Every member of the animal kingdom has a reliable defense mechanism that helps it against potential predators.

The bombardier beetle, for instance, mimics the action of a machine gun when it releases streams of foul-smelling liquid repeatedly from its rear to ward off its enemies.

Scientists from the University of Arizona (UA) and the Massachusetts Institute of Technology (MIT) say they have finally discovered how this little bug manages to do this feat.

Wendy Moore, an assistant professor from UA's Department of Entomology, teamed up with MIT students Eric Arndt and Christine Ortiz in a research that looked into the anatomy of the bombardier beetle and how it protects itself from attack.

"Understanding how these beetles produce—and survive—repetitive explosions could provide new design principles for technologies such as blast mitigation and propulsion," Moore said.

During their study, Moore and her team captured the bombardier beetle's behavior on camera as it defended itself.

When under threat, the bombardier beetle prepares a chemical mixture in a reaction chamber located in its stomach. The bug then heats up this compound just before it ejects its explosive load toward its attacker. The rancid smell of the chemical mixture often distracts potential predators, allowing the beetle to escape.

The researchers noted that the bombardier beetle is capable of firing its load repeatedly and accurately similar to how a machine gun operates, complete with "gun smoke" after each round.

The reaction chamber in the beetle's stomach is made of a combination of chitin, cuticle, waxes and proteins. The material also helps protect the bug from high temperatures, toxic chemicals, and even from high pressure created when it releases its defensive chemicals.

According to Moore, an earlier study conducted by Cornell University and MIT 25 years ago found that each of the bombardier beetle's blasts is a series of very rapid micro-pulses. The researchers, however, were not able to pinpoint what exactly causes the discharges to be pulsed similar to a machine gun.

"Previous researchers suggested that the pulses were caused by muscle contractions or by a fluttering of the exit duct during the explosions," she said.

To understand the firing mechanism of the bombardier beetle, the researchers examined the inside of the reaction chamber where the chemical mixture is formed. They discovered some of the chamber's regions have thin walls that are not as sturdy as others. One of the regions located near the mixing valve seems particularly thin.

Moore and her team theorized that the force of the reactions inside the beetle's stomach displaces the thin cuticle, causing it to impinge upon the valve and shutting off the flow of the chemicals temporarily.

The researchers recorded the behavior of live Brachinus elongatulus beetles, a type of bombardier beetle typically found in southern Arizona.

Out of the 500 total bombardier beetles observed by the team, 14 individual beetles were able to generate 30 discharges. The recordings from the experiment helped the researchers break down how the bug fires it load.

Their findings showed that whenever chemicals pass through one of the valves inside the reaction chamber, they are combined with enzymes. This action results in a sudden burst of water vapor, oxygen gas and heat, and subsequently ejects the toxic spray out of the beetle's rear.

The burst in the reaction chamber expands the thin cuticle wall, causing each chemical release to be pulsed.

"It turns out the expansion membrane of the reaction chamber acts as a passive closure mechanism, which is something that had not been described or even predicted before this study," Moore explained.

The University of Arizona and the Massachusetts Institute of Technology study is published in the journal Science.

Photo: David Hill | Flickr