How Can the Bombardier Beetle Blast Hot Gas From Its Rear?
― It Uses the Same Enzyme That Makes Hydrogen Peroxide Fizz on a Cut
Pinch it, and you hear a little "pop" as your fingertip suddenly stings with heat. What the bombardier beetle sprays is a liquid at almost 100°C. There is no fire and no battery inside its body. It uses an enzyme that is, in fact, also found in our own blood.
On a summer or autumn night, you turn over a stone at the edge of a field. A yellow-and-black mottled beetle scurries out. It is about 1.5 centimetres long. The moment you pick it up, there is a small bang, and something hits your finger.
Your fingertip stings with heat, and later you see the skin has turned brown. A sharp smell lingers in your nose. This is the beetle known in Japan as the "heppiri-mushi", the Pheropsophus jessoensis bombardier beetle.
Now think back to childhood. You put hydrogen peroxide on a scraped knee, and it bubbled up white. Do you remember? These two scenes are, in fact, linked by the same chemical reaction.
Only two things let it spray hot gas
The liquid that makes the heat sits quietly in a pouch inside the belly. If nothing is mixed, nothing happens. Heat appears only at the moment the liquid is sent into a small chamber just before the exit.
The chamber wall releases an enzyme that makes the reaction vastly faster. The same kind of enzyme is found in human blood and liver, and it is what makes hydrogen peroxide fizz.
Humans and beetles hold the same tool. What differs is how it is used. People use the enzyme to tidy up quietly inside the body. The bombardier beetle fires it as a weapon. Let's take them in turn.
Reason 1: A "two-room system" that heats up only when mixed
At the tip of the beetle's abdomen there are two rooms, one large and one small. Look at Figure 1. The large pouch on the left holds a liquid called hydrogen peroxide, plus another ingredient. Hydrogen peroxide is the same substance as in the hydrogen peroxide solution you put on cuts.
Inside this pouch, the liquid stays calm, because hydrogen peroxide breaks down only slowly. Between the pouch and the small chamber there is a valve that muscles open and close.
When an enemy attacks, the valve opens and the ingredients flow into the small chamber on the right. The moment they touch the enzyme from the chamber wall, the hydrogen peroxide breaks down all at once. Oxygen gas and a great deal of heat are produced, and the liquid heats up to boiling. Pushed by the expanding gas, the liquid shoots out of the exit with a "pop".
The spray is not one long blast. High-speed cameras show that it is a chain of hundreds of tiny shots per second. When the heat pushes the chamber open, the valve shuts. When the spray leaves and the pressure drops, more ingredients flow in again. This repeats. Firing in small bursts is thought to stop the beetle's own body from overheating.
Reason 2: The same enzyme is working inside your body too
One of the stars in the small chamber is an enzyme called catalase. Its job is to split hydrogen peroxide into water and oxygen. This enzyme is plentiful in human blood and liver.
When you put hydrogen peroxide on a scrape, it fizzes because catalase in the blood at the wound breaks down the hydrogen peroxide and makes bubbles of oxygen. On clean skin, there is hardly any fizzing.
So why does the human body have this enzyme? In fact, our cells make small amounts of hydrogen peroxide as a by-product of breathing and metabolism. Left alone, it would damage the cells, so catalase breaks it down and clears it away at once. One enzyme is said to break down several million hydrogen peroxide molecules per second, which makes it one of the fastest enzymes known.
In the human body, hydrogen peroxide is very dilute and is cleared away as soon as it forms. So nothing gets hot. The bombardier beetle does the opposite. It packs a pouch full of concentrated hydrogen peroxide and hits it with the enzyme all at once. It is the same reaction, but by changing the amount and the speed, the cleaning job becomes a weapon.
How concentrated must it be to reach boiling? Try changing the concentration of hydrogen peroxide with the slider in Figure 2. At the strength of the hydrogen peroxide solution you buy (about 3%), even if no heat escaped, it would only reach about 40°C. The beetle's pouch is said to be about 25%, which is enough to boil and still have heat to spare.
Researchers at Kobe University fed bombardier beetles to toads. Swallowed beetles sprayed inside the stomach, and the toads sometimes vomited them up. About 40% were brought back up, the quickest after 12 minutes and the slowest after more than 100 minutes. Most of the beetles that were vomited up simply walked away.
The spray contains components that irritate the skin and stain it brown. It is best not to grab the beetle with bare hands. If it does get on you, don't rub. Wash it off with soap and water. If it gets in your eyes, rinse with water at once and see an eye doctor.
Summary
The bombardier beetle stores hydrogen peroxide and other ingredients in a large pouch, then opens a valve to send them into a small chamber. The moment they meet the enzyme there, the hydrogen peroxide breaks down all at once, producing gas and enough heat to boil. The same kind of enzyme exists in the human body, where it normally works quietly as a cleaner that protects cells.
Even a cleaning tool becomes a weapon if you change its concentration and speed.
The bombardier beetle turned the fizz of hydrogen peroxide into a "100°C punch".
For what happens at a wound, see How Does a Cut Close Up?, and for how an insect makes light with a chemical reaction, see our article on fireflies.
- Prepare two small cups. Put thin slices of raw potato in one and cooked potato in the other.
- Pour about a teaspoon of hydrogen peroxide solution into each, and compare how much it fizzes.
- If only the raw one fizzes strongly, that is the enzyme at work. Heating destroys the enzyme, so the cooked one hardly fizzes.
Take care not to get the solution in your eyes, and rinse it away with water when you finish. If you have a thermometer, measure the temperature too. At about 3% concentration, and with some heat escaping, it barely warms up. You can feel how concentrated the beetle's liquid must be.
For those who want more ― terms, formulas and links to textbooksEach part is labelled with its level, from junior-high science to university specialist courses
- Junior highCovered in junior-high science
- High schoolCovered in high-school chemistry or biology
- High school+Advanced high-school material, or textbook sidebars
- UniversityNot taught in high school; university specialist courses (biochemistry, chemical thermodynamics)
- ResearchNot taught as settled fact even at university; researchers are still investigating it
Junior highTerms: this phenomenon has names
- Breakdown of hydrogen peroxide: The reaction in which hydrogen peroxide splits into water and oxygen. In junior high, you meet it in the experiment of adding manganese dioxide and collecting the oxygen.
- Catalysts and enzymes: A substance that speeds up a reaction without changing itself is a catalyst. A protein catalyst made by living things is called an enzyme, and catalase is one of them.
- Exothermic reaction: A reaction that gives off heat as it proceeds, raising the temperature around it. The breakdown of hydrogen peroxide is a classic example.
- Bombardier beetle (Pheropsophus jessoensis): Japan's best-known "heppiri-mushi". It is a ground beetle, and hundreds of species worldwide are said to have the same spray mechanism.
Junior highHigh schoolChecking with a formula: does 1 g of liquid really reach boiling?
Suppose we take 1 g of the liquid from the pouch and break it down all at once. Assuming no heat escapes, we calculate how high the temperature rises. Here we ignore the heat from the other ingredient and estimate from the hydrogen peroxide alone.
| In symbols | Q ＝ m × w × h ／ ΔT ＝ Q ÷ ( m × c ) |
| In words | Heat released ＝ amount of liquid × fraction of hydrogen peroxide × heat of breakdown per gram. Temperature rise ＝ heat released ÷ (amount of liquid × specific heat) |
| Where it comes from | Conservation of energy. It is a balance that assumes all the heat from the reaction goes into warming the liquid. Anything beyond 100°C goes into turning water into steam (heat of vaporisation). |
| Symbol | Meaning and unit |
|---|---|
| Q | Heat released (J, joules) |
| m | Amount of liquid (g) |
| w | Fraction of hydrogen peroxide in the liquid (by weight) |
| h | Heat released when 1 g of hydrogen peroxide breaks down (J/g) |
| c | Specific heat of the liquid. Taken as the same as water, 4.2 J/(g・℃) |
| ΔT | Temperature rise (℃) |
| Heat released when 1 mol of hydrogen peroxide breaks down | About 98000 J |
| Mass of 1 mol of hydrogen peroxide | 34 g |
| Fraction of hydrogen peroxide in the pouch | About 25% (a 1969 analysis) |
| Starting temperature | 20℃ |
| Heat needed to turn 1 g of water into steam | About 2260 J |
| Heat of breakdown per gram, h | 98000 ÷ 34 ≒ 2882 J/g |
| Hydrogen peroxide in 1 g of liquid | 1 × 0.25 ＝ 0.25 g |
| Heat released, Q | 0.25 × 2882 ≒ 721 J |
| Heat capacity of 1 g of liquid (m × c) | 1 × 4.2 ＝ 4.2 J/℃ |
| Temperature rise it should give, ΔT | 721 ÷ 4.2 ≒ 172 ℃ |
| Heat to raise from 20℃ to 100℃ | 4.2 × 80 ＝ 336 J |
| Heat left over after boiling | 721 － 336 ＝ 385 J |
| Water turned to steam by that heat | 385 ÷ 2260 ≒ 0.17 g |
The sum says the temperature would climb 172℃ from 20℃. In reality, boiling starts at 100℃, and the leftover heat turns about one-sixth of the liquid into steam. The spray is at almost 100℃ because of this "ceiling set by boiling".
| Fraction of hydrogen peroxide in the store-bought solution | About 3% |
| Heat released by 1 g of liquid | 0.03 × 2882 ≒ 86 J |
| Temperature rise | 86 ÷ 4.2 ≒ 20 ℃ |
| Fraction needed to boil | 336 ÷ 2882 ≒ 0.117 |
Even with no heat lost, the store-bought solution rises only about 20℃, stopping at about 41℃. Reaching boiling needs a concentration of about 12% or more, and the beetle stores roughly twice that.
High schoolHigh school+Heat of reaction, and what it means for an enzyme to speed things up
High schoolIn high-school chemistry, you study the breakdown of hydrogen peroxide as an example of heat of reaction (enthalpy change). In biology, catalase is used as an example of an "enzyme with an optimum temperature". Raw liver or vegetables fizz, but boiled ones do not, because heat destroys the protein's shape, which is called "denaturation".
High school+An enzyme does not change the amount of heat a reaction gives off. It changes only "how fast it goes". Without a catalyst, hydrogen peroxide breaks down slowly over months, and the heat escapes bit by bit. Because the enzyme makes it break down in a very short time, the heat builds up before it can escape and reaches boiling. The speed itself creates the heat. The other ingredient (hydroquinones) is oxidised by a different enzyme, peroxidase, into benzoquinones, which cause the irritation and the brown colour. This reaction also gives off heat and helps make the spray hot.
UniversityEnzyme speed and the pulsing spray
Catalase is said to reach a turnover number (the number of substrate molecules one enzyme molecule can break down per second) of several million. It is a classic example of a "diffusion-limited" enzyme, whose reaction rate is set almost entirely by how fast the substrate diffuses. Enzyme reaction rates are organised by the Michaelis–Menten equation. The pulsing of the spray was reported in 1990 from high-speed photography as working much like pulse combustion (a pulsejet) in engineering. In 2015, synchrotron X-ray imaging of living beetles showed that the valve closes passively under pressure, so the pulsing continues naturally.
📖 Derivations and further reading (in Japanese): Catalase (Wikipedia) ／ Specific heat capacity (Wikipedia)
ResearchWhat is still not well understood
- How did this two-room system evolve? Some close relatives have only a pouch that stores the ingredients, and it is thought the system grew more complex step by step. But the order in which parts were added is still debated.
- Why can the chamber wall withstand its own heat? The chamber is known to be lined with a hard membrane, but the details of how it survives repeated spraying are still being studied.
- Control of the spray's speed and direction. The tip of the abdomen can swivel, and the beetle is said to aim in almost any direction. Why the shape of the spray differs between species is not yet well understood.
So this article, too, is "an explanation based on what is known so far". The calculation above is also an estimate that leaves out the heat from the other ingredient and the heat that escapes into the body.
Links to textbooks (by level)
| Level | Subject and unit | Where in this article |
|---|---|---|
| Junior high | Science: producing oxygen, chemical change and heat | Hydrogen peroxide breaking down to give oxygen and heat |
| High school | Chemistry: heat of reaction / Biology: enzymes | Calculating the heat of breakdown, catalase and denaturation |
| High school+ | Chemistry: reaction rates and catalysts | The view that fast reaction makes heat build up |
| University | Biochemistry: enzyme kinetics | Turnover number, diffusion limit, pulsed spray |
| Research | Evolutionary biology, biomechanics | Evolution of the two-room system, heat resistance |
| ― | Links to daily life | Fizz of hydrogen peroxide on a wound, first aid after touching a bombardier beetle |
- Aneshansley, D. J., Eisner, T., Widom, J. M., Widom, B. (1969) Biochemistry at 100°C: Explosive secretory discharge of bombardier beetles (Brachinus). Science 165: 61–63.
- Dean, J., Aneshansley, D. J., Edgerton, H. E., Eisner, T. (1990) Defensive spray of the bombardier beetle: a biological pulse jet. Science 248: 1219–1221.
- Arndt, E. M., Moore, W., Lee, W.-K., Ortiz, C. (2015) Mechanistic origins of bombardier beetle (Brachinini) explosion-induced defensive spray pulsation. Science 348: 563–567.
- Sugiura, S., Sato, T. (2018) Successful escape of bombardier beetles from predator digestive systems. Biology Letters 14: 20170647.
- ミイデラゴミムシ (Wikipedia, in Japanese; Pheropsophus jessoensis)
※This article is a popular-science explanation for general readers. The figures given are rough estimates meant to help you understand the mechanism. Do any hydrogen peroxide experiments together with an adult, and keep it away from eyes and mouth. If the beetle's spray gets in your eyes, or skin problems persist, consult a medical professional.