patrick Coin (from Wikipedia).
The bombardier beetles have long fascinated scientists and natural historians alike due to their extraordinary line of defence against predators such as ants, frogs and birds. When threatened, they squirt a hot stream of liquid chemicals onto their aggressor accompanied by a loud popping sound. The chemicals, hydroquinones and hydrogen peroxide, are secreted by a pair of glands. Each gland consists of a reservoir and a reaction chamber connected by a valve. The reaction chamber is connected to the outside world via a nozzle and an exit valve. The reservoir contains an aqueous solution of hydroquinones and hydrogen peroxide, while the reaction chamber is filled with a mixture of catalase and peroxidases dissolved in water. Muscles squeeze the content of the reservoir into the reaction chamber, where extremely fast reactions occur. These reactions results in free oxygen and generate enough heat to bring the liquid to the boiling point. The valve to the reservoir closes due to the pressure of the released gasses and the liquid is expelled explosively through the nozzle and exit valve at the tip of the abdomen. By rotating the abdominal top, the beetles can aim the nearly boiling liquid in any direction. A direct hit is fatal for other insects and can cause considerable pain even in humans.


The engineer Novid Behesthi and professor of combustion theory Andy McIntosh, both from Leeds University in the United Kingdom, have modelled the reactions in the bombardier beetle to explore its biomimetic potential. They used previous findings by the biologist Tom Eisner at Cornell University, US that the spray is not a continuous stream, but a series of  mini explosions that generate a pulse jet, to model the physics of the discharge with a computational fluid dynamics (CFD) model.  The model shows that much of the behaviour of the system is controlled by the connecting vale and the exit valve, which first opens after a certain pressure threshold has been reached. The nozzle diameter, furthermore, is crucial for the exit velocity of the fluid, which can reach more than 11 m/s. In conclusion the researchers discovered the following line of events occurring during a discharge.