Bombardier Beetle - Evolution of The Defense Mechanism

Evolution of The Defense Mechanism

When a bombardier beetle is threatened by a predator or an offensive invader of any kind, at the appropriate point of approach, the bombardier beetle swings its tail end around, and hot, noxious fluid heated to 100 °C (212 °F) is explosively released from twin combustion tubes into the face of the enemy. Various quinones are commonly produced by cells in the skin of insects to harden their skin into a cuticle, and as they taste bad to predators, many insects secrete them to deter predators. Where there are indentations in the cuticle, these vary to form little sacs that store the deterrent quinone. Where predators develop resistance to this chemical, other related chemicals such as hydroquinone develop, and in many beetles, specialised cells secrete hydroquinone from glands connected by ducts to a reservoir sac, which can be closed off by muscles to stop leakage.

While all carabid beetles have this sort of arrangement, in some cases, hydrogen peroxide, which is a common by-product of the metabolism of cells, is mixed in with the hydroquinone, and some of the catalases that exist in most cells makes the process more efficient. The chemical reaction produces heat and pressure, which pushes out the discharge when the insect is attacked, as in the beetle Metrius contractus, which produces a foamy discharge. In other bombardier beetles, the muscles stopping leakage have a flap forming a valve to ensure that the pressure pushes the discharge out, and muscles controlling the outlet have developed nozzles that can direct an explosive reaction to squirt the deterrent chemicals at an attacker. The exact sequence is unknown, and it is common for features with one purpose to become useful for other purposes, a process called exaptation. More detailed scenarios have been developed showing a series of small changes that could have led to this mechanism.

The beetle's explosive defensive system has become a focus for creationists. Intelligent design proponent Michael Behe contends that the system is an example of irreducible complexity and therefore could not have evolved, an argument which is rejected by scientists as logically and factually flawed.