How a Bombardier Beetle Defends Itself

Survival techniques of insects and small animals in a fiercely competitive environment take many forms. For example, chameleons have developed the ability to
change color to match their surroundings and the butterfly Limenitis has evolved into a form that mimics the poisonous and unpleasant-tasting monarch butterfly (Danaus). A less passive defense mechanism is employed by bombardier beetles (Brachinus), which repel predators with a “chemical spray.”

The bombardier beetle has a pair of glands at the tip of its abdomen. Each gland consists of two compartments. The inner compartment contains an aqueous solution of hydroquinone and hydrogen peroxide,
and the outer compartment holds a mixture of enzymes. (Enzymes are biological molecules that can speed up a reaction.) When threatened, the beetle squeezes some fluid from the inner compartment into the outer compartment, where, in the presence of the enzymes, an exothermic reaction takes place:

To estimate the heat of reaction, let us consider the
following steps:

Recalling Hess’s law, we find that the heat of reaction for (a) is simply the sum of those for (b), (c), and (d). Therefore, we write :

The large amount of heat generated is sufficient to bring the mixture to its boiling point. By rotating the tip of its abdomen, the beetle can quickly discharge the vapor in the form of a fine mist toward an unsuspecting predator. In addition to the thermal effect, the quinones also act as a repellent to other insects and animals. One bombardier beetle carries enough reagents to produce 20 to 30 discharges in quick succession, each with an audible detonation.