Bombardier beetles – explosions of smoke in your face

I was very positively surprised by the response to my previous blogpost about Epomis. In fact, it now seems that this post is the most popular one on the blog, even more than the ones about the botfly and my NZ accident. How do I top it? Only time will tell. In the meantime, I wanted to mention some of the other ground beetles (family Carabidae) that share the habitat with Epomis. You see, when you start flipping stones and pieces of wood scattered around rain-pools you encounter many carabids. But one group really stands out in appearance, and, as much as it is hard to believe, in sound: the bombardier beetles.

Bombadier Beetle (Brachinus crepitans), Golan Heights, Israel

Bombadier Beetle (Brachinus crepitans), one of the cutest species of ground beetles. Golan Heights, Israel

Aggregation of bombardier beetles (Brachinus alexandri) and other beetles under a rock. Central Coastal Plain, Israel

An aggregation of several beetle species found under a rock. Bombardier beetles (Brachinus alexandri) can be seen on the right. Also appearing in this photo: Chlaenius aeneocephalus (Carabidae, metallic colors), and Cossyphus rugulosus (Tenebrionidae) – beautiful beetles camouflaged as seeds! Central Coastal Plain, Israel

Bombardier beetles is a large group comprised of several Carabidae tribes. Here I refer mainly to species of the genus Brachinus. These are small to medium sized beetles, usually with striking aposematic coloration: the body and limbs are bright orange, while the elytra (wing covers) are usually dark green or brown, sometimes with a metallic sheen. These colors serve as a reminder for potential enemies that these beetles can deploy a powerful weapon: an explosion of hot chemicals, which can be aimed at almost any direction.

Bombardier beetles (left - Brachinus alexandri; right - Brachinus berytensis). When disturbed, the beetle shoots hot chemicals through their anus in an explosion that harms the attacker. Central Coastal Plain, Israel

Two common species of bombardier beetles from Israel: left – Brachinus alexandri; right – Brachinus berytensis

Much has been written about the mechanics and evolution of the beetles’ chemical defense. In short, when provoked the beetle releases two chemicals, hydroquinone and hydrogen peroxide, into a chamber in its abdomen. This mixture, when comes in contact with a catalyst, turns highly combustible due to the oxidation of hydroquinone and the breakdown of hydrogen peroxide to oxygen and water. The chemical reaction starts inside the chamber with temperatures reaching 100°C, and the high-pressure buildup causes the explosion. Then all the beetle has to do is to aim its “nozzle” and fire! The result is a smoke cloud of chemicals at extremely high temperatures. It can momentarily paralyze or even kill arthropod enemies, such as ants and spiders. To us humans (=entomologists who collect the beetles with bare hands) the damage it causes is not so severe, usually nothing but a small stain of burnt tissue, but the effect is coupled with a startling popping sound, and that might be enough for the beetle to escape from a large predator. This complex defense mechanism was used by creationists as an example for intelligent design in debates against evolution. However, it can be easily demonstrated that by gradually increasing the concentration of hydrogen peroxide this defense could evolve in incremental steps without risking the beetles’ existence. If you are still confused, I highly recommend watching Richard Dawkins explaining it here.

Damage to skin caused by bombardier beetle (Brachinus berytensis) chemical defense

Damage to skin caused by bombardier beetle’s (Brachinus berytensis) chemical defense. Not much.

I feel that I must stop here for a brief public service announcement: There are several videos showing the beetle’s defense (you can google them), almost all of them depict the beetle being held in place with either glue or a pair of tweezers. I would like to argue that unless this is being done for research purposes, these actions border on animal cruelty. Sure, it is strange to hear such a statement coming from someone who fed live amphibians to beetles. Still, I want to stress that in the case of the bombardier beetles this is highly unnecessary. The beetles will still put up the same “show” if poked or gently lifted, without causing them much stress and damage, as can be seen from this short video I took almost a decade ago (I mean it, this is a really old video, so please do not judge the quality):

The species shown in the video is Brachinus bayardi, one of the largest species found in Israel:

Bombardier beetle (Brachinus bayardi), Central Coastal Plain, Israel

Bombardier beetle (Brachinus bayardi), Central Coastal Plain, Israel. These beetles are strictly nocturnal, and can be found running on muddy banks of rain-pools in search of prey.

While the chemical defense of the bombardier beetle alone is interesting enough, there is another aspect in their life history that is fascinating. In most species, the adult bombardier beetles are predators of small, soft-bodied invertebrates, but as larvae they feed solely on pupae of other beetles found in the same humid habitat, usually diving beetles (family Dytiscidae) and water scavenger beetles (family Hydrophilidae). This makes them parasitoid insects – their larvae are completely dependent on another insect for completion of their development, usually with fatal consequences to the host. While most parasitoid insects are wasps and flies, in beetles this way of life is relatively uncommon, with only a handful of beetle families exhibiting a parasitoid life history. Despite searching for years, I have yet to find larvae of bombardier beetles, and my attempts to obtain larvae from captive adults has failed so far. I hope this will change one day.

5 thoughts on “Bombardier beetles – explosions of smoke in your face

  1. Here are some photos depicting their larval natural history:

    interestingly, their Asian relatives, Pheropsophus, are also parasitoids in their larval stage, feeding on the eggs of mole crickets. They are known in Chinese as “farting carabids”, and can be quite common around crop fields.

    Like their Carabus relatives, these beetles are also drawn to alcoholic drinks like beer, and can be caught in large numbers with pitfall traps using beer as an attractant.

    • Thank you for the link, I added it to the text.
      Pherosophus are really impressive beetles. I know of one species occurring in the Israeli desert, but despite searching for it many times (even with the help of pitfall traps) I have never been able to find it.
      By the way, you have very nice photos on your Flickr, thank you for sharing!

  2. I am loving the Carabid posts! 😀 These beetles are quite interesting, I love how amazing and complex their defense system is. Too bad they are parasitoids, makes it really hard to breed in captivity.

    On a side note, I really love those Cossyphus rugulosus, I so wish they were in the U.S., I would totally want to try to breed them!

    Keep up the great blog posts! 🙂

    • Thank you! It is possible to breed Brachinus, but one has to establish a working colony of a hydrophilid species first to obtain pupae.
      Cossyphus are amazing when seen moving… hard to believe that they are not seeds. Unfortunately as far as I know larvae of Cossyphus were never described and are currently unknown, meaning they probably have a complex biology.

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