The beetle apparently does not need this gun as it is related to other beetles without such a defense mechanism..
Mol Phylogenet Evol. 2002 Aug;24(2):228-48. Related Articles, Links
Erratum in:
Mol Phylogenet Evol. 2003 Feb;26(2):334-6..
Phylogenetic relationships of the carabid subfamily Harpalinae (Coleoptera) based on molecular sequence data.
Ober KA.
Department of Entomology, 410 Forbes Building, Interdisciplinary Program in Insect Science, University of Arizona, Tucson, AZ 85721, USA. kober@uconnvm.uconn.edu
The carabid subfamily Harpalinae contains most of the species of carabid beetles. This subfamily, with over 19,000 species, radiated in the Cretaceous to yield a large clade that is diverse in morphological form and ecological habit. While there are several morphological, cytological, and chemical characters that unite most harpalines, the placement of some tribes within the subfamily remains controversial, as does the sister group relationships to this large group. In this study, DNA sequences from the 28S rDNA gene and the wingless nuclear protein-coding gene were collected from 52 carabid genera representing 31 harpaline tribes in addition to more than 21 carabid outgroup taxa to reconstruct the phylogeny of this group. Molecular sequence data from these genes, along with additional data from the 18S rDNA gene, were analyzed with a variety of phylogenetic analysis methods, separately for each gene and in a combined data approach. Results indicated that the subfamily Harpalinae is monophyletic with the enigmatic tribes of Morionini, Peleciini, and Pseudomorphini included within it. Brachinine bombardier beetles are closely related to Harpalinae as they form the sister group to harpalines or, in some analyses, are included within it or with austral psydrines. The austral psydrines are the sister group to Harpalinae+Brachinini clade in most analyses and austral psydrines+Brachinini+Harpalinae clade is strongly supported.
In addition there are more primitive i.e. ancestral versions of the bombadier beetle defense mechanism in some species...
J Exp Biol. 2000 Apr;203 Pt 8:1265-75. Related Articles, Links
Spray mechanism of the most primitive bombardier beetle (Metrius contractus).
Eisner T, Aneshansley DJ, Eisner M, Attygalle AB, Alsop DW, Meinwald J.
Department of Neurobiology and Behavior, Cornell University, Ithaca, NY 14853, USA. te14@cornell.edu
The bombardier beetle Metrius contractus discharges its defensive secretion as a froth that clings to its body. When attacked from the rear, it allows the froth to build up over the gland openings near the abdominal tip; when attacked from the front, it conveys the secretion forwards along special elytral tracks. M. contractus has two-chambered defensive glands typical of bombardier beetles, and its secretion, like that of other bombardiers, is quinonoid and hot. Its frothing mechanism, however, is unique for bombardiers and possibly illustrative of the ancestral glandular discharge mechanism of these beetles. M. contractus, thus, could be the least derived of extant bombardiers.
Now why exactly is it impossible for this to have evolved?