The Bombardier Beetle

Part One · The Spray

A bombardier beetle is a small, ordinary-looking insect with an extraordinary method of defending itself. Most of the more than five hundred known kinds are no longer than half an inch, and many wear fairly plain colors, such as dark blue or black wing covers over an orange head and legs. They belong to the large family of ground beetles, and they are found on most of the continents, usually in fields, woodlands, and stream banks where the soil stays damp. During the day they rest underneath stones and fallen logs, and at night they emerge to hunt, running swiftly across the ground on their long legs. Their name is a clue to their secret, because a bombardier is a soldier who drops bombs, and these beetles fire back.

When a bombardier beetle is grabbed by an ant, pinched by a spider, or pecked at by a bird, it answers with a spray of hot liquid. The spray shoots out of the tip of the beetle's abdomen with a sharp pop that people can hear from a short distance away, followed by a small puff of vapor that looks a little like smoke. The liquid is almost as hot as boiling water, at roughly one hundred degrees Celsius, or two hundred and twelve degrees Fahrenheit, and it also contains chemicals that sting and smell terrible. For most animals that were considering the beetle as a meal, this unpleasant surprise is enough to make them change their minds immediately. The spray is not meant to destroy an enemy, but only to convince it that the beetle is not worth the trouble.

What makes the spray so remarkable is that the beetle manufactures it on demand, inside its own body, from two ingredients that it keeps apart until the moment of danger. A pair of glands in the abdomen drain into a storage chamber, and that chamber holds a mixture of two chemicals, called hydroquinone and hydrogen peroxide. Hydrogen peroxide is the same substance that people keep in brown bottles to clean small scrapes, although the beetle's batch is considerably stronger. Left alone, these two chemicals combine only very slowly, and they cause hardly any trouble, and as long as they sit together in the storage chamber, the beetle is carrying nothing more dangerous than a pair of bottles of cleaning fluid. Because the ingredients are kept in separate, protected compartments, the beetle can carry its whole supply safely wherever it goes.

The action begins when the beetle squeezes the muscles around its storage chamber and forces a little of the liquid through a tiny valve into a second, much smaller room, called the reaction chamber. The walls of this room are lined with special proteins called enzymes, which are the substances that living bodies use to speed up chemical changes. The instant the liquid touches them, the peroxide breaks apart into water and oxygen gas, and the other chemical is converted into a sharp-smelling substance called a quinone. The change releases a great deal of heat, and the temperature of the liquid shoots up to the boiling point in a fraction of a second, while the oxygen gas builds up enough pressure to blast the hot mixture out of the beetle.

This raises a very good question, which is why the beetle does not cook itself, and scientists have found several parts of the answer. First, the heat is produced only in the small reaction chamber, which has tough, thick walls, and never in the big storage chamber that holds most of the supply. Second, the beetle fires in pulses rather than in one long stream, so the hot chamber empties and cools down between blasts. Careful photographs taken with high-speed cameras show that the beetle can make hundreds of these tiny pulses every second, which is exactly what produces the popping sound. The beetle works a bit like an engine that fires many small bursts in a row, instead of releasing one huge explosion. Without these safeguards, the same reaction would be far too violent for any living body to survive.

Part Two · The Beetle and Its World

A beetle that could only spray straight backward would be easy to avoid, so the bombardier has a way to aim. The tip of its abdomen works like the turret of a tank, and it can swivel toward almost any direction. A beetle can fire sideways at an enemy that is gripping one of its legs, and some can even point the nozzle forward, over their own backs, to hit an attacker standing in front of them. The beetle may not see its target clearly, but it can feel exactly where the ant is holding on, and it uses that sense of touch to line up the shot. In just a few seconds, a beetle can send out several sprays. This accuracy makes the weapon far more useful than a simple squirt would be, since the beetle can respond to an attack from any side.

The spray is a very effective defense, and it works against many animals that are far bigger than the beetle. Ants, spiders, birds, and frogs all tend to leave bombardier beetles alone after a single experience with the hot, stinging mist, and some of them seem to remember the lesson. The beetle does not waste its supply, either, because after a few sprays it must refill its storage chamber before it can fire again at full strength, and refilling takes some time. For that reason the beetle uses its weapon only when it is truly in danger, and the rest of the time it relies on its speed and its hiding places to stay out of trouble. A defense that works in a fraction of a second is precisely what a small insect on the ground needs.

The chemical ingredients are similar from one kind of bombardier beetle to another, but the shape of the spray is not. Some of the species in Africa and Asia are among the largest, at more than an inch long, and they make a louder pop than the small ones. Certain kinds give off a continuous jet, while others, such as the ones studied most carefully, shoot in rapid pulses. Researchers who investigate the beetles have measured the sounds, the heat, and the speed of the liquid as it leaves the nozzle. They have found that the spray moves outward at a speed of several yards every second, which is quick enough to reach a target a short distance away. These differences help scientists tell the many species apart and understand how each one has adapted to its own surroundings.

Scientists are not only curious about bombardier beetles, because they are also borrowing ideas from them. Engineers have studied the way the beetle keeps its mixing room small, controls the flow with a valve, and fires in pulses, and they have used these ideas to think about designing new kinds of sprayers. A fine mist can be useful for putting out fires, for delivering medicine, and for spreading liquids evenly. A creature that fits on the end of your finger has, without meaning to, given engineers a lesson in how to build a tiny, controlled explosion that is also safe for its owner. Studying a living animal in order to solve a human problem is a practice that scientists call biomimicry. Nature solved this problem long before people had invented a word for it.

The most astonishing fact about the bombardier beetle may be how ordinary its setup looks from the outside. There are no armor plates, no enormous claws, and no bright warning flag, only a small beetle walking across the ground. Inside its body, however, there is a tiny chemistry laboratory that mixes, heats, and fires a boiling spray hundreds of times a second without ever hurting its owner. If you go outside after a rain and gently lift a flat stone in damp soil, you might find a ground beetle hiding underneath it. Look quietly for a moment, and then set the stone back exactly where it was. You may have just met a real bombardier. It is a good reminder that some of the most impressive science in the world is happening quietly beneath our feet.