Research sheds light on beetle’s explosive rear end

Bombardier beetles defensively expel boiling chemicals from their abdomens 

The bombardier beetle is a unique creature with a powerful defense mechanism which mimics a machine gun. When threatened, the beetle discharges a jet of boiling vapour and an explosive chemical called benzoquinone toward its target.
New research has revealed how bombardier beetles produce their potent chemical defense. Researchers at the Massachusetts Institute of Technology and the Phoenix-based University of Arizona published a paper in the journal Science in May which examined how these insects manage to exhibit such a sophisticated defense mechanism.
The study used a specialized X-ray device and a high-speed camera to internally examine the abdomens of hundreds of bombardier beetles undergoing defensive explosions.
The experiment revealed that the secret of the beetles’ explosive spray is hidden in their pygidial glands. The bombardier beetle has two pygidial glands, each of which contains a reservoir chamber, a reaction chamber, and an exit channel which opens at the abdomen tip. The reservoir chamber contains a mixture of reactive chemicals, and the reaction chamber contains enzymes which accelerate chemical reactions. A valve separates the reservoir chamber and the reaction chamber, and when the beetle is under threat this valve opens and its muscles contract to expel the liquids from these chambers.
When the chemicals combine, the resulting explosion generates temperatures of up to 100 C within the beetle. These explosions create a high amount of pressure which causes the chemical concoction to spray out of the beetle’s exit channel. This spraying occurs at a rate of 368 to 735 pulses per second.
The bombardier beetle has many effective ways of ensuring its own body doesn’t explode while creating these high-pressure chemical mixtures. The study’s authors believe the pulsing nature of the spray helps protect the interior of the beetles by allowing the reaction chamber to cool down before the next pulse. The pygidial glands are also protected by a layer of cuticle made of chitin, protein and wax. This thick cuticle is essential for the beetles to withstand the toxic chemicals, extreme heat and extreme pressures which they create during defensive expulsions.
According to the study, the bombardier beetle’s complex expulsion system is one of the most powerful defense mechanisms in the animal kingdom. It also says the bombardier’s evolutionary shift from expelling a continuous spray – which still exists in a closely related beetle species – to its current pulse spray granted the insect a more effective system of defense. Understanding how these beetles reproduce and adapt to their environments may help provide novel designs in blast mitigation and propulsion technologies as well.
The bombardier beetle is typically found in dry, sunny grasslands in southern Europe and northern Africa. Its name is derived from the Latin word “crepitans,” meaning “crackle,” in reference to the noise it makes when the volatile liquid spray is expelled.

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