Insect exocrine glands

Impact of environmental pollution on ant (Camponotus japonicus) development and labial gland disease

Metallic pollutants can have harmful impacts on ant morphology and physiology. We studied the occurrence of labial gland disease in Camponotus japonicus from two polluted areas (traffic pollution and industrial pollution) and one non-polluted area. We further analyzed the metal levels (Al, As, Cd, Co, Cr, Cu, Fe, Hg, Mn, Ni, Pb and Zn) and morphological characteristics (head width, body mass, and other morphological traits) of both diseased and healthy workers. Our results showed that labial gland disease was only present in polluted areas, indicating that pollution stress makes ants more vulnerable to infections. Our research revealed that diseased ants in polluted areas accumulate higher levels of metals in their bodies and have lower dry weight and residual body mass compared to healthy ants in non-polluted environments, negatively impacting their development. We evaluated the importance of these elements and found that Cu has the greatest impact on the health risk of C. japonicus. Our study underscores the significant impact of environmental pollution on ant morphology and physiology and raises concerns about the broader ecological implications.

The Budding Neuroscience of Ant Social Behavior

Ant physiology has been fashioned by 100 million years of social evolution. Ants perform many sophisticated social and collective behaviors yet possess nervous systems similar in schematic and scale to that of the fruit fly Drosophila melanogaster, a popular solitary model organism. Ants are thus attractive complementary subjects to investigate adaptations pertaining to complex social behaviors that are absent in flies. Despite research interest in ant behavior and the neurobiological foundations of sociality more broadly, our understanding of the ant nervous system is incomplete. Recent technical advances have enabled cutting-edge investigations of the nervous system in a fashion that is less dependent on model choice, opening the door for mechanistic social insect neuroscience. In this review, we revisit important aspects of what is known about the ant nervous system and behavior, and we look forward to how functional circuit neuroscience in ants will help us understand what distinguishes solitary animals from highly social ones.

Trail pheromone identification in the ant Crematogaster scutellaris

In this work, we identified the trail pheromone of the ant Crematogaster scutellaris. We combined gas chromatography-mass spectrometry analysis of extracts from the hind tibia, the location of the respective glands, with automated trail following assays. The study found tridecan-2-ol to be the strongest discriminator between hind tibia and other body part extracts. Tridecan-2-ol elicited trail-following behaviour at concentrations of 1 ng/µL. A separation of the enantiomers showed responses to (R)-tridecan-2-ol already at 0.001 ng/µL and only at a 1000-fold higher concentration for (S)-tridecan-2-ol, suggesting that only the R enantiomer is used by C. scutellaris in its natural environment. We also found strong behavioural responses to 2-dodecanol, a substance that was not detectable in the hind tibia extract of C. scutellaris, but which has been reported to be the trail pheromone of the related species C. castanea. We discuss the contribution of these results to the 'dissection and reconstruction' of strategies and mechanisms underlying the social organization of ants.

Chemical Components of Dufour's and Venom Glands in Camponotus japonicus (Hymenoptera, Formicidae)

The Dufour's and venom glands are the most developed glands connected to the female reproductive organs, playing important roles in defense, foraging, information exchange, and reproduction in ants. The main chemical secretions of these glands vary among species and even among castes of the same species. In this study, we analyzed the chemical components of the Dufour's and venom glands in different castes of Camponotus japonicus (original worker, minor worker, major worker, gyne, and queen) using gas chromatography-mass spectrometry (GC-MS) with two sample processing methods (hexane solution and solid-phase microextraction). The secretion of the Dufour's gland is characterized by a high ratio of alkanes, with n-undecane being the dominant secretion in all castes except the original workers. The venom gland's secretion mainly includes alkanes, acids, ketones, and alcohols, with formic acid and n-undecane being the dominant components. Additionally, the chemical composition and proportion of the main components vary significantly among castes, which may be closely related to the division of labor in their social life. This study provides basic information to further understand the function of these two glands in the social life of ants.

Comparative morphology and ultrastructure of the labial gland among castes of Camponotus japonicus (Hymenoptera: Formicidae)

The labial gland is a major exocrine gland in the thoracic cavity of ants, which is connected to the mouth area and opens at the base of the labium. The labial gland in the various castes and males of Camponotus japonicus were examined using dissection, light microscopy, scanning and transmission electron microscopy. Our findings reveal clear caste differences in the appearance of the labial gland, and suggest a relationship between gland structure and its behavioral role in queens, males and workers: queens possess the largest reservoir sac; males have the smallest; workers have abundant secretion droplets in the gland cells of the tubules; epithelial thickness of gland cells varies considerably between workers and reproductive ants. The apical cell region has a high density of microvilli and polymorphic mitochondria, whereas the central cell region is rich in rough endoplasmic reticulum (RER) which means its main secretions are proteinaceous compounds. We believe that the labial gland has a different function in the castes of C. japonicus: the gland of workers and queens may be used to feed the larvae; workers participate in trophallaxis in the nest, while the males are not involved in these activities. Calculations of the relative size of the labial gland for each caste and males indicated that minor workers have the relative highest developed labial gland, supporting a role in trophallaxis. In addition, we found a phenomenon of enlarged labial gland in minor workers, which accounted for almost 22%, but the reason for this is unknown.

Linking the Morphology of Sternal Glands to Rubbing Behavior by Vespa soror (Hymenoptera: Vespidae) Workers During Recruitment for Group Predation

The activities of social insect colonies are supported by exocrine glands and the tremendous functional diversity of the compounds that they secrete. Many social wasps in the subfamilies Vespinae and Polistinae have two sternal glands-the van der Vecht and Richards' glands-that vary in their features and function across the species in which they are found. Field observations suggest that giant hornets use secretions from the van der Vecht gland to chemically mark targeted nests when workers initiate group attacks on social insect prey. However, descriptions of giant hornets' sternal glands and details about their recruitment behavior are lacking. We describe the morphology of the sternal glands of the giant hornet Vespa soror du Buysson and consider their potential to contribute to a marking pheromone. We also assess the gastral rubbing behavior of workers as they attacked Apis cerana F. (Hymenoptera: Apidae) colonies. V. soror workers have well-developed van der Vecht and Richards' glands on their terminal gastral sternites, with morphologies that robustly support the synthesis, storage, and dissemination of their secretory products. Observations confirm that the van der Vecht gland is exposed during gastral rubbing, but that the Richards' gland and glands associated with the sting apparatus may also contribute to a marking pheromone. Workers briefly but repeatedly rubbed their gasters around hive entrances and on overhead vegetation. Colonies were heavily marked over consecutive attacks. Our findings provide insight into the use of exocrine secretions by giant hornets as they recruit nestmates to prey colonies for group attacks.

The ultrastructure of the intramandibular gland in soldiers of the termite Machadotermes rigidus (Blattodea: Termitidae: Apicotermitinae)

Machadotermes is one of the basal Apicotermitinae genera, living in tropical West Africa. Old observations suggested the presence of a new gland, the intramandibular gland, in Machadotermes soldiers. Here, by combining micro-computed tomography, optical and electron microscopy, we showed that the gland exists in Machadotermes soldiers only as an active exocrine organ, consisting of numerous class III cells (bicellular units made of secretory and canal cells), within which the secretion is produced in rough endoplasmic reticulum, and modified and stored in Golgi apparatus. The final secretion is released out from the body through epicuticular canals running through the mandible cuticle to the exterior. We also studied three other Apicotermitinae, Indotermes, Duplidentitermes, and Jugositermes, in which this gland is absent. We speculate that the secretion of this gland may be used as a general protectant or antimicrobial agent. In addition, we observed that the frontal gland, a specific defensive organ in termites, is absent in Machadotermes soldiers while it is tiny in Indotermes soldiers and small in Duplidentitermes and Jugositermes soldiers. At last, we could also observe in all these species the labral, mandibular and labial glands, other exocrine glands present in all termite species studied so far.

Comparison of the External Morphology of the Sternal Glands for Hornets in the Genus Vespa

Many social wasps in the speciose subfamilies Polistinae and Vespinae have two sternal glands-the van der Vecht gland and the Richards gland-that are not found in other insects. The presence of these glands has been confirmed in only 6 of 22 hornet species (genus Vespa) and images of their fine structure have not been produced. Here; we characterize the external morphology associated with both glands for workers of nine Vespa species using scanning electron microscopy. All hornets had similar gland configurations; although gland-associated external features differed among species. Scaled for size, glands were equivalently sized for the giant hornets (V. mandarinia and V. soror) and their closest phylogenetic relatives (V. tropica and V. ducalis). Relative size of gland-associated structures was reduced by half for V. simillima; V. velutina; and V. affinis workers. The remaining species (V. crabro and V. analis) had intermediately sized features. Differences among species in external gland structure were best explained by selective pressures related to predatory behavior, rather than defense of nests against ants. However, a lack of information about how Vespa workers use their van der Vecht and Richards glands limits a comparative interpretation of the function of their external gland morphology.

Morphology and ultrastructure of the epithelial femoral gland in cicadas (Hemiptera: Cicadidae)

Exocrine glands in the legs of social insects are found throughout all leg segments, but studies of exocrine glands in legs of solitary insects are very limited. We discovered a novel gland at the apex of the fore, mid and hind femurs from six representative species of Cicadidae, which we propose to name as the epithelial femoral gland. The epithelial femoral gland is located between the paired apodemes and the articulation membrane within the apex of the femur, which faces the proximal articulation region of the tibia. The epithelial femoral gland in the midlegs is less developed than that in the fore- and hindlegs within a species. The glandular cells belong to class-1, which contain a large amount of rough endoplasmic reticulum, secretory vesicles and Golgi bodies, indicating that these cells may produce a proteinaceous secretion. Details of the epithelial femoral gland at the ultrastructural level suggest that it may function to produce nourishing substances to the joint between femur and tibia. The less developed epithelial femoral gland in the midlegs and the slight difference in the glands between fore- and hindlegs within a species could be related to the functional differentiation of the corresponding legs in cicadas. Further studies of exocrine glands in the legs of cicadas and other Cicadomorpha insects may improve our understanding of the structural and functional divergence of legs in hemipteran insects.

Morphology of the novel ventral scape gland in the ant genus Strumigenys

We describe a novel ventral scape gland in all 19 species of the genus Strumigenys that we examined, confirming the postulated existence of this gland by Bolton (1999). The gland occurs in workers and queens; it belongs to 'class-1' and is formed by a layer of epithelial secretory cells that line the ventrodistal tegumental cuticle of the antennal scape. The gland is characterized by its bowl-shape, by the ultrastructural characteristics of microvillar arrangement and by the presence of nerves in between the epithelial cells. These features make this gland structurally very similar to 3 other epithelial glands that occur exclusively in Strumigenys. Besides the epithelial ventral scape gland, also scattered class-3 glandular cells are found. The function of both glands in the scape remains unclear, although a role in prey attraction cannot be excluded.

Salivary glands in workers of Ruptitermes spp. (Blattaria, Isoptera, Termitidae, Apicotermitinae): a morphological and preoteomic approach

Salivary glands are omnipresent in termites and occur in all developmental stages and castes. They function to produce, store, and secrete compounds, ranging from a feeding function to defensive mechanisms. Here, we provide a complete morphological overview of the salivary glands in the soldierless species Ruptitermes reconditus and R. xanthochiton, and the first proteomic profile of the salivary glands in a Neotropical Apicotermitinae representative, R. reconditus. Salivary glands from both species were composed of several acini, roughly spherical structures composed of two types of central cells (type I and II) and peripheral parietal cells, as well as transporting ducts and two salivary reservoirs. Central cells were richly supplied with electron-lucent secretory vesicles and rough endoplasmic reticulum, a feature of protein-secreting cells. Parietal cells of Ruptitermes spp. had conspicuous characteristics such as electron-lucent secretory vesicles surrounded by mitochondria and well-developed microvilli. Moreover, different individuals showed variation in the secretory cycle of salivary acini, which may be related to polyethism. Ultrastructural analysis evidenced a high synthesis of secretion and also the occurrence of lysosomes and autophagic structures in central cells. Proteomic analysis of the salivary glands revealed 483 proteins divided into functional groups, highlighting toxins/defensins and compounds related to alarm communication and colony asepsis. Soldierless termites are quite successful, especially due to morphological adaptations of the workers, including unknown modifications of exocrine glands. Thus, according to our morphological and proteomic findings, we discuss the potential roles of the salivary gland secretion in different social aspects of the sampled species.

Genetic basis of chemical communication in eusocial insects

Social behavior is one of the most fascinating and complex behaviors in humans and animals. A fundamental process of social behavior is communication among individuals. It relies on the capability of the nervous system to sense, process, and interpret various signals (e.g., pheromones) and respond with appropriate decisions and actions. Eusocial insects, including ants, some bees, some wasps, and termites, display intriguing cooperative social behavior. Recent advances in genetic and genomic studies have revealed key genes that are involved in pheromone synthesis, chemosensory perception, and physiological and behavioral responses to varied pheromones. In this review, we highlight the genes and pathways that regulate queen pheromone-mediated social communication, discuss the evolutionary changes in genetic systems, and outline prospects of functional studies in sociobiology.

The basitarsal sulcus gland, a novel exocrine structure in ants

The basitarsus of the mid- and/or hindlegs of several Amblyoponinae ants shows a deep longitudinal groove or sulcus on its anterior face in workers and queens. Histological examination reveals this sulcus is associated with a conspicuous novel epithelial gland, which brings the number of exocrine glands in the legs of ants to 25. The ultrastructural characteristics of the gland show the presence of a well-developed smooth endoplasmic reticulum. This is indicative for the elaboration of a non-proteinaceous and thus possibly pheromonal secretion. Behavioural observations show that this secretion is collected by the tarsomeres and spread onto the brood and nest, suggesting a role in nestmate recognition. A similar basitarsal sulcus gland was also found in Nothomyrmecia, Paraponera and Tetraponera, which represents both a wide phylogenetic and ecological distribution, as it includes arboreal, ground-dwelling as well as subterranean taxa.

Transmission Electron Microscopy as a Tool to Study the Toxicological Effects of Thiamethoxam in Workers of Atta sexdens (Myrmicinae, Attini)

Thiamethoxam is a neonicotinoid that has been used to control insect pests. The literature reports a few behavioral studies evaluating the toxic effect of thiamethoxam in ants; however, there are scarce studies at the cellular level. The present research evaluated the effects of thiamethoxam in labial (LG) and mandibular glands (MG), fat bodies (FB), and Malpighian tubules (MT) of workers of Atta sexdens, using transmission electron microscopy. The duct and secretory cells of LG were profoundly affected, then the production of saliva can be compromised, as well as its quality and subsequent use. In MG, reservoir and canaliculi cells presented slight alterations; however, MG secretory cells presented vacuoles containing lamellar structures, increased lipid production, and a large amount of mitochondria, which may lead to organ's malfunctioning. The FB cell alterations do not seem enough to cause significant changes that lead to cell death. Prominent changes in MT, such as loss of the electron-dense concentric ring, increased smooth endoplasmic reticulum, loss of basal infolds, vacuoles containing mineralized granules, and lamellar structures associated with mitochondria, suggest that their excretory function is compromised. In conclusion, thiamethoxam acts not only in the nervous system but also contributes to systemic toxicity on the target organism.

Glandular innovations for a tunnelling life: Silk and associated leg glands in Melissotarsus and Rhopalomastix queen and worker ants

As in other Hymenoptera, adult ants cannot secrete silk, unlike the larvae that spin a cocoon prior to metamorphosis. Fisher and Robertson (1999) first showed the existence of a silk gland in the head of adult Melissotarsus beccarii workers, and we confirm this with detailed histology and ultrastructural comparisons of both queens and workers. This African genus exhibits extreme morphological adaptations (legs, head shape and mandibular muscles) for tunnelling behaviour inside living trees, that underlie an obligate mutualism with scale insects. Rhopalomastix is its sister genus distributed across Asia, and we show that queens and workers also have a silk gland. This lineage of minute workers relies on silk to secure their network of tunnels against other arboreal ants. We show striking differences between these genera in the anatomy and ultrastructure of the cells that secrete silk, especially numerous vacuoles and an unexpectedly branched end apparatus in Melissotarsus. Moreover, the legs of Melissotarsus are much more specialized for tunnelling, and this includes highly expanded basitarsi. The latter house the novel 'Delage-Darchen gland', and we document its anatomy and ultrastructure, suggesting a proteinaceous secretion to harden roofs made of silk combined with wood fragments. The restriction of the Delage-Darchen gland to Melissotarsus, combined with a modified silk gland (an almost three-fold increase in the number of secretory cells, and ultrastructural differences suggestive of higher secretory activity), are evidence of an outstanding evolutionary divergence relative to Rhopalomastix. Synthesis of silk by adults is a significant innovation among ants, but its augmented production in Melissotarsus makes them better adapted for the hazards of arboreal life.

Novel exocrine glands in the foreleg coxae of Discothyrea ants

Workers, queens and males of all examined Discothyrea species of the 'sauteri group', that have laterally expanded frontal lobes and well-developed antennal scrobes, are characterized by two hairy areas on the outer surface of their procoxae. Histological and ultrastructural examination of Discothyrea sauterirevealed that each of these areas is associated with a novel exocrine gland: the proximal procoxal gland is formed by a cluster of 15 round secretory cells of 34 μm with numerous mitochondria, smooth endoplasmic reticulum and Golgi apparatus.Their ducts have a diameter of 0.5-1 μm.The distal procoxal gland contains 50 secretory cells of 22 μm with numerous vacuoles and lamellar inclusions, and narrow ducts with a diameter of only 0.15-0.2 μm.The differences in ultrastructural appearance and duct diameter indicate that both glands produce a different but probably pheromonal secretion. The function of these novel procoxal glands could not yet be determined, although observation of D.sauteri workers and queens shows that they make frequent and peculiar leg movements, in which the foreleg basitarsus rubs over the coxal hairy areas. The foreleg basitarsus then rubs the ipsilateral hindleg basitarsus and antenna. As a last step of the sequence, the hindleg basitarsus strokes the gaster.In addition to the occurrence of these novel procoxal glands, histological examination of D. sauteri also revealed the presence of yet another novel but smaller procoxal base gland. Ants of the 'testacea group', that have less developed frontal lobes and no antennal scrobes, do not have procoxal hairy areas, although a distinct sculpturation with small pores may occur in the corresponding areas. The related Proceratium japonicum, that has a similar lifestyle as Discothyrea, does not have any of the procoxal glands and does not display the peculiar leg movements as reported for D. sauteri.

The oral gland, a new exocrine organ of termites

Termites have a rich set of exocrine glands. These glands are located all over the body, appearing in the head, thorax, legs and abdomen. Here, we describe the oral gland, a new gland formed by no more than a few tens of Class I secretory cells. The gland is divided into two secretory regions located just behind the mouth, on the dorsal and ventral side of the pharynx, respectively. The dominant secretory organelle is a smooth endoplasmic reticulum. Secretion release is under direct control of axons located within basal invaginations of the secretory cells. The secretion is released through a modified porous cuticle located at the mouth opening. We confirmed the presence of the oral gland in workers and soldiers of several wood- and soil-feeding species of Rhinotermitidae and Termitidae, suggesting a broader distribution of the oral gland among termites. The oral gland is the smallest exocrine gland described in termites so far. We hypothesise that the oily secretion can either ease the passage of food or serve as a primer pheromone.

Novel thoracic glands in the ant Myopias hollandi

Besides the common labial and metapleural glands, four novel exocrine glands are described in the thorax of both workers and queens of the ponerine ant Myopias hollandi. From anterior to posterior, these glands were designated as the propleural pit gland, the posterolateral pronotal gland, the anterolateral propodeal gland and the metasternal process gland. They all correspond with class-3 glands, that are made up of bicellular units that each comprise a secretory cell and a duct cell. In the propleural pit gland, the ducts are characterized by a gradually widening diameter, while in the three other glands the ducts show a portion which displays a balloon-like expansion, that on semithin sections stains very dark. For none of these novel glands the function is known as yet, although ultrastructural examination indicates that they produce a non-proteinaceous and therefore possibly pheromonal secretion.

Chemical Ecology and Sociality in Aphids: Opportunities and Directions

Aphids have long been recognized as good phytochemists. They are small sap-feeding plant herbivores with complex life cycles that can involve cyclical parthenogenesis and seasonal host plant alternation, and most are plant specialists. Aphids have distinctive traits for identifying and exploiting their host plants, including the expression of polyphenisms, a form of discrete phenotypic plasticity characteristic of insects, but taken to extreme in aphids. In a relatively small number of species, a social polyphenism occurs, involving sub-adult "soldiers" that are behaviorally or morphologically specialized to defend their nestmates from predators. Soldiers are sterile in many species, constituting a form of eusociality and reproductive division of labor that bears striking resemblances with other social insects. Despite a wealth of knowledge about the chemical ecology of non-social aphids and their phytophagous lifestyles, the molecular and chemoecological mechanisms involved in social polyphenisms in aphids are poorly understood. We provide a brief primer on aspects of aphid life cycles and chemical ecology for the non-specialists, and an overview of the social biology of aphids, with special attention to chemoecological perspectives. We discuss some of our own efforts to characterize how host plant chemistry may shape social traits in aphids. As good phytochemists, social aphids provide a bridge between the study of insect social evolution sociality, and the chemical ecology of plant-insect interactions. Aphids provide many promising opportunities for the study of sociality in insects, and to understand both the convergent and novel traits that characterize complex sociality on plants.

Breaking the cipher: ant eavesdropping on the variational trail pheromone of its termite prey

Predators may eavesdrop on their prey using innate signals of varying nature. In regards to social prey, most of the prey signals are derived from social communication and may therefore be highly complex. The most efficient predators select signals that provide the highest benefits. Here, we showed the use of eusocial prey signals by the termite-raiding ant Odontoponera transversaO. transversa selected the trail pheromone of termites as kairomone in several species of fungus-growing termites (Termitidae: Macrotermitinae: Odontotermes yunnanensis, Macrotermes yunnanensis, Ancistrotermes dimorphus). The most commonly predated termite, O. yunnanensis, was able to regulate the trail pheromone component ratios during its foraging activity. The ratio of the two trail pheromone compounds was correlated with the number of termites in the foraging party. (3Z)-Dodec-3-en-1-ol (DOE) was the dominant trail pheromone component in the initial foraging stages when fewer termites were present. Once a trail was established, (3Z,6Z)-dodeca-3,6-dien-1-ol (DDE) became the major recruitment component in the trail pheromone and enabled mass recruitment of nest-mates to the food source. Although the ants could perceive both components, they revealed stronger behavioural responses to the recruitment component, DDE, than to the common major component, DOE. In other words, the ants use the trail pheromone information as an indication of suitable prey abundance, and regulate their behavioural responses based on the changing trail pheromone component. The eavesdropping behaviour in ants therefore leads to an arms race between predator and prey where the species specific production of trail pheromones in termites is targeted by predatory ant species.

Ultrastructure of the mandibular gland of the ant Myrmoteras iriodum

The mandibular gland in workers of the formicine ant Myrmoteras iriodum differs from other ants both in its general morphology and ultrastructural organization. The secretory cells appear in a pseudo-epithelial arrangement that gives them a clear polarity. At their apical side, the cells are characterized by a large cup-like extension of the reservoir, from which a bulbous invagination connects to a branched end apparatus. At the basal side, the cells show a labyrinth of basal invaginations, while the lateral cell contacts show clear interdigitations. The cytoplasmic composition reveals the presence of numerous round or elongate inclusions that contain crystalline material. Microtubules are abundant, and locally fibrillar regions are found. The function of the mandibular gland in M. iriodum has not yet been documented, and should be studied using gland extracts and behavioural observations.