Bombardier Beetle

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The extraordinary bombardier beetle emits a hot spray to ward off any would be predator – and usually wins. The spray is a mixture of caustic chemicals, hot water and steam and is blasted out of a special nozzle which can be pointed in any direction!

Special defence system with moveable tank turret! 

Bombardier beetles (Carabidae Brachinini) are found mainly in warm countries such as parts of Asia, Africa, Australia, USA (Florida, California). But they are also found in Europe and small colonies have even been observed in the southern part of England. They are usually not far from water and hide during the day under rocks. The bombardier beetle (figures 1, 2) ejects a mixture of chemically heated steam and noxious chemicals out of its back end through a special turret which can be moved in any direction (even  twisting over its back and pointing forwards – see figure 3). The whole system is used to ward off predators such as ants, birds, spiders and frogs (figure 2). The beetle generally wins and stuns its opponent!

How does it do this? The chemicals do not come out as a continuous stream. Professor Tom Eisner in 1999 produced a seminal paper on the beetle and showed that a series of explosions is produced by the two chemicals hydroquinone and hydrogen peroxide in the presence of two catalysts: catalase and peroxidase. (A catalyst makes the reaction go much faster but does not actually undergo chemical change itself). In a clever experiment, Eisner filmed a firing tethered African bombardier beetle, and then played it back in slow motion. Through this he showed that about 500 explosions were given off per second and that repeated blasts, each lasting 2–3 seconds, were emitted from the beetle (figure 3).

This is like ring an automatic machine gun with repeated bursts.1  The author (Professor Andy McIntosh), inspired by the beetle, realised there was a clever design to be discovered, and work began at Leeds University UK in collaboration with Eisner. We showed that these blasts were controlled by a unique valve system, where not only was there an inlet valve that closed under high pressure, but that there was also an outlet valve which opens at high pressure (figure 4). As explained on page 86, this leads to a violent flash evaporation event where almost instantaneously the liquid (mostly water) expands to steam. Because a given mass of steam occupies about 1,700 times the volume for the same mass of water, this ejection is with great force and carries with it much of the remaining water as well, along with the caustic chemicals. The spray has been shown to reach easily twenty times the body length of the beetle in distance.2 3 4 (See the sequences in the film of the David Attenborough series ‘Life’,5 which shows a bombardier beetle successfully warding off an ant attack).

Tiny Combustion chamber

Dissections of the beetle’s rear end have shown a lot more detail about its sophisticated chemical defence system. Before the two chemicals react in the tiny 1mm long combustion chamber, they travel down a very thin tube together where the catalysts either are secreted or possibly are in crystalline form.

The catalysts act on the hydrogen peroxide which then converts to water/steam, thus liberating an oxygen atom for every molecule of peroxide. This then combines with hydrogen molecules released from the hydroquinone. The heat from the strong hydrogen/oxygen reaction causes the rest of the chemicals to react, and the expanding steam causes the vapour explosion.

Initial investigations of the chamber itself suggest that the chamber structure is of special heat-resistant material so that the beetle does not cook itself! The tubes both leading in and out of the 1 mm. combustion chamber, as well as the chamber itself, are all totally separate to the digestive tracts of the beetle.

The valve system is a passive response system, such that the valves are operated by changes in pressure. Referring to figure 4, when the combustion chamber is empty (top diagram) and at atmospheric pressure, the inlet tube is open allowing the reactants to enter the chamber, and the exit tube is closed
by a membrane that blocks the bottom part of the tube. Once the chamber is full and the chemicals react (middle diagram) the extremities of the chamber itself, which is shaped like a boxing glove, pinch the inlet tube shut. As the chemical reaction in the chamber progresses, heat is generated and the pressure in the chamber increases until the membrane is forced open near the bottom of the exit tube (bottom diagram). The hot pressurized fluid is then ejected, the pressure in the chamber then drops, the inlet reopens allowing more reactants into the chamber, and the process is repeated until all of the reactants have been exhausted.

This process is called ‘pulse combustion’ and is used by some engines to give thrust. The most infamous example of this was the V1 ’Doodlebug’ Flying Bomb (figure 5) of World War II which was used to great effect in 1944 against London and the English southern counties by Hitler. In the case of the V1, the fuel was petrol burning in air. At that time few appreciated that a similar combustion system was already in use by the bombardier beetle, not for propulsion, but for spraying its attackers.

Bio-inspiration from the bombardier beetle

Research which began at the University of Leeds has continued to develop a spray system (figure 6) which is based on the technique used by the beetle. The testimony of the author (Professor Andy McIntosh) is that, contrary to the allegation that a belief in creation closes down research, it was precisely because I knew the beetle chamber was designed that led me to these investigations.

It was clear there were design features to be understood and this has led to a patented spray facility which heats water in a special chamber (approximately 20 times the size of the bombardier beetle chamber) where inlet and outlet valves are controlled electronically to open and close at an assigned time. This is an active control system using no chemistry in contrast to the passive system of the beetle which uses chemical heating. However, the valve system itself is very similar to that used by the beetle, and one of the prototypes is displayed in figure 6. In 2010, our work won the Times Higher Education award for the most outstanding contribution to innovation and technology. Possible uses for this are for developing spray systems for fuel injectors in car and truck engines, for fire extinguishers and for fragrancers used in meeting rooms. The work is actively being developed for a fire extinguisher which can deal with forest fires and has the great advantage of shooting steam a considerable distance. Steam is particularly effective against wood fires since it eliminates the oxygen near the fire.

Design features of the bombardier beetle

Any system involving combustion has to be very carefully designed because combustion is dangerous! And it is clearly an example of irreducible complexity since the combustion system will not work unless all the design features are in place. Some of the unanswered questions arising from the bombardier beetle research are: In what form are the catalysts? How does the beetle sense the direction of attack? How does the moveable turret work that directs the exhaust? How are the chemicals hydrogen peroxide and hydroquinone produced? The very fact that many of these questions are still not answered shows the irreducible complexity of this creature and its evident design! 

1 Aneshansley, D.J. & Eisner, T., Spray aiming in the bombardier beetle: photographic evidence, Proceedings of the National Academy of Sciences USA, 96, pp.9705-9709, 1999. 

2 Beheshti, N. and McIntosh, A.C., “The bombardier beetle and its use of a pressure relief valve system to deliver a periodic pulsed spray”, Bioinspiration and Biomimetics (Inst of Physics), 2, 57–64, 2007. 

3 McIntosh, A.C., “Combustion, re, and explosion in nature – some biomimetic possibilities”, Proc. IMechE Vol. 221 Part C: J. Mechanical Engineering Science, 1157–1163, 2007. 

4 McIntosh, A.C. and Beheshti, N, “Insect inspiration”, Physics World (Inst of Physics), 21(4), 29–31, April 2008. 

5 BBC “Life”, series 6 “Insects”, Martha Holmes, Rupert Barrington, David Attenborough (narrator) 2009.