The scenario below shows a possible step-by-step evolution of the bombardier beetle mechanism from a primitive arthropod.
Quinones are produced by epidermal cells for tanning the cuticle. This exists commonly in arthropods. [Dettner, 1987]
Some of the quinones don’t get used up, but sit on the epidermis, making the arthropod distasteful. (Quinones are used as defensive secretions in a variety of modern arthropods, from beetles to millipedes. [Eisner, 1970])
Small invaginations develop in the epidermis between sclerites (plates of cuticle). By wiggling, the insect can squeeze more quinones onto its surface when they’re needed.
The invaginations deepen. Muscles are moved around slightly, allowing them to help expel the quinones from some of them. (Many ants have glands similar to this near the end of their abdomen. [Holldobler & Wilson, 1990, pp. 233-237])
A couple invaginations (now reservoirs) become so deep that the others are inconsequential by comparison. Those gradually revert to the original epidermis.
In various insects, different defensive chemicals besides quinones appear. (See Eisner, 1970, for a review.) This helps those insects defend against predators which have evolved resistance to quinones. One of the new defensive chemicals is hydroquinone.
Cells that secrete the hydroquinones develop in multiple layers over part of the reservoir, allowing more hydroquinones to be produced. Channels between cells allow hydroquinones from all layers to reach the reservior.
The channels become a duct, specialized for transporting the chemicals. The secretory cells withdraw from the reservoir surface, ultimately becoming a separate organ.
This stage – secretory glands connected by ducts to reservoirs – exists in many beetles. The particular configuration of glands and reservoirs that bombardier beetles have is common to the other beetles in their suborder. [Forsyth, 1970]
Muscles adapt which close off the reservior, thus preventing the chemicals from leaking out when they’re not needed.
Hydrogen peroxide, which is a common by-product of cellular metabolism, becomes mixed with the hydroquinones. The two react slowly, so a mixture of quinones and hydroquinones get used for defense.
Cells secreting a small amount of catalases and peroxidases appear along the output passage of the reservoir, outside the valve which closes it off from the outside. These ensure that more quinones appear in the defensive secretions. Catalases exist in almost all cells, and peroxidases are also common in plants, animals, and bacteria, so those chemicals needn’t be developed from scratch but merely concentrated in one location.
More catalases and peroxidases are produced, so the discharge is warmer and is expelled faster by the oxygen generated by the reaction. The beetle Metrius contractus provides an example of a bombardier beetle which produces a foamy discharge, not jets, from its reaction chambers. The bubbling of the foam produces a fine mist. [Eisner et al., 2000]
The walls of that part of the output passage become firmer, allowing them to better withstand the heat and pressure generated by the reaction.
Still more catalases and peroxidases are produced, and the walls toughen and shape into a reaction chamber. Gradually they become the mechanism of today’s bombardier beetles.
The tip of the beetle’s abdomen becomes somewhat elongated and more flexible, allowing the beetle to aim its discharge in various directions.
Note that all of the steps above are small or can easily be broken down into smaller steps. The bombardier beetles’ mechanism can come about solely by accumulated microevolution. Furthermore, all of the steps are probably advantageous, so they would be selected. No improbable events are needed. As noted, several of the intermediate stages are known to be viable by the fact that they exist in living populations.
The scenario above is hypothetical; the actual evolution of bombardier beetles probably did not happen exactly like that. The steps are presented sequentially for clarity, but they needn’t have occurred in exactly the order given. For example, the muscles closing off the reservior (step 9) could have occurred simultaneously with any of steps 6-10. Determining the actual sequence of development would require a great deal more research into the genetics, comparative anatomy, and paleontology of beetles. The scenario does show, however, that the evolution of a complex structure is far from impossible. The existence of alternative scenarios only strengthens that conclusion.
A few other points regarding this scenario should be stressed:
Parts of an integral system need not be created specifically for that system, and features used for one purpose can be used for another purpose. The quinones which originally served to darken the cuticle later became used for defense. The muscles which control the valve and squeeze the reservior could easily be adapted from muscles which already existed in the beetle’s abdomen.
Complexity can diminish as well as increase. In the proposed scenario, most of the invaginations in which quinones appeared later disappeared. In other cases, a structure could orginally develop with a complex supporting structure which later decreases or disappears.
Two or more parts can evolve a little at a time in conjunction with each other. The strength of the reaction chamber walls and the amount of catalases increased together. One did not have to be present in its final form before the other existed.
Any of these points makes it possible for complexity, even irreducible complexity, to evolve gradually. Many people will still have trouble imagining how complexity could arise gradually. However, complexity in other forms arises in nature all the time; clouds, cave formations, and frost crystals are just a few examples. Most important, nature is not constrained by any person’s lack of imagination.