Bombardier Beetles and the Argument of Design

by

Copyright © 1997-2003

[Originally Posted: 1997]

[Links and References Updated: May 30, 2003]

fundamental tenet of creationism is that all life looks designed, and a commonly cited example of this design is the bombardier beetle. Supporting such a claim requires an examination of the bombardier beetle and of what "design" really means. Upon examination of these issues, however, the bombardier beetle shows evidence of evolution and seriously challenges the concept of design.

This article first looks at bombardier beetles and what makes them special; then it examines how they relate to different concepts of design-- specifically, complexity, pattern, and purpose.

What Are Bombardier Beetles?

Bombardier beetles include those ground beetles in the four tribes Brachinini, Paussini, Ozaenini, and Metriini [Aneshansley et al, 1983]--over 500 species altogether [Lawrence & Britton, 1991]. The genus Brachinus is the most widely distributed.

A bombardier beetle
(Image from The Tree of Life. Copyright © 1997
by David R. Maddison Used by permission.)

Bombardier beetles are remarkable creatures, truly deserving the attention they have received. They earned their common name from their ability to defend themselves against predators by firing a mixture of boiling-hot toxic chemicals from special glands in their posterior. In at least one species, the spray even takes the form of a pulse jet. [Dean et al., 1990] (Other species spray an unpulsed stream; most species haven't been investigated so closely.)

The mechanism of their spray works thus: Secretory cells produce hydroquinones and hydrogen peroxide (and perhaps other chemicals, depending on the species), which collect in a reservoir. The reservoir opens through a muscle-controlled valve onto a thick-walled reaction chamber. This chamber is lined with cells that secrete catalases and peroxidases. When the contents of the reservior are forced into the reaction chamber, the catalases and peroxidases rapidly break down the hydrogen peroxide and catalyze the oxidation of the hydroquinones into p-quinones. These reactions release free oxygen and generate enough heat to bring the mixture to the boiling point and vaporize about a fifth of it. Under pressure of the released gasses, the valve is forced closed, and the chemicals are expelled explosively through openings at the tip of the abdomen. [Aneshansley & Eisner, 1969; Aneshansley et al, 1983; Eisner et al, 1989]

Much creationist literature gives an inaccurate account of the process. Based on an admittedly sloppy translation of a 1961 article by Schildknecht and Holoubek, [Kofahl, 1981] Duane Gish claimed that hydrogen peroxide and hydroquinones would explode spontaneously if mixed without a chemical inhibitor, and that the beetle starts with a mix of all three and adds an anti-inhibitor when he wants the explosion. [Weber, 1981] In fact, the two do not explode when mixed, as others have demonstrated. [Dawkins, 1987, p. 86-87] (Schildknecht did propose a physical inhibitor which kept the mixture from degrading in undisected beetles; in fact, the degradation he saw was probably simply a result of exposure to the air.) Gish still used the mistaken scenario after being corrected by Kofahl in 1978. [Weber, 1981] The same mistake is also repeated in books by Hitching in 1981, Huse in 1983 and 1993, and twice in a creationist magazine in 1990 [Anon, 1990a, b].

In a creationist children's book, Rue does a better job describing the chemistry but gets the physical mechanism wrong instead, saying the liquid shoots through the firing chamber and doesn't explode until outside the beetle. "If it exploded inside, it would blow any Bombardier Beetle to smithereens." [Rue, 1984, p. 23] In fact, it is because the explosion occurs inside the firing chamber that its force can be directed against a threat.

One must wonder how much weight an argument of design carries if the people making it don't know what the design looks like.

Complexity

Just knowing what something looks like doesn't tell us whether it looks designed; for that, we must also know what "design" means. Although it's rarely defined, the most important aspect of design as it relates to creationism appears to be complexity. As Richard Lumsden says,

Systems that are of high complexity, that is functionally integrated multicomponent systems, systems that are of high specificity where only one or very few of many possible arrangements of these components works, and systems which are of low probability, at least spontaneous occurrence . . . these are the hallmarks of purposefully designed engineered systems. [Lumsden, 1995]

However, the theory of evolution also allows complex, functionally integrated, low-probability systems to arise via gradual variation and selection. For example, Darwin explained how, under his theory, a few photosensitive cells might evolve gradually into human eyes. [Darwin, 1872, chpt. 6] For complexity to be a problem for evolution, it must show some property that rules out gradual development. Michael Behe proposes such a property with the concept he calls "irreducible complexity," which he defines as "a single system composed of several well-matched, interacting parts that contribute to the basic function, wherein the removal of any one of the parts causes the system to effectively cease functioning." [Behe, 1996, p. 39] Although Behe leaves open the questions of whether bombardier beetles are irreducibly complex, Gish expresses the concept succinctly with reference to them when he says, "How are you going to explain that step-by-step by evolution by natural selection? It cannot be done!" [quoted in Weber, 1981]

Gish is wrong; a step-by-step evolution of the bombardier system is really not that hard to envision. The scenario below shows a possible step-by-step evolution of the bombardier beetle mechanism from a primitive arthropod.

1. Quinones are produced by epidermal cells for tanning the cuticle. This exists commonly in arthropods. [Dettner, 1987]
   
   
2. 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])
   
   
3. 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.
   
   
4. 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])
   
   
5. A couple invaginations (now reservoirs) become so deep that the others are inconsequential by comparison. Those gradually revert to the original epidermis.
   
   
6. 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.
   
   
7. 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.
   
   
8. 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]
   
   

9. Muscles adapt which close off the reservior, thus preventing the chemicals from leaking out when they're not needed.
   
   
10. 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.
    
    
11. 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.
    
    
12. 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]
    
    
13. The walls of that part of the output passage become firmer, allowing them to better withstand the heat and pressure generated by the reaction.
    
    
14. 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.
    
    
15. 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.

Pattern

Another aspect of design is the appearance of some kind of pattern. Again, though, evolution also predicts patterns--especially a nested heirarchical organization of characteristics--and that's the pattern we see. For example, among arthropods, insects share a set of traits that set them apart from other arthropods (six legs, three body regions, one pair of antennae, etc.); among insects, beetles are distinguished by their own set of characteristics; among beetles, the suborder Adephaga has a unique set of traits; likewise for ground beetles as a subset of Adephaga, bombardier beetles as a subset of that, and all the subgroups within them [Erwin, 1970]. Such an organization shows up not only when looking at morphological characteristics, but the same pattern shows up when looking at biochemistry, embryology, genetics, and even behavior. Although no genetic studies of bombardier beetles have been done, I can confidently predict that genetic similarities will closely match the morphological similarities which have already been worked out.

Evolution also predicts patterns of distribution, with the more similar species and groups of species generally occurring closer together. Such patterns are observed. [Erwin, 1970, pp. 184-208]

Creationism, on the subject of design, says little except that similar forms were created for similar functions and different forms were created for different funtions, [Morris, 1985, p. 70] or, briefly, that form follows function. However, that does not describe the pattern we see in nature.

The same function often takes different forms. Many ground beetles have habits and habitats quite similar to centipedes, but the two groups look nothing alike. One group of bombardier beetles (the paussines) uses the same chemical mechanism to shoot their defensive spray as other bombardier beetles, but they have a totally different method of aiming. Brachinine bombardier beetles have their gland openings at the tip of their abdomen and simply bend their abdomen to aim; paussines have their gland openings more to the side, shoot from only the chamber on the desired side, and if they want to shoot forward, move their abdomen slightly so that the opening is adjacent to a flange on their elytra that deflects the ejecta forward. [Eisner and Aneshansley, 1982] Pygidial glands are used for defense not just by bombardier beetles but by virtually all beetles in the suborder Adephaga, but the structure of the glands and the chemicals they secrete vary significantly among different families and genera of beetles. [Forsyth, 1970; Kanehisa & Murase, 1977; Moore, 1979; Eisner et al., 1977]

The same form is sometimes used for different functions. I know of no good examples among bombardier beetles, but rove beetles show an example. Many species exude defensive chemicals from the tip of their abdomen. Beetles of the genus Stenus have another use for those chemicals. When threatened while foraging on water, they touch their abdominal glands to the surface of the water. The chemicals disrupt the surface tension, which rapidly propels the beetle up to several meters. [Eisner, 1970, p. 200]

Finally, some forms have no function. Some species of bombardier beetles (and many other insects, for that matter) cannot fly but still have vestigal flight wings. [Erwin, 1970, pp. 46, 55, 91, 114-115, 119] Some may argue that the wing stubs have an as yet unknown function, but even in the remote chance that functions can be found for all vestigal wings, the situation merely changes to the previous case of different functions for the same form.

Creationists also claim that life forms were created in distinct "kinds," but these kinds also fail to appear in any tangible form. Different species are not always perfectly reproductively isolated. Some species of bombardier beetles are so similar that even experts would have trouble telling them apart. At higher levels of classification, it is a simple matter to find groups that are unambiguously isolated, but these groups are all nested within each other (a consequence of common descent), so it is entirely arbitrary which groups to label as different kinds. Would you identify "kind" with species, species group, subgenus, genus, subtribe, tribe, division, family, order, phylum, or some level of classification between these levels? If you decide that a certain amount of variation and no more is acceptable within a kind, it is always possible to find a natural grouping which includes only slightly more variation, and thus which could be reached by microevolution. Probably the best evidence for the lack of natural kinds is the inability of creationists themselves to decide what they mean.

In summary, the patterns of similarities and differences we see in nature are just what we would expect from descent with modification; they don't match what we would see in a creation where form follows function. "Kinds" are arbitrary and man-made; they can't be determined from nature.

Purpose

Finally, another aspect of design is purpose. But purpose may be even harder to distinguish than design is. It may seem obvious that the purpose of a bombardier beetle's defense mechanism is to protect it against predators--and indeed it is effective at such defense [Eisner, 1958]--but that is only our view; without reading the beetle's mind, we can't know what its purpose is. In fact, the bombardier mechanism is probably just a reflex, since it doesn't fire at some predators (such as some human collectors) and it does fire at some non-predators (such as a pair of forceps wielded by an experimenter). Ultimately, statements of purpose are statements of our own beliefs and nothing more.

Besides, an appearance of purpose is consistent with the theory of evolution. The theory says that surviving organisms have developed strategies that succeed; those which acquired failing strategies aren't around any more. Because the strategies succeed, they appear to us to have the purpose of what they succeed at. The bombardier beetle's defense doesn't work because that's its purpose; we attribute that purpose because the beetle's defense works.

Some people's belief is that the bombardier beetle's defense, whether reflexive or not, shows God's purpose. But claiming to know the mind of God is a form of hubris. The Bible makes it clear (for example, Job 37:5, Eccl. 11:5, Is. 55:8) that we can't understand God's ways.

For many creationists, purpose leads to an inextricable contradiction. They say the beetle's defense mechanism was designed, but designed for what? They also say that death wasn't part of the original design, but came later with original sin [Morris, 1985, p. 211]. If the bombardier beetle's defense was part of the original creation, it had no purpose; if it came later, it wasn't designed. And the problem involves more than their defense mechanism. All bombardier beetles are predators, and are thus themselves agents of death. Even as larvae, they are predaceous; at least two species are ectoparasitoids of the pupae of other beetles, slowly devouring and ultimately killing their helpless hosts [Erwin, 1967]. Was that aspect of its life cycle designed with the rest of the beetle?

Other Comments

In order to tell whether something looks designed, you must first be able to distinguish designed from undesigned. This immediately begs the question of what is undesigned. If you believe God created everything, then nothing is undesigned, and the claims of appearance of design fail for lack of comparison. Alternately, you can claim that only certain selected parts of the universe were designed by God.

Conclusions

Do bombardier beetles look designed? Yes; they look like they were designed by evolution. Their features, behaviors, and distribution nicely fit the kinds of patterns that evolution creates. Nobody has yet found anything about any bombardier beetle which is incompatible with evolution.

This does not mean, of course, that we know everything about the evolution of bombardier beetles; far from it. But the gaps in our knowledge should not be interpreted as meaningful in themselves. Some people are apparently uncomfortable with the idea of uncertainty, so uncomfortable that they try to turn the unknown into the unknowable. There has never been any evidence that bombardier beetles could not have evolved, but just because they couldn't explain exactly how the beetles evolved, lots of people jumped to the conclusion that an explanation was impossible. In fact, their conclusion says a lot more about themselves than about the beetles. To make such a conclusion based only on a lack of knowledge is a kind of arrogance.

Does evolution disqualify an intelligent designer? A lot of people reject the idea of evolution because they think it takes away any role for God to play in the creation of life. Such is the case, however, only for people who require God's role to fit certain narrow preconceptions of what "intelligent design" must mean. Millions of people around the world have no trouble believing in God and accepting evolution at the same time. Evolution only contradicts a man-made God that operates under man-made constraints.

Finally, remember that the general arguments used here apply to a lot more than bombardier beetles. Creationists have argued for an appearance of design in everything from bacteria flagella to butterfly metamorphosis. Those arguments all share the same fallacies; they are all based on a combination of ignorace combined with a concept of design that is indistinguishable from evolution. If a kind of design incompatible with evolution were found in biology, nobody would be more excited than the professional biologists. As yet we haven't found such a design.

References

A French translation of an earlier version version of this document can be found at http://laurent.penet.free.fr/bombardier.html#reponse.