Bomb(ardier)s away!

The astute of you may have noticed that we’re running a little bit behind here, post-wise. So, I’m going to switch things around a little bit, and give us another break from the periodic table. It’s still Chemistry Wednesday, but we’re going to take a look at another piece of applied chemistry, this time, chemical ecology. Or chemical entomology, if you want to call it that. Or exploding beetles of doom, if you want to be awesome and call it that.

Definitely exploding beetles of doom.

Yes, the bombardier beetle, an organism so bizarre that its very existence is a fundamental part of the intelligent design argument - the beetle displays such an off-the-wall defense mechanism that it couldn’t possibly have evolved. Well, actually, it could have, and it did, and we’ll see why and how in a moment. But first, the chemistry part. And, in the interests of full disclosure, I must say that the guy who discovered all of this was one of my favorite professors in college. Hats off, Dr. Eisner.

Well, actually, first a crash course in the beetles. Bombardier beetles belong to the Carabidae family. These are ground beetles - they’re quick and nimble scuttlers, but not particularly strong or fast flyers. When Carabidae beetles get in trouble, they need a response other than taking wing, and boy, do bombardiers ever have a response.

Imagine that you are an ant. There, on the ground ahead of you, is a large, potentially succulent bombardier beetle. It’s got small, tightly furled wings - it’s going to be a ponderous process of getting airborne, and you’re a quick little thing. So, you run at the beetle. Bang! You’ve suddenly been hit with an extremely well-targeted, 100 degree Centigrade, chemically irritating fluid. What just happened?

The ant's eye view of what just happened.

Say hello to benzoquinones! And some other things. Benzoquinone, specifically 1,4 benzoquinone, is an extremely strong irritant, one that can cause blisters, blindness and respiratory damage in humans (and most other life forms). Clearly, being able to squirt this stuff at an attacker is a good survival strategy. It’s like taking the painful, itchy experience of encountering a poison sumac, cubing the pain, and subjecting anything that gets near the tree, not just touches it. Bombardier beetles aren’t overly fond of benzoquinone, mind. Part of the precision inherent in spraying is to allow the beetles to avoid accidentally spraying themselves.

Bombardiers avoid any negative effects of carrying this stuff around inside of them by, well, not carrying it around inside them. Instead, bombardiers carry precursors to benzoquinones, and combine these precursors when threatened, in order to synthesize the irritant spray. Bombardiers have a specialized arrangement of glands at the rear of the body - two glands empty into a reaction chamber, which then empties into a firing mechanism. The reaction chamber, as you might expect, is made of rather resistant tissue.

With a name like "explosion chamber", it would almost have to be.

One of the two glands contains hydroquinones (a chemical that can be an irritant in sufficiently high doses, and is involved in regulating melanin) and hydrogen peroxide, which appears in everything from topical antiseptics to rocket fuel. The other chamber contains a mixture of enzymes. When the two glands are emptied into the reaction chamber, enzymes break hydrogen peroxide down into hydrogen and oxygen. The oxygen then converts the hydroquinones to quinones by removing two hydrogen atoms from the hydroquinone molecule.

Not pictured: heat

Ok, that explains the synthesis of a toxic spray, but not the explosive burning part. Ah, but it does. Remember that bit where hydrogen peroxide was split in half, generating oxygen? As we’ll see next week, oxygen is a very reactive element. That’s where the explosion comes from. The heat is the result of the reaction being exothermic. Exothermic reactions release heat. Sometimes, the heat is barely perceptible, and sometimes it heats the resultant solution to the boiling point. Really, it’s just a matter of magnitude. And for the record, the reaction is controlled, so there’s no risk of the beetle exploding - the reaction chamber can withstand the force of the explosion, particularly since said explosion is directed outward.

Now, we’ve taken a look at how the chemistry works. How on Earth did all of this evolve? Well, as tends to be the case in evolution, bombardier beetle spray arose through a sequence of relatively minor tweaks to existing mechanisms. Let’s run this backwards.

There is a beetle in the Carabidae family that synthesizes the same irritating mix of quinones, but rather than shooting them out at high temperature, exudes them as a high-temperature foam which bubbles off the resistant carapace of the beetle (the foaming action should be familiar to anyone who has ever used hydrogen peroxide to clean out a cut). The reaction is still exothermic and explosive, but it does not take place within a reaction chamber that empties into a firing mechanism. You might see a flaw with this method, however. By exuding the irritant as a foam, the beetle can only deter predators that have made it onto the body - a really determined predatory might be able to hang on. Beetles which could spray the exudation would have a higher chance of surviving encounters with a predator. It’s a relatively minor evolutionary matter for the reaction chamber to become more rigid, and for an aiming mechanism to develop.

 

It's the Windows 98 of bombardiers. Almost there.

All right, but how do we get to the hot, toxic exudation in the first place? Well, any number of insects secrete hydroquinones as defensive compounds. Most insects also produce small amounts of hydrogen peroxide as a byproduct of metabolic reactions. Over time, a beetle which initially secreted hydroquinone via a gland and duct arrangement may have begun to combine hydrogen peroxide with hydroquinone, generating both a more toxic secretion, and a secretion with more temperature oomph. It’s easy to see the evolutionary progression from a beetle which exudes a hydroquinone solution to a beetle which combines hydroquinone with hydrogen peroxide before exuding it.

Hydroquinone, in turn, is a tweak on a more primitive defensive compound, quinone. Quinone was not originally a defensive compound - it had other, structural roles to play in insects, but eventually began to be used as defensive secretions, as they have an unpleasant taste. What was that original structural role? That would be protective pigmentation - various quinone derivatives are widely used in order to provide some protection from solar radiation. You might be familiar with one of those derivatives - melanin.

So, however exotic the chemistry of the bombardier beetle may be, just remember, it’s just an evolutionary hop, skip and jump away from human skin tone. That’s wicked cool.