Bombardier beetles are a family of ground beetles comprising over 500 individual species. When aggravated they have the ability to blast predators with a noxious mix of chemicals from glands in their abdomen, hence the name 'bombardier' beetle (a bombardier was a soldier in charge of heavy artillery).
Cells in the beetle secrete hydroquinones and hydrogen peroxide which collect in a reservoir that opens through a valve controlled by a muscle into a reaction chamber. The reaction chamber has thick walls lined with cells secreting catalases and peroxidases. When the hydroquinones and hydrogen peroxide are forced through the valve into the reaction chamber, the hydrogen peroxide is broken down by the catalases and peroxidases and oxidized into p-quinones. This process releases free oxygen and generates heat that brings the mixture to boiling point and vaporizes about 20% of it. The heat and vapor creates pressure that forces the valve closed, and squirts the chemicals through openings at the tip of the abdomen.
So complex and exact is the defence mechanism of the bombardier, involving secretion of hydroquinones and hydrogen peroxide in a 'reaction chamber' inside the very bowels of the beetle, that many creationists have championed the bug as evidence that evolutionary theory is a fallacy.
Experimental evidence shows that hydrogen peroxide and hydroquinone do not explode spontaneously, as creationist writers have sometimes claimed, if mixed without a chemical inhibitor. The liquid explodes inside the beetle. This is the reason that its force can be directed against a threat. If the liquid exploded outside the beetle, it would be a less effective defence because the beetle would not be able to direct its force against its attacker.
Evolutionary biologists have postulated evolutionary accounts of the origin these traits.
One such process could be:
- 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 reservoir.
- 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 reservoir, 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.
- Bomby the Bombardier Beetle, by Hazel Rue, ISBN 0-932766-13-7
- Armitage, Mark H. and Mullisen, Luke, Preliminary observations of the pygidial gland of the Bombardier Beetle, Brachinus sp. Journal of Creation 17(1):95–102, April 2003
- Dawkins, Richard, 1987. The Blind Watchmaker, Norton, NY p. 86-87
- Aneshansley, Daniel J. & T. Eisner, 1969. Biochemistry at 100C: explosive secretory discharge of bombardier beetles (Brachinus). Science 165: 61-63.
- Aneshansley, D.J., T.H. Jones, D. Alsop, J. Meinwald, & T. Eisner, 1983. Thermal concomitants and biochemistry of the explosive discharge mechanism of some little known bombardier beetles. Experientia 39: 366-368
- Eisner, Thomas, George E. Ball, Braden Roach, Daniel J. Aneshansley, Maria Eisner, Curtis L. Blankespoor, & Jerrold Meinwald, 1989. Chemical defense of an Ozaenine bombardier beetle from New Guinea. Psyche 96: 153-160.
- Bombardier Beetles and the Argument of Design by Mark Isaak, 1997