The amazing bombardier beetle
This article is adapted from the author’s contribution to the book Wonders of Creation: Design in a fallen world (creation.com/s/10-2-627 (NZ, UK, SA) and creation.com/10-2-661 (AU, US, CA)) which he co-authored with Dr Stuart Burgess, and is edited by Brian Edwards.
The extraordinary insect known as the bombardier beetle (see fig. 1) 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 with moveable tank turret
Bombardier beetles (Carabidae brachinini) are found mainly in warm climates such as parts of Asia, Africa, Australia, and the USA. But they are also found in Europe, and small colonies have even been observed in southern England. They are usually not far from water, and hide during the day under rocks.
The bombardier beetle’s mixture of chemically heated steam and noxious chemicals is emitted out of its back end (see fig. 2) through a special ‘turret’ which can be moved in any direction (even twisting over its back and pointing forwards—see fig. 3). The whole system is used to ward off predators such as ants, birds, spiders, and frogs—usually successfully, stunning its opponent.
How does it do this?
The chemicals do not come out as a continuous stream. Professor Tom Eisner in 1999 co-authored a seminal paper on the beetle and showed that a series of explosions is produced by combining the two chemicals hydroquinone and hydrogen peroxide in the presence of two catalysts: catalase and peroxidase.1 (A catalyst makes a reaction go much faster but is not consumed in the reaction.) In a clever experiment, Eisner filmed a tethered firing 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, similar to a machine gun fired in repeated bursts, these were emitted in brief bursts of 2–3 seconds each. It can do this repeatedly, sometimes 4–5 times before the now-depleted chemical system takes a few minutes to recover.
Inspired by the beetle, I realized there was a clever design to be discovered. In discussions with Eisner, I began work on it at Leeds University (UK). We showed that these blasts were controlled by a unique valve system, where high pressure causes an inlet valve to close and an outlet valve to open (see fig. 4). This leads to a violent vapour explosion (flash evaporation) event where almost instantaneously a substantial proportion of the liquid (mostly water) expands to steam. A given mass of steam occupies about 1,600 times the volume of the same mass of water, so this ejection is with such force that it carries with it much of the remaining water as well, along with the caustic chemicals. The spray has been shown to reach about 20 cm—that is about 200 times the length of the tiny 1-mm–long combustion chamber.2,3,4 (See the sequences in the David Attenborough series Life,5 which shows the 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, they travel down a very thin tube together where the catalysts are either secreted or possibly are in crystalline form.
The catalysts catalase and peroxidase act on the hydrogen peroxide and the hydroquinone. The hydrogen peroxide then converts to water/steam, thus liberating an oxygen atom for every molecule of peroxide and 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 a vapour explosion.
The valve system is a passive response system, such that the valves are operated by changes in pressure. When the combustion chamber is empty (see fig. 4 above, left panel) 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 (see fig. 4 middle) 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 (see fig. 4 right).
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. Both tubes leading in and out of the combustion chamber, as well as the chamber itself, are totally separate to the digestive tracts of the beetle.
When the hot fluid is ejected, the pressure in the chamber 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 (see fig. 5) of WW2 used by Hitler in 1944 against London and the English southern counties. In the case of the V1, the fuel was petrol (gasoline) 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!
Bioinspiration from the bombardier beetle
The research which began at the University of Leeds has enabled us to develop a spray system based on the technique used by the beetle. Contrary to the allegation that a belief in creation closes down research, it was precisely my conviction that the beetle chamber was designed that led me to make 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. We found that just as with the beetle, for particular valve settings the spray could shoot out to a maximum distance roughly 200 times the 2-cm–long heating chamber we were using— about four metres!
Copying beetle design wins award
Our design has 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 (see fig. 6). In 2010, our work won the Times Higher Education award for the most outstanding contribution to innovation and technology. It has already been used for developing spray systems for fuel injectors in car and truck engines. The invention 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 with a fine spray of water droplets is particularly effective against wood fires since it eliminates the oxygen near the fire. Other possible uses are for pharmaceutical sprays for those having difficulty inhaling medications, and room fragrancers.
Roadblock to evolution
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. Which means that it could not have evolved step by step, since a partly evolved system would not offer any advantage—in fact it would be an impediment to the creature’s survival, and be eliminated by natural selection!
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?
However, what we do understand of the interdependence of the beetle chemistry, the combustion mechanism, and the twin valve system, indicates superb engineering design!
References and notes
- Eisner, T. and Aneshansley, D.J., Spray aiming in the bombardier beetle: photographic evidence, Proc. National Academy of Sciences (USA) 96(17):9705–9709, 17 Aug 1999. Return to text.
- 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. Return to text.
- McIntosh, A.C., Combustion, fire, and explosion in nature—some biomimetic possibilities, Proc. IMechE Part C: J. Mechanical Engineering Science 221:1157–1163, 1 Oct 2007. Return to text.
- McIntosh, A.C. and Beheshti, N., Insect inspiration, Physics World 21(4):29–31, 2008. Return to text.
- BBC Life, series 6 ‘Insects’, Martha Holmes, Rupert Barrington, David Attenborough (narrator), 2009. Return to text.