The spray is produced from a reaction between two hypergolic chemical compounds, hydroquinone and hydrogen peroxide, which are stored in two reservoirs in the beetle's abdomen. When the aqueous solution of hydroquinones and hydrogen peroxide reaches the \"vestibule\" (Eisner's word), catalysts facilitate the decomposition of the hydrogen peroxide and the oxidation of the hydroquinone. Heat from the reaction brings the mixture to near the boiling point of water and produces gas that drives the ejection. The damage caused can be fatal to attacking insects. Some bombardier beetles can direct the spray in a wide range of directions.
Bombardier beetles inhabit all the continents except Antarctica. They typically live in woodlands or grasslands in the temperate zones but can be found in other environments if there are moist places to lay their eggs.
Most species of bombardier beetles are carnivorous, including the larva. The beetle typically hunts at night for other insects, but will often congregate with others of its species when not actively looking for food.
When the beetle feels threatened it opens a valve which allows the aqueous solution from the reservoir to reach the vestibule. The catalases lining the vestibule wall facilitate the decomposition of hydrogen peroxide, as in the following theoretical reaction:
This reaction is very exothermic, and the released energy raises the temperature of the mixture to near 100 C, vaporizing about a fifth of it. The resultant pressure buildup forces the entrance valves from the reactant storage chambers to close, thus protecting the beetle's internal organs. The boiling, foul-smelling liquid is expelled violently through an outlet valve, with a loud popping sound. The beetles' glands store enough hydroquinone and hydrogen peroxide to allow the beetle to release its chemical spray roughly 20 times. In some cases this is enough to kill a predator. The main component of the beetle spray is 1,4-benzoquinone, an irritant to the eyes and the respiratory system of vertebrates.
The flow of reactants into the reaction chamber and subsequent ejection occur in a series of about 70 pulses, at a rate of about 500 pulses per second. The whole sequence of events takes only a fraction of a second. These pulsations are caused by repeated microexplosions which are the results of the continuous pressure on the reservoir and the oscillatory opening and closing of the valve that controls access to the reaction chamber. This pulsed mechanism is beneficial for the beetles' survival because the system uses pressure instead of muscles to eject the spray at a constant velocity, saving the beetle energy. Also, the reintroduction of new reactants into the vestibule where enzymes are stored, reduces the temperature of the chamber, thereby protecting the peroxidases and catalases from thermal denaturation.
Typically the beetle turns its body so as to direct the jet towards whatever triggered the response. The gland openings of some African bombardier beetles can swivel through 270 and thrust between the insect's legs, discharging the fluid in a wide range of directions with considerable accuracy.
Bombardier beetles, which exist on every continent except Antarctica, have a pretty easy life. Virtually no other animals prey on them, because of one particularly effective defense mechanism: When disturbed or attacked, the beetles produce an internal chemical explosion in their abdomen and then expel a jet of boiling, irritating liquid toward their attackers.
The liquid these beetles eject is called benzoquinone, and is actually a fairly common defensive agent among insects, Arndt says. But bombardier beetles are unique in their ability to superheat the liquid and expel it in an intense, pulsating jet.
The explosive mechanism used by the bombardier beetle generates a spray that is not only much hotter than that emitted by other insects that use the same chemical irritant, but also propels the jet five times faster. Both the speed and the heat serve to make the spray even more effective against potential predators, Arndt says.
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.
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:
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.
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 100C. 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.
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.
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.
Toads are predators of bombardier beetles. In an experiment, scientists fed bombardier beetles to Japanese common toads (Bufo japonicus) to study how the beetles and toads reacted. Yasunori Koide via Wikimedia Commons (CC BY-SA 4.0)
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.
Frogs and toads are important predators of carabid beetles (Larochelle, 1974a; Larochelle, 1974b). However, bombardier beetles have rarely been found in the gut contents and faeces of frogs and toads (Larochelle, 1974a; Larochelle, 1974b; Sarashina, Yoshihisa & Yoshida, 2011; except Mori, 2008), suggesting that bombing prevents toads and frogs from swallowing and ingesting beetles (Eisner & Meinwald, 1966; Dean, 1980a; Eisner, 2003; Sugiura & Sato, 2018). Still, only a few studies have investigated the factors that cause anuran predators to stop preying on bombardier beetles (Dean, 1980b). Elucidating these ecological factors would contribute to a better understanding of the evolution of anti-predatory defences in insects.
(A) 0 ms. (B) 100 ms. (C) 175 ms. (D) 325 ms. (E) 1,100 ms. The frog stopped the attack immediately after its tongue touched the beetle. No bombing sounds were heard (see Video S3). Credit: Shinji Sugiura.
When dead beetles were used (n = 28), 24 frogs (85.7%) rejected the dead beetles without swallowing them (Fig. 1); 20 frogs (71.4%) stopped attacking the beetles after their tongues touched the dead beetles (Video S5), and four frogs (14.3%) spat out the beetles after taking the beetles into their mouths (Fig. 1). Only four frogs (14.3%) swallowed the dead beetles. Similar to the experiment using live beetles, 87.5% of the frogs that took beetles into their mouths (n = 7/8) were initially deterred when their tongues first touched the beetles, but continued with their predatory behaviour soon afterwards. The frogs that did not swallow beetles ate other prey (i.e., T. molitor larvae) soon thereafter.
The proportion of dead beetles swallowed by frogs (14.3%) was higher than that of live beetles (7.1%). However, the GLM results indicated that the frog swallowing rates of live and dead beetles did not significantly differ (Table 1). Whether beetles were swallowed or not was associated with beetle size, but not frog size (Table 1). Beetles were more likely to be swallowed as beetle size decreased (Fig. 4A). The interaction of frog and beetle weight was not significant (Fig. 4A). 59ce067264