Bombardier beetles and their caustic chemical cannon

In the video above, Max Barclay, Senior Curator in Charge of the Museum's beetle collection, explains the science behind a bombardier beetle's detonations.

What is a bombardier beetle?

There are hundreds of species of bombardier beetles living around the world, found on every continent except Antarctica. They are small ground beetles (Carabidae) that can typically be found in leaf litter and under stones in woodlands and grasslands.

Bombardier beetles and some of their relatives are known for using chemical defence strategies.

'A lot of beetles use chemicals simply because they smell or taste disgusting,' explains Max.

In the UK, Charles Darwin was once on the receiving end of a chemical defence deployed by a ground beetle species closely related to bombardier beetles. He detailed the experience in a letter to naturalist Leonard Jenyns in 1846:

'Under a piece of bark I found two Carabi (I forget which) and caught one in each hand, when lo and behold I saw a sacred Panagaeus cruxmajor; I could not bear to give up either of my Carabi, and to lose Panagaeus was out of the question, so that in despair I gently seized one of the Carabi between my teeth, when to my unspeakable disgust and pain the little inconsiderate beast squirted his acid down my throat…'

Max continues, 'There are a number of related beetles that have derived a system of mixing two chemicals together. Bombardier beetles are the ones that have taken it to the greatest extreme.'

Chemical defence mechanisms vary among bombardier beetle species. Some emit chemicals subtly, such as the foamy secretion of Metrius contractus that clings to the beetle's body as it is released from the abdomen. If attacked from the front, it can move the foam towards its head along tracks on its hardened outer wings (elytra).

But the best-known bombardier beetles are those that deploy explosions to defend themselves. With an audible pop, these beetles spray a concoction of boiling, irritating chemicals at predators that get too close. The beetles have plenty of ammo and can rapidly fire their chemicals over and over again.

How do bombardier beetles survive their own explosions?

In explosive bombardier beetle defences, the reaction of the two chemicals mixing together is highly exothermic. The spray released from the beetle is thought to be up to a scalding 100°C. But how does such a small creature manage to carry around such violently reacting chemicals?

Max explains, 'The two chemicals - one is hydrogen peroxide and the other is a hydroquinone - are stored in separate little sacs. The beetle has a chamber at the back of the abdomen in which it mixes them.'

The beetle will only mix the two chemicals at the exact moment it needs to defend itself, and the mixture is almost instantaneously ejected with force out of the tip of the abdomen. The tough reaction chamber at the rear end of the beetle protects the rest of the insect's internal organs from taking damage.

'Even if there were an accidental detonation it would come out of the back end of the beetle, go "pop" and just fire a charge off into nothing. It wouldn't do the beetle any harm,' says Max.

The hot fluid the beetles produce is an irritant, released in a volley of rapid pulses rather than as one continuous stream. Some beetles have exceptional aim.

The African bombardier beetle (Stenaptinus insignis) can twist its abdomen to fire its spray in almost any direction in response to a threat, even targeting sites on its own back. Scientists have suggested that this incredible marksmanship may have evolved to give the beetles a fighting chance against foes like ants that can attack from any direction.

Diners' remorse for hungry toads

Bombardier beetles have evolved an undeniably excellent defence mechanism, but they can occasionally be snapped up by a larger predator. When this happens, however, some are primed for a last stand.

One of the predators that bombardier beetles have to be on the lookout for are toads. Toads are ambush predators, easily catching and swallowing animals smaller than themselves, usually invertebrates such as beetles.

If their prey is chemically defended, toads may simply spit out their meal. But if they do swallow a toxic animal, toads can turn their stomachs inside out to vomit what they have eaten. 

A study found that bombardier beetles have a surprisingly high survival rate when swallowed by toads, and that this appears to the be the result of their explosive defences.

Scientists fed adult Asian bombardier beetles (Pheropsophus jessoensis) to two species of toad. While the toads would quickly catch and swallow the beetles, 43% vomited them out between 12-107 minutes later.

The scientists determined that the beetles were deploying their chemical defence whilst inside the toads' stomachs. This encouraged the toads to abandon their latest meal by everting their stomachs. Beetles that had their chemical spray reserves depleted before ingestion were all digested by the toads. 

It was found that there appears to be a relationship between the chances of escape and the size of predator and prey. Larger beetles more frequently escaped from the toads, and smaller toads were more likely to have a vomiting response.

The ejected beetles were noticeably covered in a lot of mucus, suggesting that they had made it into the toad's digestive system. However, how they avoid being digested is not known for certain. It may be that bombardier beetles have evolved a high tolerance to the gastric juices of amphibians, or that the chemicals the beetles emit might reduce the toad's ability to digest them.

In this study, all the beetles that survived being swallowed were active when t­­hey were ejected, with over 93% surviving for at least two weeks after the experiment concluded. Additionally, swallowing the beetles wasn't fatal for any of the toads involved.