Evolution
The evolution of chemical defenses is widespread in various forms of life, from antibiotics in fungal species to poisons and toxins found in plants, amphibians, reptiles, mammals and of course insects. In fact, the order Coleoptera has evolved chemical defenses at least 30 separate times (Commonwealth Scientific and Industrial Research
Organisation [CSIRO] 1991). These defensive compounds are commonly secreted by special glands on various parts of the body, but can also be found in the hemolymph (released via reflexive bleeding or crushing of the cuticle). Many arthropods have also independently evolved the ability to produce defensive 1-4-benzoquinones such as millipedes, earwigs, termites, grasshoppers, cockroaches and more. Bombardier beetles differ however, in that they do not directly store these quinones in glands, but produce them with explosive chemical reactions at the moment of release (Eisner,
Eisner & Siegler 2005).
The most primitive Bombardier beetle Metrius contractus however, does not spray its defensive quinones but rather secretes them in a froth that clings to its body and can be moved to the front of its body via special tracks on the elytra. The pygidial glands are typical of other Bombardier beetles despite this different excretion mechanism. This is quite probably very similar to the secretion methods used by ancestors of explosive discharging Bombardier beetles (Eisner 2000).
As the Bombardier beetles' quinone spray is used in defense, it is likely that these mechanisms evolved from predation pressure. The evolutionary arms race between prey and predator has been well documented. Given the evidence that the Bombardier beetles' defense mechanism is effective against ants, spiders, frogs and birds it is logical to assume that predation pressure from organisms similar to these promoted the evolution of the Bombardier beetles' amazing defense mechanism.
To more closely investigate the evolution of the pygidial glands and the Bombardier beetles' hot explosive discharge, phylogenetic analysis and inspection of the fossil record can be done. Also studies could be done to compare the mitochondrial DNA of extant species to identify phylogeny. Phylogeny of Coleopterans inclusive of Bombardier beetles can be found at the Tree of Life Web Project (http://tolweb.org/Coleoptera/8221).
An interesting article on the evolutionary history of the Bombardier beetle was published by Mark Isaak (1997) which theorised a hypothetical but logical series of events that could have led to the evolution of these marvelous Coleopterans (http://www.talkorigins.org/faqs/bombardier.html).
The most primitive Bombardier beetle Metrius contractus however, does not spray its defensive quinones but rather secretes them in a froth that clings to its body and can be moved to the front of its body via special tracks on the elytra. The pygidial glands are typical of other Bombardier beetles despite this different excretion mechanism. This is quite probably very similar to the secretion methods used by ancestors of explosive discharging Bombardier beetles (Eisner 2000).
As the Bombardier beetles' quinone spray is used in defense, it is likely that these mechanisms evolved from predation pressure. The evolutionary arms race between prey and predator has been well documented. Given the evidence that the Bombardier beetles' defense mechanism is effective against ants, spiders, frogs and birds it is logical to assume that predation pressure from organisms similar to these promoted the evolution of the Bombardier beetles' amazing defense mechanism.
To more closely investigate the evolution of the pygidial glands and the Bombardier beetles' hot explosive discharge, phylogenetic analysis and inspection of the fossil record can be done. Also studies could be done to compare the mitochondrial DNA of extant species to identify phylogeny. Phylogeny of Coleopterans inclusive of Bombardier beetles can be found at the Tree of Life Web Project (http://tolweb.org/Coleoptera/8221).
An interesting article on the evolutionary history of the Bombardier beetle was published by Mark Isaak (1997) which theorised a hypothetical but logical series of events that could have led to the evolution of these marvelous Coleopterans (http://www.talkorigins.org/faqs/bombardier.html).