Metrius contractus on a leaf feeding. Image taken by and used with permission from Sarah Crews.

Ingenious Chemistry

Reaction system of Metrius contractus labeled with location of main reactants. Image published by Beheshti and McIntosh (2007).Nobody does chemistry like Metrius contractus does chemistry. Not even other bombardier beetles have the same chemical make up in their sprays (Eisner et al. 1977). The initial chemical reactions of almost all bombardier beetles are the same, but in the final result, Metrius contractus shows several key differences (Eisner et al. 2000). First, it is essential to understand the basic chemistry present across bombardier beetles in general. In the reservoir chamber, the chemicals that are present are hydroquinone and hydrogen peroxide in solutions at concentrations of 25% and 10% by mass respectively (Beheshti and McIntosh 2007). Quinone is formed in aqueous solution initially and then stored in more of a lipoidal form as a hydrocarbon; therefore, both aqueous and lipoidal phases are found in the reservoirs (Eisner et al. 2000). As seen in the discussion of form and function, enzymes are also necessary for these reactants to do anything. Enzymes are present in the reaction chamber. More specifically most of the enzymes are catalase and peroxidase (Beheshti and McIntosh 2007).

Steps of the reaction that occurs in the jets of Metrius contractus. Image published by Beheshti and McIntosh (2007).Upon mixture of these enzymes with reactants from the reservoir chamber, there is a production of oxygen gas and water from the peroxides (Schwarcz 2010). Benzoquinone is also formed from the reaction between the oxygen gas and hydroquinone (Schwarcz 2010). Not only is there a production of irritating compounds, but the overall reaction occurring in the reaction chamber is ridiculously exothermic as seen by the fact it has a change in enthalpy of -794.2kJ/kg solution (Beheshti and McIntosh 2007). This is exactly how the spray is able to heat up to 100°C in most bombardier beetles (Beheshti and McIntosh 2007). In Metrius contractus there is a notably lower temperature of spray that is generally closer to 55°C (Eisner et al. 2000). This can be attributed to an evolutionary difference in how its jets function (Eisner et al. 1977). Finally, after releasing the advanced chemical mixture, the channel leading outside of the body is closed, and the reservoir is refilled with peroxides and quinone preparing the beetle for the next attack (James et al. 2012).

Structure of main quinones found in the spray of bombardier beetles.Click for more information on Quinone! Image created by Sarah Lloyd (2013). Information from Eisner et al. (2000).Aside from various similarities that relate Metrius contractus to other organisms which are discussed in detail on the Classification page, it also has some very thrilling unique characteristics relating to its chemistry. Eisner et al. (2000) discusses the numerous anomalies in the chemical make up of its spray. The first strange discovery in this study was the presence of 2-chlorobenzoquinone. This compound was the second most concentrated in the spray which is unbelievable considering that compound is hardly ever found in nature (Eisner et al. 2000). It is an incredibly difficult compound to form. In the same study, it was found that the primary quinone produced by Metrius contractus is 1,4-benzoquinone. This is different from every other bombardier beetle as normally 2-methyl-1,4-benzoquinone is the primary quinone. While they sound very similar, these are quite different chemicals and make a huge difference in the overall spray properties (Eisner et al. 2000).Meet the chemistry master! Image taken by and used with permission from Mark Leppin. It is for this reason that the spray of Metrius contractus is so different from other beetles and tends to hang around in the air making a cloud of toxic spray (Eisner et al. 2000). Lastly in this study, it was found that the spray of Metrius contractus also contains conjugated dienes which are not only also highly unlikely to find in nature, but would never be expected to be in the same space as 1,4-benzoquinone without reacting. A special mechanism using stereochemistry to prevent these compounds from completely reacting with each other has been developed over time by these beetles (Eisner et al. 2000).

Metrius contractus is the chemical mastermind of the natural world, and that has some astonishing applications. Feeling flabbergasted by these organic chemist braniac beetles? Be sure to refer to the Explanation of Terms page. If you want to know more about the chemical quinones found in bombardier beetles, take a look at what the U.S. Environmental Protection Agency has to say about them.

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