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Pygidial glands are of great importance to ground beetles for defense against predators, especially for the species that live in subterranean habitats. The purpose of our study is to better understand the chemistry of the pygidial gland secretions of subterranean ground beetles, as well as the function and structure of the glands. We studied both the chemical composition of the pygidial gland secretion and morphology of the glands in adults of the troglophilic ground beetle species Laemostenus (Antisphodrus) cavicola (Schaum, 1858). The chemical composition of its defensive secretion was revealed using gas chromatography-mass spectrometry (GC-MS), while pygidial gland morphology of the beetle was investigated using bright-field microcopy. In total, seven chemical compounds were detected in the secretion mixture. Formic acid was the most dominant compound, followed by dodecyl acetate and undecane. Other chemicals were present in minor amounts. The morphological structure of the pygidial glands of L. (A.) cavicola was compared with the structure of the glands of the related congeneric troglophilic species Laemostenus (Pristonychus) punctatus (Dejean, 1828). Summary data on the semiochemicals that have been recorded so far in subterranean ground beetle species are presented, and the differences in the chemical composition of the secretions between and among troglobitic and troglophilic species are discussed. So far, forty-four compounds have been detected in four subterranean ground beetle species (two troglobites belonging to the tribe Trechini and two troglophiles belonging to the tribe Sphodrini). The results of this study indicate the great diversity of chemicals in the pygidial gland secretions of subterranean ground beetles.

The pygidial glands of carabids produce strong-smelling vapours. In this study, we examined the chemical composition of the gland secretions and the structure of the glands in five species of Carabini ground beetles (one species from the subtribe Calosomatina and four species from the subtribe Carabina): Calosoma (Calosoma) maderae (Fabricius, 1775), Carabus (Carabus) granulatus Linnaeus, 1758, C. (Limnocarabus) clathratus Linnaeus, 1760, C. (Carabus) ulrichii Germar, 1823, and C. (Procerus) gigas Creutzer, 1799. Additionally, we tested the antibacterial potential of the pygidial gland secretions of the two latter species against 11 bacterial strains. In order to detect the chemical content of the secretions, we used gas chromatography–mass spectrometry (GC–MS). The secretion extracts were applied against selected strains of medically important bacteria. We used bright-field microscopy to examine the morphology of the glands. We discovered a total of 11 chemical compounds in the pygidial gland extracts of the ground beetles we analysed. Ten of these compounds were identified as seven carboxylic acids, two hydrocarbons, and one aromatic aldehyde, while one chemical remained unidentified. Most of the components were isolated from the secretion of C. (L.) clathratus (nine), while the lowest number of compounds was found in C. (P.) gigas (two). Methacrylic acid was the most dominant compound by percentage in all five species, while angelic acid was also detected in all samples. As expected, salicylaldehyde was exclusively found in the species of the genus Calosoma Weber, 1801. The secretion of C. (P.) gigas was shown to achieve the highest level of antibacterial activity against Pseudomonas aeruginosa, Salmonella enterica, and S. typhimurium (even the same level as the positive control streptomycin), while the secretion of C. (C.) ulrichii achieved the highest antimicrobial potential against Staphylococcus epidermidis, S. aureus, Listeria monocytogenes, and Bacillus cereus. The most noticeable difference in the structure of the glands between the two genera is that the reservoir in Calosoma is more significantly narrowed as it leads into the efferent duct, compared to that of Carabus.

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