Spray mechanism of crepidogastrine bombardier beetles (Carabidae; Crepidogastrini)

Abstract

The defensive glandular apparatus of primitive bombardier beetles of the tribe Crepidogastrini (Carabidae) is described for the first time. As exemplified by two African species (Crepidogaster ambreana and C. atrata), the apparatus conforms to the basic bombardier plan, in that the glands are bicompartmented and the secretion is quinonoid (it contains 1,4-benzoquinones and hydrocarbons), hot, and discharged audibly. In a number of morphological respects the crepidogastrine apparatus resembles that of the classical bombardiers of the tribe Brachinini (rather than that of bombardiers of the paussoid lineage), reinforcing the view, already held on taxonomic grounds, that the Crepidogastrini and Brachinini are closely related. That the Crepidogastrini may be primitive relative to Brachinini is underscored by the finding that, unlike brachinines, crepidogastrines do not pulse their secretory emissions. Moreover, they discharge their secretion as a mist, rather than forcibly in the form of jets.

A mathematical model of the defence mechanism of a bombardier beetle

“…To explore how different species may display the mechanism, different parameter values were tested. Bombardiers come in many sizes: for example, Crepidogaster spp., which exhibit a cool (638C) mist-like ejection, have a body length of less than 5 mm [2], and Stenaptus insignis, which exhibit pulsed discharges, are up to 20 mm long [11]. Similar ranges have been seen in SEM images of the beetle glands and reaction chambers: for example, M. contractus, with an external body length of approximately 10 mm, have a reaction chamber with length 0.7 mm [12], whereas the smaller Crepidogaster spp.…”

“…Similar ranges have been seen in SEM images of the beetle glands and reaction chambers: for example, M. contractus, with an external body length of approximately 10 mm, have a reaction chamber with length 0.7 mm [12], whereas the smaller Crepidogaster spp. have a reaction chamber with length closer to 0.3 mm [2]. Figure 5 shows an example of the sensitivity of the different behavioural modes to the beetles internal measurements: on the top row, all the beetle's internal length measurements were increased by a factor of 2, and on the bottom row, they were all decreased by a factor of 0.5.…”

“…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).…”

“…Similarly, the discharge temperature of some individuals of the tribe Crepidogastrini (Carabidae) is 658C [2], secretions of Metrius contractus have been measured at 558C on average [8] and Mystropomus regularis has a relatively cold discharge of only 34-478C [9]. The duration of the ESD varies across studies but in general the rsif.royalsocietypublishing.org J R Soc Interface 10: 20120801 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.…”

“…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.…”