text: Holy smokes! Researchers found the bombardier beetle (pictured) mimics a machine gun by mixing chemicals in an internal 'reaction chamber' A beetle with a machine gun built into its rear end sounds like something from a comic or sci-fi film - or the stuff of nightmares. But researchers have discovered the Bombardier beetle mimics this powerful weapon by mixing chemicals in an internal 'reaction chamber' in its stomach. When threatened, the diminutive bug can then repeatedly fire streams of foul-smelling liquid from its rear, complete with 'gun smoke'. It can even precisely aim the nozzle at the attacker. This behaviour has been caught on camera by Wendy Moore and her colleagues at the University of Arizona. Many beetles secrete foul-smelling or bad-tasting chemicals from their abdomens to ward off predators, but bombardier beetles take it a step further. When threatened, they combine chemicals in an explosive chamber in their abdomen to simultaneously heat and propel their defensive load as a boiling hot spray. Assistant Professor Moore, from the Department of Entomology at the university, has published a paper along with her team that not only reveals the beetle's firing apparatus in never-before-seen detail, but also solves a long-standing mystery of how the animals achieve their incredible rapid-fire capabilities. 'Understanding how these beetles produce - and survive - repetitive explosions could provide new design principles for technologies such as blast mitigation and propulsion,' she said. The study was led by doctoral student Eric Arndt and Christine Ortiz in the Department of Materials Science and Engineering at Massachusetts Institute of Technology (MIT) in Cambridge. Arndt and Ortiz teamed up with Professor Moore to study the anatomy of the beetle's reaction chamber, which is constructed of cuticle, a composite of chitin, proteins and waxes. The material, which also makes up the exoskeleton of insects, protects the beetle from the toxic chemicals, high temperatures and high pressures during explosions. A real hot shot! The research, including video footage (shown above) not only reveals the beetle's firing apparatus in never-before-seen detail, but also solves a long-standing mystery of how the bugs achieve their rapid-fire capabilities . The mechanism of the bombardier beetle's spray pulsation had not been understood because previous studies, relying on external observations, could not probe exactly what happens inside the reaction chamber of the chemical gun. African bombardier beetles can accurately hit targets in any direction. Researchers at Cornell University snapped another type of Bombardier beetle, Stenaptinus insignis' attack in 1999, to reveal they use the explosive defensive mechanism to buy time when they are under attack. The beetle is quite vulnerable on the ground because unlike most insects such as bees and flies, it can't take off instantly. This means that the slow process of unfurling its wings could cost the insect its life if under attack from ants, for example. So it attacks its attackers in turn, giving it time to get airborne. The spray itself contains p-benzoquinones - compounds known for their irritant properties. The chemicals are stored separately in the creature's abdomen and then combine explosively, explaining why they are ejected at such high speed and at 100 C. 'Twenty-five years ago, a team of scientists from Cornell University and MIT discovered that each blast from the bombardier beetle is actually a series of extraordinarily fast micro-pulses,' Assistant Professor Moore said. 'What wasn't known is what causes each discharge to be pulsed, like 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.' Using enzymes that digest muscle and fat, the researchers cleaned the reaction chamber, which enabled them to examine it under a microscope. While studying the shape of the reaction chamber, they noticed that some regions of the chamber wall were thin and not as strong as others, and that one region near the mixing valve looked particularly thin. They predicted that this thin cuticle would be displaced by the force of the reactions in such a way that it would impinge upon the valve - temporarily shutting off the flow of reactants. To investigate this hypothesis, the team directly observed the internal dynamics of spray pulsation in live Brachinus elongatulus beetles, a type of bombardier beetle, which is found throughout southern Arizona. Of 500 beetles, the researchers were able to record 30 discharges from 14 individuals. This was enough to provide high-definition, high-speed video of what happens inside a bombardier beetle's reaction chamber when it goes off. All guns blazing: 'Twenty-five years ago, a team of scientists from Cornell University and MIT discovered that each blast from the bombardier beetle is actually a series of extraordinarily fast micro-pulses,' Assistant Professor Moore said, which led to the machine gun analogy. A stock image of a machine gun is shown . It turns out the pulses are generated in a passive way, not through an active process involving muscle contraction, as previously thought. As the chemicals pass through a valve into the reaction chamber, they mix with enzymes and explosively liberate oxygen gas, water vapor and heat, propelling a hot, noxious spray down the nozzle and out the exit pore. Each explosion inside the chamber forces a highly elastic region of the chamber wall - the thin cuticular area Moore found in her earlier dissections - to expand and impinge on the valve that separates the two chambers, temporarily cutting off the flow of reactants and resulting in a pulsed delivery. '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,' Professor Moore said. 'We also discovered that the chamber's anatomy varies between female and male beetles.' While it is still not completely clear why it might be beneficial to the beetle to have a pulsed delivery, she suspects that dividing the spray into very short, rapid pulses allows the beetle to sustain the energy of the blast and its small body size at the same time. 'By having a pulsed delivery, these small beetles produce a relatively large amount of defensive spray, which they can aim precisely and with great force and speed,' she said. 'This is truly one of the most remarkable and elegant defensive mechanisms documented to date.' When threatened, the bombardier beetle repeatedly fires streams of liquid from its rear, complete with 'gun smoke'. The expulsion of fluid is shown in the images above, which come from a 1999 study that found another bug, Stenaptinus insignis, uses its weapon to 'buy time' when under attack because it can't fly away .
target: Bombardier beetle mimics machine gun using chemicals in its stomach .
  As chemicals pass into the abdomen they mix with enzymes and explode .
  Each explosion causes the foul-smelling liquid to be forced from its rear .
  It 'pulses' repeatedly from the beetle's rear - and the bug can even aim .