Bombardier beetles are ground beetles (Carabidae) whose defence mechanism has earned them a special place in entomology. They spray their attackers with a noxious and often hot spray that is ejected with force from their abdomen, in some cases with great accuracy [1]. There are many species of bombardier from the tribes Brachinini, Crepidogastrini, Metriini, Mystropomini, Paussini and Ozaenini [2], and their collective name derives from the audible pop that often accompanies the ejection [3] (table 1).
Table 1.
Table 1.
Variables and parameters.
A bombardier beetle stores reactant chemical compounds in a reservoir and, when attacked, contracts muscle that forces these reactants into a rigid-walled reaction chamber containing catalytic enzymes. In this chamber, the compound starts to react exothermically, increasing the chamber pressure and ultimately forcing the reactants out of glands in the beetle's abdomen at temperatures of up to 100°C, in what has been termed an explosive secretory discharge (ESD) [6]. A system of valves stops these reactants from entering the beetle's greater anatomy. The total ejection lasts for only a fraction of a second but is easily enough to ward off most predators.
The defence mechanism of the bombardier beetle has fascinated entomologists for close to two centuries since they were first discovered by the naturalist J. O. Westwood in 1839. Individuals of the Brachinus genus (species unknown) were amongst the first to be examined [6], and it was found that the reservoir solution contained 10 per cent hydroquinone and 25 per cent hydrogen peroxide with heat of reaction of 48.5 kcal per mol hydroquinone [5]. Although the range of chemicals found in the secretions of different bombardiers is remarkably similar, there is a marked difference between species in the proportions of the different constituent parts [7].
While the outlet temperature of the discharge is often quoted as being 100°C (the maximum outlet temperature calculated by Aneshansley et al. [6]), it has also been seen to vary over a considerable range depending on species. For instance, Goniotropis nicaraguesis has an outlet flow with a much lower temperature (average 65°C, maximum observed temperature 81°C) [1]. Similarly, the discharge temperature of some individuals of the tribe Crepidogastrini (Carabidae) is 65°C [2], secretions of Metrius contractus have been measured at 55°C on average [8] and Mystropomus regularis has a relatively cold discharge of only 34–47°C [9]. The duration of the ESD varies across studies but in general the discharge has either finished (in the case of jet discharges) or peaked (for secretory/mist discharges) after 30 ms [2,6,10].
Additionally, the manner in which beetles discharge varies between species. Many species, e.g. those of the genus Brachinus and Stenaptus insignus, eject their discharge as a spray or jet which can often be aimed with very high accuracy [11]. Other species, e.g. M. contractus, have a more diffuse, mist-like secretion and in some cases also produce a froth that can build-up on the individual's body. Of particular interest is the outlet flow of S. insignus, which is not continuous but pulsed [10]. Again, the duration of the entire discharge is short (2.6–24.1 ms), but each discharge contains between 2 and 12 pulses (average 7) at a frequency of between 400 and 800 Hz [10]. The discharge velocities of this pulsed flow are, in general, an order of magnitude higher than those of non-pulsed flow in other beetles, averaging 12 ms−1 with a range between 3.25 and 19.5 ms−1 [10]. Despite the diversity of ESD behaviour, the prevailing literature [1,2,12]. with only a single exception [13] states that the internal mechanism is remarkably similar between species, consisting of a thin and flexible reservoir and separate rigid-walled reaction chamber. Figure 1 shows schematic of the bombardier gland system.
