Pygidial Glands in Carabidae, an Overview of Morphology and Chemical Secretion

Predator community structure is an important selective element shaping the evolution of prey defence traits and strategies. Carabid beetles are one of the most diverse families of Coleoptera, and their success in terrestrial ecosystems is related to considerable morphological, physiological, and behavioural adaptations that provide protection against predators. Their most common form of defence is the chemical secretion from paired abdominal pygidial glands that produce a heterogeneous set of carboxylic acids, quinones, hydrocarbons, phenols, aldehydes, and esters. This review attempts to update and summarise what is known about the pygidial glands, with particular reference to the morphology of the glands and the biological function of the secretions.

Primary Metabolism co-Opted for Defensive Chemical Production in the Carabid Beetle, Harpalus pensylvanicus

Of the approximately one million described insect species, ground beetles (Coleoptera: Carabidae) have long captivated the attention of evolutionary biologists due to the diversity of defensive compounds they synthesize. Produced using defensive glands in the abdomen, ground beetle chemicals represent over 250 compounds including predator-deterring formic acid, which has evolved as a defensive strategy at least three times across Insecta. Despite being a widespread method of defense, formic acid biosynthesis is poorly understood in insects. Previous studies have suggested that the folate cycle of one-carbon (C1) metabolism, a pathway involved in nucleotide biosynthesis, may play a key role in defensive-grade formic acid production in ants. Here, we report on the defensive gland transcriptome of the formic acid-producing ground beetle Harpalus pensylvanicus. The full suite of genes involved in the folate cycle of C1 metabolism are significantly differentially expressed in the defensive glands of H. pensylvanicus when compared to gene expression profiles in the rest of the body. We also find support for two additional pathways potentially involved in the biosynthesis of defensive-grade formic acid, the kynurenine pathway and the methionine salvage cycle. Additionally, we have found an array of differentially expressed genes in the secretory lobes involved in the biosynthesis and transport of cofactors necessary for formic acid biosynthesis, as well as genes presumably involved in the detoxification of secondary metabolites including formic acid. We also provide insight into the evolution of the predominant gene family involved in the folate cycle (MTHFD) and suggest that high expression of folate cycle genes rather than gene duplication and/or neofunctionalization may be more important for defensive-grade formic acid biosynthesis in H. pensylvanicus. This provides the first evidence in Coleoptera and one of a few examples in Insecta of a primary metabolic process being co-opted for defensive chemical biosynthesis. Our results shed light on potential mechanisms of formic acid biosynthesis in the defensive glands of a ground beetle and provide a foundation for further studies into the evolution of formic acid-based chemical defense strategies in insects.

Pygidial glands of Harpalus pensylvanicus (Coleoptera: Carabidae) contain resilin-rich structures

The pygidial gland system is a key innovation in adephagan beetles, producing, storing, and spraying defensive chemical compounds. As the source of defensive chemical production and storage, the pygidial gland system experiences severe chemical stress which challenges the integrity of the entire gland system. Here, we utilize autofluorescence-based confocal laser scanning microscopy to examine the morphology of pygidial gland secretory lobes and collecting ductules in a common Pennsylvanian harpaline species, Harpalus pensylvanicus. The glandular units are composed of type-III exocrine cells which empty into resilin-rich ductules, which themselves lead into a larger resilin-rich collecting duct, and ultimately the pygidial reservoir pump. We also utilize histological staining with toluidine blue and brightfield imaging to provide additional support for the presence of resilin in the collecting duct, as toluidine blue has been shown to stain resilin without metachromasia. We hypothesize that the high resilin content of the collecting ducts might be a widespread key evolutionary adaptation to prevent damage caused by physical and chemical stress generated in pump-containing insect exocrine gland systems.

Carabidae Semiochemistry: Current and Future Directions

Ground beetles (Carabidae) are recognized for their diverse, chemically-mediated defensive behaviors. Produced using a pair of pygidial glands, over 250 chemical constituents have been characterized across the family thus far, many of which are considered allomones. Over the past century, our knowledge of Carabidae exocrine chemistry has increased substantially, yet the role of these defensive compounds in mediating behavior other than repelling predators is largely unknown. It is also unclear whether non-defensive compounds produced by ground beetles mediate conspecific and heterospecific interactions, such as sex-aggregation pheromones or kairomones, respectively. Here we review the current state of non-exocrine Carabidae semiochemistry and behavioral research, discuss the importance of semiochemical research including but not limited to allomones, and describe next-generation methods for elucidating the underlying genetics and evolution of chemically-mediated behavior.

Molecular Diversity of Compounds from Pygidial Gland Secretions of Cave-Dwelling Ground Beetles: The First Evidence

Three adult cave-dwelling ground beetle species were induced to discharge secretions of their pygidial glands into vials. Dichloromethane extraction was used to obtain the secretions. In total, 42 compounds were identified by GC/MS analysis. Pheggomisetes ninae contained 32 glandular compounds, Laemostenus (Pristonychus) punctatus 13, whereas Duvalius (Paraduvalius) milutini had nine compounds. Caproic, oleic, palmitic, and stearic acids were present in the samples of all analyzed species. Undecane was predominant in the extract of L. punctatus. Palmitic acid was the major component in the secretion of D. milutini. Finally, the most abundant compounds in P. ninae secretion were heptacosene and nonacosadienes. Herein, we present the first data on the identification of pygidial gland secretion components in both troglophilous and troglobite cave-dwelling ground beetles. Some compounds are reported for the first time in the secretions of ground beetles and other higher or lower taxa. The adaptation to underground life has not led to a reduction or changes in the chemical defense mechanism in the analyzed troglophilous and troglobitic Platyninae and Trechinae taxa.

Defensive secretions of the carabid beetle Chlaenius cordicollis: chemical components and their geographic patterns of variation

The defensive secretion of the ground beetle Chlaenius cordicollis is predominantly 3-methylphenol. Adult C. cordicollis were collected in Pennsylvania and Manitoba and induced to discharge defensive secretion in a vial. The headspace was sampled by solid phase microextraction, and samples were analyzed by gas chromatography-mass spectrometry. Five alkylphenolic compounds were detected: all beetles secreted 3-methlyphenol, 2,5-dimethylphenol, and 3-ethylphenol, and most beetles from each locality secreted detectable amounts of 2,3-dimethlyphenol and 3,4-dimethylphenol. In about 80% of beetles, we detected small amounts of the alkoxyphenolic compounds 2-methoxy-4-methylphenol and 2-methoxy-5-methylphenol. Multivariate compositional analysis of relative peak areas of alkylphenolic compounds revealed geographic variation and sexual dimorphism in defensive secretions. Compared with samples from Manitoba, relative peak areas of samples from Pennsylvania were lower for 2,3-dimethylphenol and higher for 3-methylphenol. Sexual dimorphism was detected only in Manitoba where, compared with samples from males, relative peak areas for samples from females were higher for 2,5-dimethylphenol and lower for 3-ethylphenol. This is the first report of geographic variation in defensive secretions of carabid beetles, and it demonstrates the need for knowledge of patterns of variation before characterizing the defensive secretions of a species as a whole.

Quantification and evidence for mechanically metered release of pygidial secretions in formic acid-producing carabid beetles

This study is the first to measure the quantity of pygidial gland secretions released defensively by carabid beetles (Coleoptera: Carabidae) and to accurately measure the relative quantity of formic acid contained in their pygidial gland reservoirs and spray emissions. Individuals of three typical formic acid producing species were induced to repeatedly spray, ultimately exhausting their chemical compound reserves. Beetles were subjected to faux attacks using forceps and weighed before and after each ejection of chemicals. Platynus brunneomarginatus (Mannerheim) (Platynini), P. ovipennis (Mannerheim) (Platynini) and Calathus ruficollis Dejean (Sphodrini), sprayed average quantities with standard error of 0.313 +/- 0.172 mg, 0.337 +/- 0.230 mg, and 0.197 +/- 0.117 mg per spray event, respectively. The quantity an individual beetle released when induced to spray tended to decrease with each subsequent spray event. The quantity emitted in a single spray was correlated to the quantity held in the reservoirs at the time of spraying for beetles whose reserves are greater than the average amount emitted in a spray event. For beetles with a quantity less than the average amount sprayed in reserve there was no significant correlation. For beetles comparable in terms of size, physiological condition and gland reservoir fullness, the shape of the gland reservoirs and musculature determined that a similar effort at each spray event would mechanically meter out the release so that a greater amount was emitted when more was available in the reservoir. The average percentage of formic acid was established for these species as 34.2%, 73.5% and 34.1% for for P. brunneomarginatus, P. ovipennis and C. ruficollis, respectively. The average quantities of formic acid released by individuals of these species was less than two-thirds the amount shown to be lethal to ants in previously published experiments. However, the total quantity from multiple spray events from a single individual could aggregate to quantities at or above the lethal level, and lesser quantities are known to act as ant alarm pheromones. Using a model, one directed spray of the formic acid and hydrocarbon mix could spread to an area of 5-8 cm diameter and persisted for 9-22 seconds at a threshold level known to induce alarm behaviors in ants. These results show that carabid defensive secretions may act as a potent and relatively prolonged defense against ants or similar predators even at a sub-lethal dose.

Orange/lemon-scented beetles: opposite enantiomers of limonene as major constituents in the defensive secretion of related carabids

The major constituent in the pygidial gland defensive fluid of the carabid beetle Ardistomis schaumii is (R)-(+)-limonene, whereas that of Semiardistomis puncticollis is (S)-(-)-limonene. This was an unanticipated result, since it is not very common to find the opposite enantiomers of the same compound among the secondary metabolites of related species. Moreover, the glandular liquid of A. schaumii contains 1,8-cineole, and that of S. puncticollis has beta-pinene, beta-phellandrene, sabinene, and p-cymene. Of about 500 carabid species that have been chemically investigated, this is the first report of the presence of such complex mixtures of monoterpenes in their defensive secretions.