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

Form and Function

 The simple exterior appearance of Metrius contractus is truly rather deceiving. Basic morphology of a bombardier beetle showing location and general structure of jets. Image published by Eisner et al. (2000).At a glance they are quite similar to other closely related bombardier beetles and only generally develop to be a mere two centimeters long (μMist spray technology 2013). Like other arthropods, they have a protective exoskeleton of chitin, multiple jointed legs, and wings (Eisner and Meinwald 1995). If bombardier beetles all have wings, why don’t they just fly away when attacked? Unlike some other arthropods such as Horseflies and The Bald-faced Hornet, they are not quite masters of flight. According to Dr. Schwarcz of the Chemical Institute of Canada, bombardier beetles do not have their wings out and readily available for use like other arthropods. They have covers over their wings that need to be moved before flight can be initiated (Schwarcz 2010). Bombardier beetles are also generally rather slow moving creatures and cannot dart away when experiencing unfavorable interactions (Eisner et al. 1977).    

Because they cannot simply fly away or swiftly run when approached by a frightening predator, evolution had to give bombardier beetles a different plan for defense in order for them to survive asA reminder of just how simple Metrius contractus may appear from the outside despite very complex inner systems! Image taken by and used with permission from Jacob Kalichman. long as they have. Fully efficient chemistry laboratories completely stocked with every needed reagent have been developed inside of the abdomens of bombardier beetles (James et al. 2012). Nature certainly has other chemistry master organisms like Stinging Nettles and The Kiss Me Quick Plant, but there is nothing quite like the tiny potent bombs inside of bombardier beetles. Metrius contractus in particular withholds quite intensive and unique chemistry inside of its reaction chambers (Eisner et al. 2000).

Before the actual process and function of the jet system is discussed, it is worth noting the fortress of physiology it takes to contain such powerful chemical reactions. The reason the beetle does not bring harm to itself in producing explosive mists of toxin up to 100°C is due to its superb structure (Beheshti and McIntosh 2007). Posterior quarters of bombardier beetles are compartmentalized into a reservoir or storage chamber for holding unreacted chemicals and a reaction chamber with numerous enzymes that are able to quicklReaction system of Metrius contractus after being infused with Potassium Hydroxide for better viewing.Letters R, r, d, and g correspond to the reservoir, reaction chamber, efferent duct, and gland respectively. The scale bar (B) is representative of one millimeter. Image published by Eisner et al. (2000).y catalyze the formation of the toxic spray when necessary (Eisner et al. 2000). There are two sets of these chambers, one on both the left and right side of the beetle (Beheshti and McIntosh 2007). A valve attached to a muscle connects the two chambers preventing the reaction from occurring when unnecessary so that futile reactions do not consume energy and resources (Beheshti and McIntosh 2007). Observe the image to the right displaying these structures where letters R, r, d, and g correspond to the reservoir, reaction chamber, efferent duct, and gland respectively. The tissues making up both of these chambers contain a cuticle layer for protection, but the storage chamber has much thinner more flexible walls (Eisner et al. 2000). This can largely be attributed to the fact that an extremely powerful oxidation happens in the reaction chamber which requires it to be thicker and more rigid to protect the rest of the body (Eisner et al. 2000). Small glands are also imbedded in the walls of the reaction chamber that release the enzymes for the reaction (Beheshti and McIntosh 2007).
Aside from the astounding chemical reaction system, the overall abdomen of bombardier beetles is also strategically structured for efficiency (Agosta 1996). An experiment run by Eisner et al. (2000) tested the ability of bombardier beetles and specifically Metrius contractus to aim their jets to hit threats. By using indicator paper and irritating the beetles from different positions, they were able to determine that the beetles were able to use their flexible abdomen to aim their sprays quite accurately. Overall, the efficiency of bombardier beetles arises from amazing structure and impressive chemistry. The functions that allow for form and chemistry to work together are remarkable. The functions of Metrius contractus certainly relate to its interactions and habitat, but its most relevant and unique functional activity is with its jets.

Metrius contractus on indicator paper displays the ability to aim its jets. Image published by Eisner et al. (2000).

As seen in work by Agosta, when alarmed, bombardier beetles open the valve connecting the reaction chamber and the storage chamber thereby releasing powerful chemicals to be catalyzed by enzymes. Almost instantly, an oxidation reaction occurs resulting in intense heat and toxic products (Agosta 1996). Reactants are prevented from flowing backwards by the muscle and valve in the connecting channel which shuts tightly due to pressure from the reaction (James et al. 2012). The heat and pressure produced in this reaction then force the toxin out of the tip of the beetle’s abdomen (Agosta 1996). The immense power behind this bombarding chemistry is so forceful that it actually makes a fairly audible sound as it bursts forth in defense (Schwarcz 2010).

While Metrius contractus is mostly extremely similar to other bombardier beetles, its form and function have several key differences. These differences are essential in the determination of its evolutionary classification (Eisner et al. 2000). Most bombardier beeSeries of images capturing just how fast Metrius contractus can react and shoot. Numbers in lower left corners are time in milliseconds after the beetle was irritated. Image published by Eisner et al. (2000).tles eject their spray in a harsh streamline fashion (Beheshti and McIntosh 2007). Metrius contractus creates more of a mist by puffing out its spray as an oozing chemical (Eisner et al. 2000). This alternate way of excretion gives insight into how more ancient bombardier beetles probably functioned and actually serves as a benefit to Metrius contractus (Eisner et al. 2000). Metrius contractus remains in a cloud of its spray for quite awhile so that its enemies don’t try to attack again (Eisner et al. 2000). Other very enthralling factoids of the pure uniqueness of Metrius contractus are heavily related to the chemistry intricately guiding its ability to defend itself (Eisner et al. 1977).

Revisit Habitat, Geography, & Interactions or move on to Reproduction.