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

Abstract

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.

1. Introduction

The carabid beetles (Coleoptera, Carabidae) include approximately 40,000 described species that are ecologically important as predators in many ecosystems and range in feeding habits from generalist to specialists [1,2]. Carabids are often used as indicators because they are extremely sensitive to environmental changes [3,4,5]. Their ecological role in the trophic web of agroecosystems [6], makes them particularly suitable for monitoring the impact of agrochemicals [5,7,8] and heavy metals [9,10,11,12,13]. Furthermore, as generalist predators, ground beetles provide important ecosystem services by lowering populations of invertebrate pests and weed seeds [14,15]. However, carabids are consumed by a number of different species, including invertebrates and insectivorous vertebrates such as birds, mammals, amphibians, and reptiles [1]. Predator–prey interactions are likely the major driving force for the evolution of defences against predators in carabid beetles. Strategies to escape predatory attacks primarily include morphological adaptations, such as cryptic or warning coloration [16,17,18,19] and dorso-ventral flattening, large eyes, and long legs to escape [20], as well as secretion of chemical repellents [21,22,23]. Ground beetles possess a pair of abdominal glands called pygidial glands that produce defensive secretions. The main function of the pygidial glands is to defend against predators, but they also engage in biological activities such as facilitating the penetration of the defensive substances into the integument of the predator and inhibiting the growth of fungi and pathogens [24,25]. A few studies to date have examined the chemical compounds of pygidial gland secretions [26,27,28,29,30] and comparatively investigated their taxonomic significance [22,31,32,33,34]. We attempt to review the current state of knowledge on the pygidial glands of carabid beetles by providing an overview of their structure and the chemical compounds of the secretion.

2. General Morphology

Forsyth [35] first proposed a comparative description of pygidial glands in 71 species from 34 tribes to define phylogenetic relationships within Carabidae. Currently, approximately 150 species from 43 tribes have been described (Table 1). The most commonly used examining technique to study pygidial gland morphology is light microscopy (LM). In addition, other techniques such as fluorescence (FM) microscopy, scanning electron microscopy (SEM) and focused ion beam/scanning (FIB/SEM) electron microscopy, (TEM) transition electron microscopy, synchrotron radiation X-ray phase-contrast micro-tomography (SR-PhC micro-CT) are also applied.

Table 1.

Summary of carabid species in which the pygidial gland morphology has been investigated and the method used for analyses. Abbreviations—CLSM: confocal laser scanning microscopy; FIB/SEM: focused ion beam/scanning electron microscopy; FM: fluorescence microscopy; LM: light microscopy; NLM: non linear microscopy; SEM: scanning electron microscopy; SR-PhC micro-CT: synchrotron radiation X-ray phase-contrast micro tomography.

§ Subfamily	Tribe	Genus	Species	Methodology	Refs
Paussinae	Metriini	Metrius	M. contractus	LM; FIB/SEM	[35,39]
		Sinometrius	S. turnai	LM; FIB/SEM	[39]
	Ozaeniini	Mystropomus	M. regularis	LM	[34]
	Paussini	Paussus	P. favieri	LM; FM; FIB/SEM	[40]
			P. laevifrons	LM	[35]
		Heteropaussus	H. jeanneli	LM	[35]
Cicindelinae	Cicindelini	Cicindela	C. campestris	LM	[47]
			C. hibrida	LM	[47]
Carabinae	Carabini	Calosoma	C. oceanicum	LM	[34]
			C. schayeri	LM	[34]
			C. senegalense	LM	[35]
			C. sycophanta	LM	[48]
		Carabus	C. (Tomocarabus) convexus	LM	[42]
			C. (Procustes) coriaceus	LM	[42,49]
			C. problematicus	LM	[35]
			C. ullrichii	LM	[49]
			C. (Megodontus) violaceus	LM	[43]
	Cychrini	Cychrus	C. caraboides rostratus	LM	[35]
	Pamborini	Pamborus	P. alternans	LM	[34]
Elaphrinae	Elaphrini	Elaphrus	E. cupreus	LM	[35]
		Blethisa	B. multipunctata	LM	[35]
Loricerinae	Loricerini	Loricera	L. pilicornis	LM	[35]
Omophroninae	Omophronini	Omophron	O. dentatum	LM	[35]
Nebriinae	Nebriini	Eurynebria	E. complanata	LM	[35]
		Leistus	L. ferrugineus	LM	[35]
		Nebria	N. brevicollis	LM	[35]
			N. psammodes	LM	[50]
	Notiophilini	Notiophilus	N. substriatus	LM	[35]
Scaritinae	Clivinini	Clivina	C. basalis	LM	[34]
			C. collaris	LM	[35]
			C. fossor	LM	[35]
		Schizogenius	S. lineolatus	LM	[31]
	Dyschiriini	Dyschirius	D. globosus	LM	[35]
	Pasimachini	Pasimachus	P. elongatus	LM	[35]
			P. subsulcatus	LM	[51,52]
	Carenini	Carenum	C. bonelli	LM	[34]
			C. interruptum	LM	[34]
			C. tinctillatum	LM	[34]
		Laccopterum	L. foveigerum	LM	[34]
		Philoscaphus	P. tuberculatus	LM	[34]
Broscinae	Broscini	Eurylychnus	E. blagravei	LM	[34]
			E. ollifi	LM	[34]
		Promecoderus	P. sp.	LM	[34]
Trechinae	Trechini	Thalassotrechus	T. barbarae	LM	[35]
		Trechus	T. obtusus	LM	[35]
	Bembidiini	Bembidion	B. lampros	LM	[28,35]
			B. rupestre	LM	[35]
Patrobinae	Patrobini	Amblytelus	A. curtus	LM	[34]
		Patrobus	P. longicornis	LM	[31]
			P. septentrionis	LM	[35]
Harpalinae	Morionini	Morion	M. simplex	LM	[31]
		Moriosomus	M. seticollis	LM	[31]
	Perigonini	Diploharpus	D. laevissimo	LM	[31]
	Loxandrini	Loxandrus	L. icarus	LM	[31]
			L. longiformis	LM	[34]
			L. velocipes	LM	[31]
		Oxycrepis	O. sp.	LM	[31]
	Sphodrini	Calathus	C. ambiguus	LM	[35]
		Pristonychus	P. terricola	LM	[35]
	Pterostichini	Abacomorphus	A. asperulus	LM	[34]
		Abaris	A. anaea	LM	[31]
		Blennidus	B. liodes	LM	[31]
		Castelnaudia	C. superba	LM	[34]
		Cratoferonia	C. phylarchus	LM	[34]
		Cratogaster	C. melas	LM	[34]
		Cyclotrachelus	C. sigillatus	LM	[31]
		Gasterllarius	G. honestus	LM	[31]
		Incastichus	I. aequidianus	LM	[31]
		Loxodactylus	L. carinulatus	LM	[34]
		Myas	M. coracinus	LM	[31]
		Notonomus	N. angusribasis	LM	[34]
			N. crenulatus	LM	[34]
			N. miles	LM	[34]
			N. muelleri	LM	[34]
			N. opulentus	LM	[34]
			N. rainbowi	LM	[34]
			N. scotti	LM	[34]
			N. triplogenioides	LM	[34]
			N. variicollis	LM	[34]
		Prosopogmus	P. harpaloides	LM	[34]
		Pseudoceneus	P. iridescens	LM	[34]
		Pterostichus	P. (Cophosus) cylindricus	LM; NLM	[44]
			P. (Monoferonia) diligendus	LM	[31]
			P. externepunctatus roccai	LM	[50]
			P. fortis	LM	[33]
			P. luctuosus	LM	[31]
			P. madidus	LM	[38]
			P. melanarius	LM	[35]
			P. melas	SR-PHC MICRO-CT	[53]
			P. (Pseudomaseus) nigrita	LM; NLM	[44]
		Rhytisternus	R. laevilaterus	LM	[34]
		Sarticus	S. cyaneocinctus	LM	[34]
		Sphodrosomus	S. saisseri	LM	[34]
		Trichosternus	T. nudipes	LM	[34]
	Platynini	Agonum	A. dorsale	LM	[35]
	Zabrini	Amara	A. aenea	LM	[35]
		Curtonotus	C. fulvus	LM	[35]
		Zabrus	Z. tenebriodes	LM	[35]
	Molopini	Abax	A. parallelepipedus (sub:A. ater)	LM	[35,49]
		Molops	M. (Stenochoromus)montenegrinus	LM; NLM	[44]
	Harpalini	Bradycellus	B. harpalinus	LM	[35]
		Diaphoromerus	D. edwardsi	LM	[34]
			H. aeneus	LM	[35]
		Harpalus	H. pensylvanicus	CLSM	[54]
		Pseudophonus	P. rufipes (sub:pubescens)	LM	[35]
	Licinini	Badister	B. bipustulatus	LM	[35]
		Dicrochile	D. brevicollis	LM	[34]
			D. goryi	LM	[34]
		Licinus	L. depressus	LM	[35]
		Syagonix	S. blackburni	LM	[34]
	Chlaeniini	Chlaenius	C. australis	LM	[34]
			C. cumatilis	LM	[35]
			C. inops	LM	[33]
			C. pallipes	LM	[33]
			C. velutinus	LM	[50]
			C. vestitus	LM	[35,50]
		Oodes	O. amaroides	LM	[31]
			O. hehpioides	LM	[35]
	Panagaenini	Craspedophorus	C. sp.	LM	[34]
		Panagaeus	P. crux-major	LM	[35]
		Psecadius	P. eustalactus	LM	[35]
		Tefflus	T. sp.	LM	[35]
	Masoreini	Masoreus	M. wetterhlii	LM	[35]
	Odacanthini	Colliuris	C. melanura	LM	[35]
			C. pensylvanica	LM	[31]
	Lebiini	Eudalia	E. macleayi	LM	[34]
		Metabletus	M. foveatus	LM	[35]
		Movmolyce	M. phyllodes	LM	[35]
	Galeritini	Galerita	G. lecontei	LM	[45]
	Anthiini	Anthia	A. artemis	LM	[35]
	Helluonini	Helluo	H. costatus	LM	[34]
	Dercylini	Dercylus (s.s.)	D. sp.	LM	[31]
	Catapieseini	Catapiesis	C. attenuata	LM	[31]
			C. sulcipennis	LM	[31]
	Dryptini	Drypta	D. dentata	LM	[35]
			D. japonica	LM	[33]
	Pseudomorphini	Sphallomorpha	S. colymbeioides	LM	[34]
Brachininae	Brachinini	Aptinus	A. bombarda	LM; FM; FIB/SEM	[41]
			A. crepitus	LM; FM; FIB/SEM	[41]
			A. displosor	LM	[35]
		Brachinus	B. crepitans	LM	[35]
			B. elongatus	LM; FM; FIB/SEM; SEM	[41,55]
			B. sclopeta	LM; FM; FIB/SEM	[41]
			B. stenoderus	LM	[33]
			P. verticalis	LM	[34]
		Pheropsophus	P. africanus	LM; FM; FIB/SEM	[41]
			P. hispanus	LM; FM; FIB/SEM	[41]
			P. lissoderus	LM	[35]
			P. occipitalis	LM; FM; FIB/SEM	[41]
			P. verticalis	LM	[34]

^§ Classification of taxa has been arranged according to Bousquet [56] and Beutel and Ribera [57].

Each pygidial gland consists of a variable number of secretory lobes (acini), collecting duct, reservoir chamber, reaction chamber, and efferent duct (Figure 1). These glands (class 3 according to the classification of Noirot and Qhennedey [36,37]) are variable in structure and have been described in several species [35,38]. The lobe or acinus, which is spherical or elongated and enveloped in a thin basal lamina, is a cluster of secretory units, connected to the collecting duct by a conducting duct that drains secretions outward. The secretory unit consists of two parts, an elongated, cube-shaped secretory cell surrounding a receiving duct and a duct cell surrounding the conducting duct [35,39,40,41]. The receiving duct is a porous tube composed of one or more layers of epicuticle located in its extracellular space and bounded by microvilli. The collecting duct has an epithelial wall of flattened cells, lined by endocuticle, and a thin layer of epicuticle that is regularly folded into spiral ridges, annular arrays, or pointed peg-like projections, that reduce the volume of the lumen to control the free flow of secretion to the reservoir chamber [35,39,41]. The entrance of the collecting duct to the reservoir chamber is of great variability. It is located at the anterior or middle position in Scaritinae, Brachininae, and some Bembidiini, Pterostichini, Amarini, Carabini, Nebriini, Metriini, and Paussini [33,35,39,40,42,43,44]. While it is located near the entrance of the efferent duct in Harpalini, Agonini, Chlaeniini, Dryptini, Anthiini, Lebiini, Trachypachini, Omophronini, Loxandrini, Catapieseini, Galeritini, and Zuphini [31,33,35,45]. The reservoir chamber is a spherical, elongate, or bilobate compartment of variable size. Interwoven muscle bundles cover the outer wall and are connected to tracheal branches. The basal membrane supports flattened epithelial cells covered by a thin uniform layer of endo- and epicuticle. The muscular contraction regulates secretion through a valve that separates the reservoir from the reaction chamber. In Paussinae, Brachininae, and Carabinae, an accessory gland is located below the valve [35]. Secretions from the reservoir chamber are mixed with secretions from the accessory glands in the reaction chamber. The efferent duct leads from the reservoir chamber to the external orifice. The close association of the pygidial glands with the tracheal branches suggests a high aerobic metabolism.

Figure 1.

Schematic drawing of a pygidial gland. cd: collecting duct; ed: efferent duct; r: reservoir chamber; rc: reaction chamber; sl: secretory lobe; VIII: eighth tergite; IX: ninth tergite (for more details of species listed in the text, see Forsyth (1972) [35]).

The external orifice is located dorso-laterally in the posterior part of the abdomen, near to the antero-lateral margin of the ninth tergite, and close to the tergo-sternal suture in Carabinae, Scaritinae, Paussinae, Elaphrinae, Broscinae, and Brachinini, or at the posterolateral margin of the eighth tergite in derived lineages, e.g., Trechinae and Harpalinae, and including Licinini, Chlaeniini, Panagaeini, Anthiini, Zabrini, Oodini, Pterostichini, and Agonini [35,46]. Differences in pygidial gland morphology between sexes have been reported in Cicindela campestris [47].

3. Excretory Mechanisms

Oozing, spraying, and crepitation are the main types of external excretory mechanisms observed in carabid beetles in response to disturbance [58]. Oozing of secretion over the cuticle of the hind segments occurs in species that have relatively weakly developed muscles on the wall of the reservoir chamber, i.e., in the tribes Nebriini, Notiophilini, Loricerini, Elaphrini, and the subfamilies Scaritinae, Cicindelinae, and Broscinae [32,35]. This is probably the plesiomorphic mode of discharge, whereas the secretion expelled by strong muscle pressure on the reservoirs is an apomorphic adaptation. The discharge of a directional secretion by turning the tip of the abdomen has been observed in many taxa that exhibit a variable secretion discharge, such as Trechini, Bembidiini, Galeritini, Carabini, Cychrini, Harpalini, Agonini, Anthiini, and Pterostichini (except the genus Abax) [32,45,59]. Bombardier beetles discharge secretion by crepitation [60,61], with the exception of Metrius contractus, which discharges its secretion using the oozing ancestral discharge mechanism [39,62]. This discharge has evolved independently in Ozaenini and Paussini on the one hand and in Brachinini on the other. In the tribe Brachinini, the explosive defence is an active enzymatic exothermic reaction that produces benzoquinones, free oxygen, water, and heat up to 100 °C [55]. The process begins with muscle contraction of the reservoir chamber, which allows stored hydroquinones and hydrogen peroxide, to move through the one-way valve, enter the reaction chamber, and mix with catalases and peroxidases produced by the accessory glands. In Paussinae, fluids are directed via a cuticular fold (Coanda flange) located at the posterolateral angle of the elytra, which serves as a launching guide for rapid anterior discharge [60,61,63]. The ability to direct the sprayed secretion has also been observed in Calosoma prominens [64].

4. Chemical Compounds of Secretion

To date, over 363 species from 45 tribes have been studied by gas chromatography-mass spectrometry (GC-MS) (Table 2) in dichloromethane or hexane extracts. The semiochemicals, listed in Table 2, belong to one of the following classes: aliphatic and aromatic carboxylic acids, phenols (m-cresol and xylenol), aldehydes, quinones, hydrocarbons, ketones, terpenes, and esters. The biosynthetic pathways of these compounds have been extensively studied in arthropods [27,65]. However, studies addressing their biogenesis in the pygidial gland of carabids are lacking. The enzymatic derivation of quinones is one of the few metabolic pathways investigated. The bombardier beetle Brachinus elongatulus has the ability to convert m-cresol to 2-methyl-1,4-hydroquinone, which is then oxidised to 2-methyl-1,4-benzoquinone (toluquinone), within 24 h in its defensive spray, when added to food or injected into the haemocoel [66]. An origin from amino acids has been demonstrated for carboxylic acids. Valine is converted into methacrylic and isobutyric acids in Carabus taedatus [67] and Scarites subterraneus [68]. Biosynthesis of both tiglic and ethacrylic acid from isoleucine via 2-methylbutyric acid has been demonstrated in Pterostichus californicus [69]. Indeed, valine and isoleucine are essential amino acids, diet-dependent and strictly regulated by the availability of resources [70].

Table 2.

Components of pygidial gland secretions in Carabidae. Classification of taxa has been arranged according to Bousquet [56] and Beutel and Ribera [57].

Subfamily	Tribe	Genus	Species	Substances *	Refs
Paussinae	Metriini	Metrius	M. contractus	H18, H19, H20, H21, H22, H23, H25, H26, H27, H28, H29, H36, H35, H37, H41, H42, H52, H53, H54, H55, H56, H62, Q2, Q3, Q8, Q11, Q13	[62]
	Ozaeniini	Arthropterus	A. sp.	Q6, Q11	[34]
		Mystropomus	M. regularis	Q2, Q6, Q11	[34]
		Pachyteles	P. longicornis	H52, Q2	[59]
			P. striola	H52, Q2	[59]
		Physea	P. hirta	H52, Q2	[59]
		Platycerozaena	P. panamensis	H52, Q2, Q8, Q11	[59]
	Paussini	Homopterus	H. arrawi	H52, Q2	[59]
		Paussus	P. favieri	Q2, Q11	[92]
Cicindelinae	Cicindelini	Cicindela	C. flexuosa	E9, E19	[93]
			C. haemorrhagica	A1, B1, E19, H52, H62	[79]
			C. marutha	E9	[79]
			C. nigrocoerulea	E19	[79]
			C. punctulata chihuahuae	A1, E19	[79]
			C. sedecimpunctata	A1, T3	[79]
			C. sexguttata	T3	[79]
			C. abdominalis, C. andrewesi, C. angulata, C. assamensis, C. aurofasciata, C. belfragei, C. bicolor, C. bigemina, C. calligramma, C. cancellata, C. cardoni, C. catena, C. celeripes, C. chloris, C. circumpicta, C. cuprascens, C. depressula, C. duodecimguttata,  C. duponti, C. erudita, C. f. generosa, C. f. manitoba, C. fabriciana, C. fastidiosa, C. fowleri, C. fulgida, C. grammophora, C. hamata, C. hamiltoniana, C. hirticollis, C. horni, C. intermedia, C. lemniscata, C. limbata, C. macra,  C. melancholica, C. minuta, C. motschulskyana, C. multiguttata, C. nevadica, C. o. rectilatera, C. obsoleta, C. ocellata ocellata, C. oregona,  C. pamphila, C. pimeriana,  C. pulchra, C. punctulata punctulata, C. purpurea, C. repanda, C. rufiventris, C. rugosiceps, C. s. lecontei,  C. s. rugifrons, C. schauppi, C. severa, C. severini, C. striatifrons, C. striolata, C. sumatrensis, C. togata globicollis, C. tranquebarica, C. venosa, C. virgula, C. westermanni, C. willistoni, C. lengi	A1	[79]
		Odontocheila	O. annulicornis, O. cayennensis, O. confuse, O. luridipes	A1, H52	[79]
		Pentacomia	P. egregia	A1, H52	[79]
Cicindelinae	Collyridini	Neocollyris	N. variitarsus	A1	[79]
	Megacephalini	Megacephala	M. carolina	A1, B8, N1	[79]
		Omus	O. audouini	B15	[79]
Carabinae	Carabini	Calosoma	C. (Campalita) chinense	C4, F27, A2	[33]
			C. externuum	C4	[28]
			C. marginalis	C4	[28]
			C. oceanicum	A2, C4, F5	[34]
			C. prominens	A2	[30,64]
			C. schayeri	A2, C4, F5	[34]
			C. sycophanta	A2, C4, C5, B1, F2, F6, F11, F17, F25, F27	[30,48]
		Carabus	C. auratus	C4, F27	[28,30,78,94]
			C. auronitens	C4, F27	[30,78,94,95]
			C. (Damastes) blaptoides	C2, C4, F27	[33]
			C. (Megodontus) caelatus	B1, C1, C4, F1, F2, F8, F11, F17, F25, F27	[43]
			C. (Tachypus) cancellatus	C4, F27	[30]
			C. cansellatus	C4, F27	[78]
			C. cyaneus	C4, F27	[30]
			C. (Tomogarabus) convexus	B1, C4, F27	[30,42,78,94]
			C. (Procustes) coriaceus	B1, C4, F27	[30,42,49,94]
			C. (Apotomopterus) dehaanii	C2, C4, F27	[33]
			C. granulatus	C4, F27	[30,78,94]
			C. intricatus	C4, F27	[94]
			C. (Platycarabus) irregularis	C4, F27	[30,94]
			C. (Apotomopterus) japonicus	C2, C4, F27	[33]
			C. (Archicarabus) montivagus	C4, F27	[43]
			C. porrecticollis	C2, C4, F27	[33]
			C. problematicus	C4, F27	[28,78]
			C. procelus	C2, C4, F27	[30,33]
			C. taedutus	C2, C4	[67]
			C. ullrichii	B1, C4, F1, F2, F11, F17, F27	[30,49,94]
			C. (M.) violaceus	B1, C4, F1, F11, F17, F25, F27	[30,43,94]
			C. (Apotomopterus) yaconinus	C2, C4, F27	[33]
		Hemicarabus	H. tuberculosus	C2, C4	[33]
Carabinae	Ceroglossini	Ceroglossus	C. buqueti	B1, C1, C2, C4, C5, F3, F11, F14, F27, H61, S1	[96]
			C. chilensis	B1, C2, C4, F3, F11, F14, F27, H61	[96]
			C. magellanicus	B1, C2, C4, F3, F11, S1	[96]
	Cychrini	Cychrus	C. caraboides rostratus	C4, F27	[28,30,94]
		Scaphinotus	S. andrewsi germari, S. andrewsi montana, S. virdus, S. webbi	C4, F27	[77]
	Pamborini	Pamborus	P. alternans, P. guerini, P. pradieri, P. viridis	C2, C4	[34]
Elaphrinae	Elaphrini	Elaphrus	E. riparius	F11, F14	[28]
Loricerinae	Loricerini	Loricera	L. pilicornis	F11, F14	[28]
Omophroninae	Omophronini	Omophron	O. limbatum	F11, F14	[28]
Nebriinae	Nebriini	Leistus	L. ferrugineus	C4, F27	[28,78]
		Nebria	N. chinensis	C2, C4, F27	[33]
			N. lewisi	C2, C4, F27	[33]
			N. livida	C4, F27	[28,78]
			N. macrogona	C2, C4, F27	[33]
			N. psammodes	C4, F27	[50]
Nebriinae	Notiophilini	Notiophilus	N. biguttatus	F11, F14	[28]
			N. impressifrons	C4, F27	[33]
Scaritinae	Scaritini	Scarites	S. aterrimus	C4, F1, F6, F13, F27	[33]
			S. subterraneus	C4, F1, F6, F11, F13, F27	[68]
			S. cutidens	C4, F1, F13, F27	[33]
			S. sulcatus	C4, F1, F13, F27	[33]
			S. terricola	C4, F1, F13, F27	[33]
	Clivinini	Ardistomis	A. schaumii	Q11, T1, T4	[75]
		Clivina	C. basalis	Q1, Q11	[34]
			C. fossor	Q1, Q6, Q2, Q11, Q12, Q13	[78]
		Semiardistomis	S. puncticollis	Q2, Q11, T2, T4, T5, T6, T7	[75]
		Schizogenius	S. lineolatus	F8, F9, F10	[31]
	Dyschiriini	Dyschirius	D. wilsoni	B9, K2, K7, T3	[97]
	Pasimachini	Pasimachus	P. subsulcatus	C4, F1, F11, F14, F17, F25, F27	[51,52]
	Carenini	Carenum	C. bonelli	C4, F1, F7, F13	[34]
			C. interruptum	C4, F1, F13	[34]
			C. tinctillatum	C4, F1, F13, F27	[34]
		Laccopterum	L. foveigerum	C2, C4, F5, F6, F7, F13, F27	[34]
		Philoscaphus	P. tuberculatus	C4, F6, F12, F13, F27	[34]
Broscinae	Broscini	Broscosoma	B. doenitzi	F2, F14	[33]
		Broscus	B. cephalotes	F11, F14	[28,98,99]
		Craspedonotus	C. tibialis	F1, F11, F14	[33]
		Eurylychnus	E. blagravei	C4, F27	[34]
			E. ollifi	C4, F17, F27	[34]
Trechinae	Trechini	Duvalius	D. (Paraduvalius) milutini	B1, F4, F5, F15, F18, F22, F23, F24, F26	[100]
		Pheggomisetes	P. ninae	A1, C1, C5, F2, F11, F12, F14, F16, F18, F22, F23, F26, H12, H16, H17, H24, H34, H38, H39, H40, H45, H48, H49, H62, H65	[100]
		Trechoblemus	T. postilenatus	F11, F14	[33]
	Bembidiini	Bembidion	B. lampros	F11, F14	[28]
			B. lissonotum	C4, F27	[33]
			B. morawitzi	C4, F27	[33]
			B. quadriguttatum	A2, F28	[78]
			B. semilunium	C4, F27	[33]
			B. stenoderum	C4, F27	[33]
		Tachys	T. sericans	F11, F14	[33]
Patrobinae	Patrobini	Amblytelus	A. curtus	C3	[34]
		Asaphidion	A. semilucidum	F2, F14	[33]
		Diplous	D. caligatus	C4, F27	[33]
			D. depressus	C4, F27	[33]
		Patrobus	P. flavipes	C4, F1, F13, F27	[33]
			P. longicornis	C1, C4, F27, K6	[31]
Harpalinae	Morionini	Morion	M. simplex	C1, C3, C4, F25, F27, H3, H4, H5, H6	[31]
		Moriosomus	M. seticollis	B1, C1, C3, E6, E19, E20, E21, H3, H5, H6, H7	[31]
	Perigonini	Diploharpus	D. laevissimo	C1, C3, E1, E3, E4, E12, H1, H3	[31]
	Loxandrini	Loxandrus	L. icarus	C1, C3, F7, H2, H3	[31]
			L. longiformis	A2	[34]
			L. velocipes	C1, C3, F7, H3, H7	[31]
	Sphodrini	Calathus	C. fuscipes	C3	[98,99]
		Dolichus	D. halensis	K8	[101]
		Laemostenus	L. punctatus	C1, C3, E6, E20, F22, F23, F26, G2, H30, H32, H65	[87,100]
		Synuchus	S. callitheres	C3	[33]
			S. cycloderus	C3, K8	[33]
			S. dulcigradus	C3, K8	[33,101]
	Pterostichini	Abacomorphus	A. asperulus	C3, C4, F1, F27	[34]
		Abaris	A. anaea	C4, F27, H3, H4, H5	[31]
		Blennidus	B. liodes	C4, F14, F27, H4	[31]
		Castelnaudia	C. superba	C1, C4, F27	[34]
		Cratoferonia	C. phylarchus	C4, F27	[34]
		Cratogaster	C. melas	C4, F27	[34]
		Cyclotrachelus	C. sigillatus	C1, C3	[31]
		Gasterllarius	G. honestus	C4, F25	[31]
		Incastichus	I. aequidianus	C1, C3	[31]
		Lesticus	L. magnus	C4, F27	[33]
		Loxodactylus	L. carinulatus	C3	[34]
		Myas	M. coracinus	C4, F25, F27	[31]
		Notonomus	N. angustibasis, N. crenulatus, N. miles, N. muelleri, N. opulentus, N. rainbowi, N. scotti, N. triplogenioides, N. variicollis	C3	[34]
		Poecilus	P. coerulescens	C4, F27	[33]
			P. cupreus	C4, F27, H8, H62, H65	[78]
			P. fortipes	C4, F27	[33]
		Prosopogmus	P. harpaloides	C4	[34]
		Pseudoceneus	P. iridescens	C4, F27	[34]
		Pterostichus	P. (Hypherpes) californicus	C2, F17, F27	[69]
			P. (Cophosus) cylindricus	C4, F27	[44]
			P. daisenicus	C2, C4, F27	[33]
			P. (Monoferonia) diligendus	C4, F27	[31]
			P. externepunctatus roccai	C4, F11, F27, H62, H65	[50]
			P. fortis	C2, C4, F27	[33]
			P. fujimurai	C2, C4, F27	[33]
			P. longinquus	C2, C4, F27	[33]
			P. luctuosus	C4, F27	[31]
			P. macer	C4, F27, H8, H62, H65	[28,78]
			P. masidai	C2, C4, F27	[33]
			P. (Ferodinius) melas	C4, F11, F17, F25, F27, H8, H62, H65	[28,44,78]
			P. metallicus	C4, F27, H8, H62, H65	[28,30,78,94]
			P. microcephalus	C2, C4, F27	[33]
			P. niger	C4, F27, H8, H62, H65	[28,30,78,94]
			P. (Pseudomaseus) nigrita	C1, C4, C5, F9, F11, F17, F27, H13, H14, H15, H57, H60, H62, H64, H65	[44]
			P. prolongatus	C2, C4, F27	[33]
			P. rotundangulus	C2, C4, F27	[33]
			P. vulgaris	C4, F27, H8, H62, H65	[30,78,94]
		Rhytisternus	R. laevilaterus	C4, F27	[34]
		Sarticus	S. cyaneocinctus	C3	[34]
		Sphodrosomus	S. saisseri	C3	[34]
		Trichosternus	T. nudipes	C4, F27	[34]
		Trigonotoma	T. lewisii	C4, F27	[33]
		Trigonognatha	T. cuprescens	C4, F27	[33]
	Platynini	Agonum	A. chalcomum	C3, K8	[33,101]
			A. daimio	C3	[33]
		Anchomenus	A. (Idiochroma) dorsalis	H14, H58, H60, H65	[28]
			A. leucopus	C3	[33]
		Colpodes	C. atricomes	C3	[33]
			C. japonicus	C3	[33]
		Loxocrepis	L. rubriola	C3	[33]
		Lorostemma	L. ogurae	C3	[33]
		Platynus	P. brunneomarginatus	C3, F18, F23, H52, H62, H65, K1, K3, K6, K8	[31]
			P. magnus	C3	[33]
			P. ovipennis	C3, F19, H52, H62, H65, K1, K3, K6, K8	[102]
			P. protensus	C3, K8	[33,101]
	Zabrini	Amara	A. chalcites	C2, C4, F1, F27	[33]
			A. chalcophaea	C2, C4, F1, F27	[33]
			A. familiaris	C4, F27, H8, H62, H65	[28,78]
			A. similata	C4, F27, H8, H62, H65	[28,78]
		Bradytus	B. ampliatus, B. simplicidens	C2, C4, F1, F27	[33]
		Curtonotus	C. giganteus	C2, C4, F1, F27	[33]
	Molopini	Abax	A. ovalis	C4, F27	[28,30,78]
			A. parallelepipedus (sub:A. ater)	C4, C5, F6, F11, F25, F27	[28,30,49,78]
			A. parallelus	C4, F27	[30,78,95]
		Molops	M. elatus	C4, F27	[28,78]
			M. (Stenochoromus) montenegrinus	C1, C4, C5, F1, F2, F5, F8, F9, F11, F17, F28	[44]
	Harpalini	Anisodactylus	A. signatus	C3, K8	[33,101]
			A. tricuspidatus	C3	[33]
		Anoplogenius	A. cyanescens	C3	[33]
		Bradycellus	B. inornatus	C3	[33]
		Diaphoromerus	D. edwardsi	C3	[34]
		Harpalus	H. atratus	C3	[98,99]
			H. capito	C3, K8	[33,101]
			H. dimidiatus	C3	[30]
			H. distinguendus	C3	[98,99]
			H. luteicornis	C3	[98,99]
			H. platynotus	C3	[33]
			H. sinicus	C3	[33]
			H. tardus	C3	[98,99]
		Platymetopus	P. flavibarbis	C3	[33]
		Pseudophonus	P. griseus	C3	[30,33,94]
			P. rufipes (sub:pubescens)	C3	[30,94]
		Stenolophus	S. agonoides	C3, H63	[33,101]
			S. difficilis	C3	[33]
			S. iridicolor	C3	[33]
		Trichocellus	T. tenuimanus	C3	[33]
		Trichotichnus	T. longitarsis	C3, H63	[33,101]
	Licinini	Dicrochile	D. brevicollis	C3	[34]
			D. goryi	C3	[34]
		Diplocheila	D. elongata	C3	[33]
			D. zeelandica	C3, H63	[33,101]
		Syagonix	S. blackburni	C3	[34]
	Chlaeniini	Callistoides	C. delciolus	B2	[33]
		Callistus	C. lunatus	Q2, Q11	[28]
			C. basalis	Q2	[28]
		Chlaenius	C. australis	B2	[34]
			C. circumdatus	B2	[33]
			C. cordicollis	B2, B4, B3, B5, B7, B11, B12	[30,91]
			C. (Chlaeniellus) inops	Q1, Q6, Q10	[33]
			C. noguchii	B2	[33]
			C. pallipes	B2	[33]
			C. (Chlaeniellus) postemus	Q1, Q6, Q10	[33]
			C. spoliatus	B2	[33]
			C. velutinus	B2, B4, B6, H22, H52, Q1, Q7, Q11	[50]
			C. vestitus	B2, Q2	[28,50]
			C. virgulifer	B2	[33]
		Epomis	E. nigricans	B2	[33]
		Macrochlaenites	M. costiger	B2	[33]
		Oodes	O. amaroides	A2, B1, C1, F11, F14	[31]
			O. americanus	B1, C1, C4, C5, F11, F2, F5, F6, F8, F14, F17, F21, F27	[89]
	Panagaenini	Dischissus	D. mirandus	B2	[33]
		Panagaeus	P. bipustulatus	B2	[98,99]
			P. japonicus	B2	[33]
		Peronomerus	P. auripilis	B2	[33]
			P. nigrinus	B2	[33]
	Odacanthini	Archiocolliuris	A. bimaculata nipponica	H63	[101]
		Colliuris	C. pensylvanica	C1, C3, H3, K6	[31]
		Odacantha	O. melanura	C3	[28]
	Lebiini	Apristus	A. grandis	C3, H63	[33,101]
		Callida	C. lepida	C3	[33]
		Lebia	L. retrofasciata	C3	[33]
		Eudalia	E. macleayi	C3	[34]
		Coptodera	C. japonica	C3	[33]
			C. subapicalis	C3	[33]
		Cymindis	C. daimio	H63	[101]
		Dolichoctis	D. striatus	C3, H63	[33,101]
		Helluomorphoides	H. clairvillei	H43, H9, H8, H65, E8, H11, E17, E14, E16, E12, E3, E15, E1, E21, E3, E22, E13, E5, E20, E7, E6, H14, H31, H58, H33	[85]
			H. ferrugineus	C3, E12	[84]
			H. latitarsis	C3, E12	[84]
		Lebidia	L. octoguttata	C3, H63	[33,101]
	Galeritini	Galerita	G. lecontei	C3, E1, E2, E3, E5, E6, E13, E12, E17, E20, G1, H8, H43, H44, H46, H62, H65, H66, H67	[45]
		Galeritula	G. japonica	C3, E1, E3, E12	[33,101]
		Planetes	P. puncticeps	C3, E1, E3, E12	[33,101]
	Anthiini	Anthia	A. thoracica	A3, C1, C3, F1, F27	[103]
		Thermophilum	T. burchelli	C1, C3, F1, F27	[103]
			T. homoplatum	A3, C1, C3, F1, F27	[103]
	Helluonini	Helluo	H. costatus	C3, E12, E13	[34]
	Catapieseini	Catapiesis	C. attenuata	C3, E1, H1, H3	[31]
			C. sulcipennis	E1	[31]
	Dryptini	Drypta	D. japonica	C3, E1, E3, E12	[33,101]
	Pseudomorphini	Sphallomorpha	S. colymbeioides	C3	[34]
Brachininae	Brachinini	Brachinus	B. chuji	Q1, Q9	[33]
			B. crepitans	Q2, Q11	[28,30]
			B. elongatus	H10, H14, H60, Q2, Q4, Q5, Q8, Q11, Q12	[41,55,66]
			B. explodens	Q2, Q11	[28,30]
			B. sclopeta	Q2, Q11	[30,41]
			B. scotomedes	Q1, Q9	[33]
			B. stenoderus	Q1, Q9	[33]
		Pheropsophus	P. africanus	N2, N3	[30,41]
			P. agnatus	C3	[30]
			P. catoirei	Q2, Q11	[104]
			P. verticalis	Q1, Q11	[34]
			P. jessoensis	Q1, Q9	[33]

* Abbreviations [Aldehydes (A)—A1: benzaldehyde; A2: salicylaldehyde; A3: iso-valeraldehyde. Benzene, substituted derivatives and phenols (B)—B1: benzoic acid; B2: cresols, (m-cresol), (3-methylphenol); B3: 2,3-dimethylphenol; B4: 2,5-dimethylphenol; B5: 3,4-dimethylphenol; B6: 3,5-dimethylphenol; B7: 3-ethylphenol; B8: mandelonitril; B9: methyl 2-hydroxy-6-methylbenzoate; B10: methyl salicylate; B11: 2-methoxy-5-methylphenol; B12: 2-methoxy-4-m-cresol; B13: 2-phenylethanol; B14: 2-phenylethyl; B15: phenylacetic acid; B16: xylenol isomer. Carboxylic acids and derivatives (C)—C1: acetic acid; C2: ethacrylic acid; C3: formic acid; C4: methacrylic acid; C5: propanoic acid (propionic acid). Fatty alcohol esters (E)—E1: decyl acetate; E2: decyl butyrate; E3: decyl formate; E4: decyl hexanoate; E5: decyl propionate; E6: dodecyl acetate; E7: dodecyl formate; E8: heptyl acetate; E9: hexadecyl acetate; E10: isopropyl ethacrylate; E11: isopropyl methacrylate; E12: nonyl acetate; E13: nonyl butyrate; E14: nonyl formate; E15: nonyl propionate; E16: 3-nonen-l-yl acetate; E17: octyl acetate; E18: 2-phenylethyl ethacrylate; E19: tetradecyl acetate; E20: undecyl acetate; E21: undecyl formate; E22: 4-undecen-l-yl acetate. Fatty acids and conjugates (F)—F1: angelic acid; F2: butyric acid; F3: n-butanoic acid; F4: capric acid; F5: caproic acid (hexanoic acid); F6: crotonic acid; F7: hexenoic acid; F8: 2-hexenoic acid; F9: 3-hexenoic acid; F10: 3,5-hexadienoic; F11: isobutyric acid; F12: isocaproic acid; F13: isocrotonic acid; F14: isovaleric acid (3-methylbutyric acid); F15: lauric acid; F16: linoleic acid; F17: 2-methylbutyric acid; F18: myristic acid; F19: nonanoic acid; F20: octanoic acid (caprylic acid); F21: 2-octenoic acid; F22: oleic acid; F23: palmitic acid; F24: pelargonic acid; F25: senecioic acid; F26: stearic acid; F27: tiglic acid; F28: valeric acid. Fatty alcohol (G)—G1: 1-decanol; G2: dodecan-1-ol. Hydrocarbons (H)—H1: C9:0; H2: C10:0; H3: C11:0; H4: C12:0; H5: C13:0; H6: C15:0; H7: C17:0; H8: decane; H9: 1-decene + 3-decene; H10: 9-docosene; H11: dodecane; H12: 3-ethyltetracosane; H13: heneicosadiene; H14: heneicosane; H15: heneicosene; H16: heptacosadiene; H17: heptacosene; H18: 5,7-heptadecadiene; H19: 7,9-heptadecadiene; H20: (6Z,9Z)-6,9-heptadecadiene; H21: (7Z,9Z)-7,9-heptadecadiene; H22: heptadecane; H23: (Z)-8-heptadecene; H24: hexacosane; H25: hexadecadiene; H26: 6,8-hexadecadiene; H27: 7,9-hexadecadiene; H28: hexadecane; H29: hexadecene; H30: 7-hexyldocosane; H31: 9-methylheneicosane; H32: 9-methyltetracosane; H33: 9-methyltricosane; H34: 11-methylheptacosane; H35: 3-methylpentadecane; H36: 4-methylpentadecane; H37: 5-methylpentadecane; H38: nonacosapentaene; H39: nonacosatetraene; H40: nonacosene; H41: nonadecane; H42: 7,9-nonadecadiene; H43: nonane; H44: 1-nonene; H45: octacosane; H46: octane; H47: pentacosadiene; H48: pentacosane; H49: pentacosene; H50: (Z)-7-pentacosene; H51: (Z)-9-pentacosene; H52: pentadecane; H53: 5,7-pentadecadiene; H54: 6,8-pentadecadiene; H55: 7-pentadecene; H56: tetradecane; H57: tricosadiene; H58: tricosane; H59: (Z)-7-tricosene; H60: (Z)-9-tricosene; H61: 11-tricosene; H62: tridecane; H63: 2-tridecane; H64: tricosatriene; H65: undecane; H66: 4-undecene; H67: 5-undecene. Ketone (K)—K1: 2-dodecanone; K2: 2-heptanone; K3: 2-heptadecanone; K4: 2-hexanone; K5: 3-hexanone; K6: 2-pentadecanone; K7: 2-pentanone; K8: 2-tridecanone. Non-metal oxoanionic compounds and organonitrogen compounds (N)—N1: hydrogen cyanide; N2: nitrites; N3: nitrous acid. Quinone (Q)—Q1: benzoquinone; Q2: 1,4-benzoquinone; Q3: 2-chloro-1,4-benzoquinone; Q4: 2,5-dimethyl-1,4-benzoquinone; Q5: 2,3-dimethyl-1,4-benzoquinone; Q6: 2-ethylquinone; Q7: ethylbenzoquinone; Q8: 2-ethyl-1,4-benzoquinone; Q9: 2-methylbenzoquinone; Q10: 2-methylquinone; Q11: 2-methyl-1,4-benzoquinone (toluquinone); Q12: methoxy-1,4-benzoquinone; Q13: 2-methoxy-3-methyl-1,4-benzoquinone. Terpenes (T)—T1: 1,8-cineole; T2: p-cymene; T3: iridodial; T4: (R)-(+)-limonene, (S)-(-)-limonene; T5: sabinene; T6: β-phellandrene; T7: β-pinene. Thioethers (S)—S1: 3-methyl-1-(methylthio)-2-butene.].

4.1. Interspecific Adaptations

The chemical composition of pygidial gland secretions exhibits interspecific variability within and among subfamilies (Table 2). This variability is the result of a trade-off between the diversity of predators in different habitats and the fitness costs of resource allocation in life traits such as behavioural defences against these enemies [2,71].

The chemicals found in secretions belong to two different functional categories, allomones and bacteriostats. Allomones are primarily involved in the secondary antipredator responses that carabids exhibit as prey to actively defend themselves against predators. Ground beetles emit volatile substances directed at specific groups of arthropods or vertebrates that act as repellents on the chemoreception of predators or interfere with physiological processes as irritants (emesis, vesication) [58]. Deterrent, toxic, and irritant properties of pygidial gland secretions are known in bombardier beetles (Brachinini), which release irritant quinones by a hot, pulsed spray mechanism [55,61,66] as an antipredator defence [72,73]. Quinones are the main class of compounds also found in the secretions of obligate or facultative myrmecophilous species belonging to Metrini, Ozaenini, and Paussini [34,74]. In the defensive secretions of Clivinini [75] and Metrius contractus [62], they are associated with complex mixtures of monoterpenes or hydrocarbons (Table 2). Saturated and unsaturated aliphatic carboxylic acids and fatty acids are widely distributed in the subfamilies Carabinae, Loricerinae, Nebrinae, Scaritinae, Scaritinae, and Harpalinae. They are recorded as a separate compound class in some species that belong to the tribes Pamborini, Elaphrini, Loricerini, Omophronini, Notiophilini, Broscini, Patrobini, Sphodrini, Pterostichini, Platynini, Harpalini, Licinini, and Lebiini. In Cychrini, irritant carboxylic acids (i.e., methacrylic acid) and fatty acids (i.e., tiglic acid) are released, associated with a stridulatory elytra-abdominal mechanism acting as an acoustic warning signal against predators [76,77]. Behavioural analyses showed that Pasimachus subsulcatus (Scaritinae) secretes a mixture of methacrylic acid and fatty acids to protect itself from lizards [51,52]. Carboxylic acids are also found in variable associations with terpenes, quinones, and hydrocarbons in Trechinae, and Harpalinae (Table 2). The repellent effect of salicylaldehyde in the secretion of Calosoma prominens has been tested against ants and vertebrates [64]. This chemical has also been detected in C. sycophanta, C. schayeri, C. oceanicum [34,48], C. prominens [64], C. chinenses [33], Loxandrus longiformis [34], and Bembidion quadriguttatum, A. flavipes [78] in a mixture with carboxylic acids. Benzaldehyde is the typical component of secretion in Cicindelinae [79,80]. It is produced via a cyanogenetic pathway that is absent in the other carabid subfamilies [81]. In tiger beetles, secretion of benzaldehyde may be associated with several antipredator characters, including aposematic camouflage, flight, and gregarious behaviour to avoid predators such as robber flies, lizards, and birds [17,82,83].

Synergism between polar volatile irritant compounds and lipophilic components of secretion has been demonstrated. Nonpolar lipophilic components from Galerita lecontei (long-chain hydrocarbons and esters) act as wetting and penetration enhancers and facilitate the spread of volatile polar compounds such as formic and acetic acids in the cuticle of predators [45]. In Helluomorphoides clairvillei, n-nonyl acetate facilitates the spread of formic acid through the epidermis or cuticle of predators [84]. The same surfactant effect has been attributed to hydrocarbons [85] for the uptake of repellent quinones in Metrius contractus [62].

The mixture of substances in glandular secretions also has biological functions. In vitro assays have shown that the pygidial gland secretion inhibits cell proliferation [86]. The mixture of aromatic (benzoic acid) and aliphatic carboxylic acids, esters, and terpenes have antimicrobial and fungicidal activity in Carabus ullrichii, C. coriaceus, Abax parallelepipedus [49], caterpillar hunter Calosoma sycophanta [48], and troglophilic and guanophilic Laemostenus (Pristonychus) punctatus [87]. Complex mixtures of monoterpenes are found in the defensive secretions of a large number of the species reported here (Table 2). Terpenes are volatile and are present in glandular secretions of many taxa, acting as chemical deterrents, trail scents, mating attractants, or alarm pheromones [22]. In carabid beetles, they have also been detected in the pupal stages of Carabus lefebvrei [88].

4.2. Intraspecific Adaptations

Little is known about intraspecific variation in secretion as a function of sex, age, and resource availability. Nevertheless, data collected to date suggest that chemical secretion plays a parsimonious role in both antipredation and mating behaviour. In Oodes americanus, defensive secretion shows qualitative differences in males and females [89]. The sexual dimorphism of carboxylic acids found in the defensive secretion of Chlaenius cordicollis depends on the reproductive status and age of both sexes and provides a means of chemical communication between the sexes [89,90]. Sex-specific variation likely protects mates during copulation, and the flower number of compounds in female secretions saves females the cost of synthesising them [90]. Although compounds that act as pheromones, such as pentacosadiene, 7-hexyldocosane, 9-methyltetracosane, have been detected in Laemostenus punctatus, Trachypachus gibbsi, and Helluomorphoides clairvillei, studies on their role in alarm or sex-aggregation reproductive behaviour are limited [27].

Intra- and inter-population variation in defensive secretion has also been documented to reflect genetic variability at the population level in responses to selective habitat pressure, as observed in Chaenius cordicollis [91], Pasimachus subsulcatus [51], and Cicindelinae [80]. On the other hand, the shift in secretion composition may have a dietary origin, as observed in species of the genus Scaphinotus [77]. These findings suggest the role of dietary chemical precursors in the biosynthesis of chemical secretions.

5. Concluding Remarks

Pygidial glands are homologous structures in the Carabidae. They show a range of morphological variations in structural elements, i.e., number of acini, the morphology of ducts and reservoir chamber, and mode of secretion discharge, among carabid species, regardless of habitat and associated ecological differences. Chemical defences are an important part of antipredator strategies in ground beetles. Prey–predator coevolution likely influences glandular secretion composition, which is the result of a trade-off between the predator diversity and the fitness costs of defending against these enemies. A great deal of interspecific diversity in the distribution of substances has been found in subfamilies. Some chemicals are readily identifiable as specific to particular taxa, while others show great species-level diversity among genera or tribes. These results are broadly consistent with previous studies in which the taxonomic distribution of compound secretion was reviewed according to habitat diversification and by mapping chemical classes in a phylogenetic context [31,33]. However, some elements need to be considered in the future interpretation of the taxonomic distribution of chemicals. The findings pertain to only the 4% of carabid species so far described, and further studies are needed to clarify differences in chemical composition in additional taxa. A large number of studies reported only the most abundant chemicals, neglecting compounds that are present in smaller percentages and have additional biological functions in the mixture, e.g., surfactants, pheromones, and antiseptic agents. In addition, the differences found in some chemical profiles may be related to the number of samples analysed as single or mixed samples or to the accuracy of the gas chromatographic equipment used in early studies. Finally, we recommend that further research should address to elucidate: (1) the biogenesis of all chemicals described in the pygidial glands and their function in an ecological context; (2) clarify the phylogenetic distribution patterns of chemicals by studying as many species as possible using comparable protocols; (3) the sexual dimorphism of the secretion with regard to the different degree of resource allocation between the sexes under the pressure of environmental selection.

Author Contributions

Conceptualisation, A.G.; data curation, F.T. and M.L.V.; validation of taxonomy, P.B.; writing—original draft preparation, A.G.; writing—review and editing: A.G., M.L.V. and F.T. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

No new data were created or analyzed in this study. Data sharing is not applicable to this article.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.