Postpharyngeal Gland Research Articles

Cuticular hydrocarbon profiles differ between ant body parts: implications for communication and our understanding of CHC diffusion.

Insect cuticular hydrocarbons (CHCs) serve as communication signals and protect against desiccation. They form complex blends of up to 150 different compounds. Due to differences in molecular packing, CHC classes differ in melting point. Communication is especially important in social insects like ants, which use CHCs to communicate within the colony and to recognize nestmates. Nestmate recognition models often assume a homogenous colony odor, where CHCs are collected, mixed, and redistributed in the postpharyngeal gland (PPG). Via diffusion, recognition cues should evenly spread over the body surface. Hence, CHC composition should be similar across body parts and in the PPG. To test this, we compared CHC composition among whole-body extracts, PPG, legs, thorax, and gaster, across 17 ant species from 3 genera. Quantitative CHC composition differed between body parts, with consistent patterns across species and CHC classes. Early-melting CHC classes were most abundant in the PPG. In contrast, whole body, gaster, thorax, and legs had increasing proportions of CHC classes with higher melting points. Intraindividual CHC variation was highest for rather solid, late-melting CHC classes, suggesting that CHCs differ in their diffusion rates across the body surface. Our results show that body parts strongly differ in CHC composition, either being rich in rather solid, late-melting, or rather liquid, early-melting CHCs. This implies that recognition cues are not homogenously present across the insect body. However, the unequal diffusion of different CHCs represents a biophysical mechanism that enables caste differences despite continuous CHC exchange among colony members.

Antennal Olfactory Physiology and Behavior of Males of the Ponerine Ant Harpegnathos saltator.

In comparison to the large amount of study on the communication abilities of females in ant societies and their associated chemical ecology and sensory physiology, such study of male ants has been largely ignored; accordingly, little is known about their olfactory sensory capabilities. To address this, we explored peripheral odor sensitivities in male Harpegnathos saltator by measuring the electrophysiological activity of olfactory sensory neurons within antennal trichoid and coeloconic sensilla using an extracellular recording technique. In an initial trial of 46 compounds, sensilla trichodea responded strongly to two alarm pheromone components, while a limited number of non-hydrocarbon odorants elicited strong responses in sensilla coeloconica. Both sensillar types responded indifferently to 31 cuticular hydrocarbons (CHCs) and synthetic long-chain hydrocarbons (HCs) typically found on insect cuticle. In a search for sensilla responding to CHCs and other compounds, we found some sensilla that responded to synthetic HCs and CHCs from virgin queen postpharyngeal glands that are potentially used in close range mate recognition. Olfactometer bioassays of male ants to 15 non-HCs correlated sensory responsiveness to the respective behavioral responses. Comparing olfactory responses between H. saltator males and females, we found that sensilla coeloconica and basiconica of workers showed greater responses and broader selectivity to all compounds. The rarity of CHC-responding trichoid sensilla in Harpegnathos males suggests a more specific role in sexual communication compared to that in females, which use CHCs in a broader communication context.

Is the Salivary Gland Associated with Honey Bee Recognition Compounds in Worker Honey Bees (Apis mellifera)?

Cuticular hydrocarbons (CHCs) function as recognition compounds with the best evidence coming from social insects such as ants and honey bees. The major exocrine gland involved in hydrocarbon storage in ants is the post-pharyngeal gland (PPG) in the head. It is still not clearly understood where CHCs are stored in the honey bee. The aim of this study was to investigate the hydrocarbons and esters found in five major worker honey bee (Apis mellifera) exocrine glands, at three different developmental stages (newly emerged, nurse, and forager) using a high temperature GC analysis. We found the hypopharyngeal gland contained no hydrocarbons nor esters, and the thoracic salivary and mandibular glands only contained trace amounts of n-alkanes. However, the cephalic salivary gland (CSG) contained the greatest number and highest quantity of hydrocarbons relative to the five other glands with many of the hydrocarbons also found in the Dufour’s gland, but at much lower levels. We discovered a series of oleic acid wax esters that lay beyond the detection of standard GC columns. As a bee’s activities changed, as it ages, the types of compounds detected in the CSG also changed. For example, newly emerged bees have predominately C19-C23n-alkanes, alkenes and methyl-branched compounds, whereas the nurses’ CSG had predominately C31:1 and C33:1 alkene isomers, which are replaced by a series of oleic acid wax esters in foragers. These changes in the CSG were mirrored by corresponding changes in the adults’ CHCs profile. This indicates that the CSG may have a parallel function to the PPG found in ants acting as a major storage gland of CHCs. As the CSG duct opens into the buccal cavity the hydrocarbons can be worked into the comb wax and could help explain the role of comb wax in nestmate recognition experiments.

Microbial Communities in Different Tissues of Atta sexdens rubropilosa Leaf-cutting Ants.

Bacterial endosymbionts are common in all insects, and symbiosis has played an integral role in ant evolution. Atta sexdens rubropilosa leaf-cutting ants cultivate their symbiotic fungus using fresh leaves. They need to defend themselves and their brood against diseases, but they also need to defend their obligate fungus gardens, their primary food source, from infection, parasitism, and usurpation by competitors. This study aimed to characterize the microbial communities in whole workers and different tissues of A. sexdens rubropilosa queens using Ion Torrent NGS. Our results showed that the microbial community in the midgut differs in abundance and diversity from the communities in the postpharyngeal gland of the queen and in whole workers. The main microbial orders in whole workers were Lactobacillales, Clostridiales, Enterobacteriales, Actinomycetales, Burkholderiales, and Bacillales. In the tissues of the queens, the main orders were Burkholderiales, Clostridiales, Syntrophobacterales, Lactobacillales, Bacillales, and Actinomycetales (midgut) and Entomoplasmatales, unclassified γ-proteobacteria, and Actinomycetales (postpharyngeal glands). The high abundance of Entomoplasmatales in the postpharyngeal glands (77%) of the queens was an unprecedented finding. We discuss the role of microbial communities in different tissues and castes. Bacteria are likely to play a role in nutrition and immune defense as well as helping antimicrobial defense in this ant species.

Allogrooming, Self-Grooming, and Touching Behavior: Contamination Routes of Leaf-Cutting Ant Workers Using a Fat-Soluble Tracer Dye.

The aim of this study was to determine whether worker self-grooming, allogrooming, and direct contact promotes the dispersal of substances among members of the colony. For this purpose, a tracer (Sudan III dye) was applied topically to a worker ant and the social interactions between the worker with the tracer and workers without the tracer were studied. Additionally, the worker heads were dissected to visualize whether or not the post-pharyngeal gland was stained. The post-pharyngeal glands from 50% to 70% of workers were stained depending on the size of the group. With the increase in the experimental group size, the frequency of interactions between workers increased, with touching being the most frequent behavior. The tracer dye was probably passed on by direct contact between workers, followed by self-grooming and allogrooming. These behaviors are responsible for the rapid dispersal of substances among colony members as observed in our experiment. The results therefore support the hypothesis that contact with substances promotes the contamination of nestmates, even in the absence of feeding, serving as a model for further studies on the contamination of workers with the active ingredients of insecticides.

Sexual selection and the evolution of male pheromone glands in philanthine wasps (Hymenoptera, Crabronidae)

BackgroundSexual selection is thought to promote evolutionary changes and diversification. However, the impact of sexual selection in relation to other selective forces is difficult to evaluate. Male digger wasps of the tribe Philanthini (Hymenoptera, Philanthinae) scent mark territories to attract receptive females. Consequently, the organs for production and storage of the marking secretion, the mandibular gland (MG) and the postpharyngeal gland (PPG), are subject to sexual selection. In female Philanthini, these glands are most likely solely subject to natural selection and show very little morphological diversity. According to the hypothesis that sexual selection drives interspecific diversity, we predicted that the MG and PPG show higher interspecific variation in males than in females. Using histological methods, 3D-reconstructions, and multivariate statistical analysis of morphological characters, we conducted a comparative analysis of the MG and the PPG in males of 30 species of Philanthini and three species of the Cercerini and Aphilanthopsini, two related tribes within the Philanthinae.ResultsWe found substantial interspecific diversity in gland morphology with regard to gland incidence, size, shape and the type of associated secretory cells. Overall there was a phylogenetic trend: Ensuing from the large MGs and small PPGs of male Cercerini and Aphilanthopsini, the size and complexity of the MG was reduced in male Philanthini, while their PPG became considerably enlarged, substantially more complex, and associated with an apparently novel type of secretory cells. In some clades of the Philanthini the MG was even lost and entirely replaced by the PPG. However, several species showed reversals of and exceptions from this trend. Head gland morphology was significantly more diverse among male than among female Philanthinae.ConclusionOur results show considerable variation in male head glands including the loss of an entire gland system and the evolution of a novel kind of secretory cells, confirming the prediction that interspecific diversity in head gland morphology is higher in male than in female Philanthini. We discuss possible causes for the remarkable evolutionary changes in males and we conclude that this high diversity has been caused by sexual selection.

Lipid Mobilization and Acyl-Coenzyme A Dehydrogenase Abundance in the Postpharyngeal Gland of a Leaf-Cutting Ant

The postpharyngeal gland (PPG) in ants has a central role in lipidaccumulation. Our objective was to investigate whether the PPG ofworkers of the leaf-cutting ant Atta sexdens rubropilosa (F.) performslipid mobilization to the hemolymph, and to verify whether lipidsin gland cells are in part oxidized to acetyl-CoA. Increased levelsof fatty acids were observed in both the PPG and the hemolymphafter 48 h of lipid supplementation. In contrast, after 120 h of lipidsupplementation, fatty acid levels were reduced in the PPG andsignificantly increased in the hemolymph. The gland cells of the PPGof workers supplemented with lipids exhibited a high amount of Acyl-CoA dehydrogenase, and the lipids were in part β-oxidized to Acetyl-CoA. These findings suggest that the PPG of A. sexdens rubropilosaworkers mobilize fatty acids from their gland cells to the hemolymph.Further work will be needed to elucidate lipid transport from thehemolymph to other organs in this organism.

THE GUILD OF SAPROBIONTIC NEMATODES ASSOCIATED WITH ANTS (FORMICOIDEA)

At least 14 different saprobiontic species of Rhabditida are intimately associated with ants on different continents. These myrmecophilous nematodes belong in particular to Diploscapter and Sclerorhabditis, and to the Oscheius Dolichura group (all "Rhabditidae"). Species of Diplogastridae and Halicephalobus (Panagrolaimidae) are rarely found in association with ants, but some records do exist. Oscheius janeti is argued here to be a separate species. Dauerlarvae of all these species invade the postpharyngeal glands of ants via the mouth (including as a result of trophallaxis) and, after some time and under suitable conditions, leave them the opposite way to complete their life cycle. These species usually propagate in the debris of ants' nests using ants as a means of dispersal (endophoresis). Species which live in certain saprobic biochores (such as slime flux) frequented by ants enter the ants to survive the unfavourable conditions of their habitat. Mostly the infestation rate is very low. Most field-collected ants contain no nematodes, and more than ten specimens are rarely found in one ant. The infestation rate increases in formicaria in the laboratory. There are some indications that dauerlarvae obtain nutrients from the carrier/host, so they may perhaps be viewed as facultatively larval parasitic. Queens, which have the potential to found a new colony, have also been documented to be infested. New nematode records from Solenopsis, from Azteca in domatia of Tococa (Melastomataceae) and Cecropia (Urticaceae), from Crematogaster in domatia of Macaranga (Euphorbiaceae), and from the fungus-growing Acromyrmex and Atta are reported. The nematode-ant relationship and distinctive organ specificity of the dauerlarvae to the postpharyngeal glands of ants has been attained by convergent evolution no less than nine times. It is expected that living together with saprobiontic nematodes in myrmecophilous plants offers some kind of benefit to ants.

The Postpharyngeal Gland: Specialized Organ for Lipid Nutrition in Leaf-Cutting Ants.

There are several hypotheses about the possible functions of the postpharyngeal gland (PPG) in ants. The proposed functions include roles as cephalic or gastric caeca and diverticulum of the digestive tract, mixing of hydrocarbons, nestmate recognition, feeding larvae, and the accumulation of lipids inside this gland, whose origin is contradictory. The current study aimed to investigate the functions of these glands by examining the protein expression profile of the PPGs of Atta sexdens rubropilosa (Hymenoptera, Formicidae). Mated females received lipid supplementation and their glands were extracted and analyzed using a proteomic approach. The protocol used combined two-dimensional electrophoresis and shotgun strategies, followed by mass spectrometry. We also detected lipid β-oxidation by immunofluorescent marking of acyl-CoA dehydrogenase. Supplying ants with lipids elicited responses in the glandular cells of the PPG; these included increased expression of proteins related to defense mechanisms and signal transduction and reorganization of the cytoskeleton due to cell expansion. In addition, some proteins in PPG were overexpressed, especially those involved in lipid and energy metabolism. Part of the lipids may be reduced, used for the synthesis of fatty alcohol, transported to the hemolymph, or may be used as substrate for the synthesis of acetyl-CoA, which is oxidized to form molecules that drive oxidative phosphorylation and produce energy for cellular metabolic processes. These findings suggest that this organ is specialized for lipid nutrition of adult leaf-cutting ants and characterized like a of diverticulum foregut, with the ability to absorb, store, metabolize, and mobilize lipids to the hemolymph. However, we do not rule out that the PPG may have other functions in other species of ants.

Morphology and ultrastructure of the pro- and postpharyngeal glands in workers of Pachycondyla sennaarensis

The pro- and postpharyngeal glands of Brachyponera sennaarensis both appear as globular formations at a general anatomical level. However, only for the propharyngeal gland do these formations correspond with spherical secretory cells with diameters of 30-40 µm. For the lobed postpharyngeal gland, in contrast, this globular appearance is caused by the bulbous protrusions of the epithelial cells. This lobed appearance and globular cell shape also occur in the postpharyngeal glands of other Ponerinae and thus may represent a phylogenetic character. At the ultrastructural level, the propharyngeal gland cells are characterized by a well-developed granular endoplasmic reticulum, which is in agreement with its presumed production of digestive enzymes. The postpharyngeal gland cells contain a well-developed smooth endoplasmic reticulum, which allows the production of a non-proteinaceous secretion.

Trajectory of Water- and Fat-Soluble Dyes in the Grass-Cutting ant Atta capiguara (Hymenoptera, Formicidae): Evaluation of Infrabuccal Cavity, Post-Pharyngeal Glands and Gaster

The diet of leaf-cutting ants is based on cultivation of their symbiotic fungus, whose successful cultivation depends on the task of incorporation and handling of vegetable substrate. This task may cause the workers to be contaminated with toxic substances and thus decrease the survival of the colony. The objective of this study was to analyze the contamination of workers of Atta capiguara as well as the dissemination trajectory of water- and fat-soluble substances. Four colonies received non-toxic baits containing water-soluble dye Rhodamine-B and three, non-toxic baits with fat-soluble dye Sudan III. The dye Rhodamine-B stained the gaster in 40.31% of workers and showed no significant difference among castes. The Sudan III stained the infrabuccal cavity in 35.41%, post-pharyngeal glands of 24.22% and gaster in only 8.44% of the workers, with no significant difference among the castes. The watersoluble dye was spread in the body of workers through the digestive system while fat-soluble dye was diverted to the post-pharyngeal glands.

Mitochondrial and peroxisomal population in post-pharyngeal glands of leaf-cutting ants after lipid supplementation

The post-pharyngeal gland (PPG) occurs in ants and some Sphecidae wasps. Among its several roles is the storage of lipids from food. In order to investigate the effect of lipids on the cell, especially on mitochondria and peroxisomes, the present study was aimed at examining the peroxisomal and mitochondrial population in the PPG of Atta sexdens rubropilosa after lipid supplementation by confocal laser scanning microscopy and transmission electron microscopy. Soybean oil provided as lipid supplement was not toxic for A. sexdens rubropilosa workers for the first 48h and 120h. However, the ultrastructural cytochemical analysis revealed an accumulation of lipid droplets in the PPGs of ants after lipid supplementation at 48h and 120h, and smaller lipid droplets in the basal membrane of the PPG epithelium, showing lipid mobilization from the PPG to the hemolymph. The lipid supplementation reduces the life expectancy of medium workers, probably due to the high lipid metabolism. Most importantly, the PPGs of medium workers of leaf-cutting A. sexdens rubropilosa is probably a specialized gland in the lipid metabolism, due to the increased mitochondrial and peroxisomal population inside cells after lipid supplementation; participation of peroxisomal population in the β-oxidation of long chain fatty acids into smaller chains and participation of mitochondrial population in the β-oxidation of fatty acids for energy, or mobilization of lipid derivatives from the PPG to hemolymph, a process that requires energy. However, the hypothesis that the PPGs convert lipids from food in aldehydes and/or hydrocarbons must be better investigated.

Exocrine glands of the antMyrmoteras iriodum

This paper describes the morphological characteristics of nine major exocrine glands in workers of the formicine ant Myrmoteras iriodum. The elongate mandibles reveal along their entire length a conspicuous intramandibular gland, which contains both class-1 and class-3 secretory cells. The secretory cells of the mandibular glands show a peculiar appearance, with a branched end apparatus, which is unusual for ants. The other major glands (pro- and postpharyngeal gland, infrabuccal cavity gland, labial gland, metapleural gland, venom gland and Dufour gland) show common features for formicine ants. The precise function of the glands could not yet be experimentally demonstrated, and to clarify this will depend on the availability of live material of these enigmatic ants in future.

Morphology, Chemistry and Function of the Postpharyngeal Gland in the South American Digger Wasps Trachypus boharti and Trachypus elongatus

Microbes pose severe threats to animals as competitors or pathogens and strongly affect the evolution of life history traits like parental care. Females of the European beewolf Philanthus triangulum, a solitary digger wasp, provision their offspring with paralyzed honeybees and embalm them with the secretion from large postpharyngeal glands (PPG) that contain mainly unsaturated hydrocarbons. This coating changes the physico-chemical properties of the prey surface, causes a reduction of water condensation and retards growth of mold fungi. Here we examined the closely related South American genus Trachypus, which shows a life-history similar to Philanthus. We investigated whether Trachypus spp. also possess PPGs and embalm larval provisions. Using histological methods and 3D reconstructions we show that Trachypus boharti and T. elongatus possess PPGs that are similar to P. triangulum but somewhat smaller. The ultrastructure of the gland epithelium suggests that the gland content is at least partly sequestered from the hemolymph. Chemical analyses using gas chromatography / mass spectrometry revealed that both the cuticle and PPGs of Trachypus contain mainly unsaturated long-chain hydrocarbons. The gland of T. boharti additionally contains long-chain ketones. The hydrocarbons from the PPG of T. elongatus occurred on prey bees excavated from nests in the field but not on conspecific control bees. While the embalming only slightly elevated the amount of hydrocarbons on prey bees, the proportion of unsaturated hydrocarbons, which is crucial for the antifungal effect, was significantly increased. The Trachypus species under study possess PPGs that are very similar to the PPG of P. triangulum with regard to morphology, ultrastructure and chemistry. Moreover, we provide clear evidence that T. elongatus females embalm their prey, presumably as a means of prey preservation. The observed differences among Trachypus and Philanthus in gland size and prey embalming may have evolved in response to divergent ecological conditions.

Mandibular and postpharyngeal gland secretions of <i>Acromyrmex landolti</i> (Hymenoptera: Formicidae) as chemical cues for nestmate recognition

Nestmate recognition is fundamental for the cohesiveness of the colony in social insects. Acromyrmex landolti is a higher Attini, a leaf-cutter ant with largest nest and morphological castes. We studied the location of the recognition signal of A. landolti on the whole ant body, the isolation effect on the capacity of nestmate recognition, and the role of the mandibular (MG) and post-pharyngeal glands (PPG) as putative sources for chemical recognition signals in this species. We performed behavioral bioassays and chemical analyses of MG and PPG glands; MG volatiles in different castes were also characterized. In order to determine the occurrence of nestmate recognition signals, behavioral tests were performed for which the whole body, body parts or gland extracts from a nestmate and an alien worker ant, were placed on the central axis of an active field trail. Besides, the isolation effect of the nestmate chemical signal persistence was evaluated by repeating the experiments with whole ant bodies after 12 h, 24 h and 48 h of isolation. The agonistic behavior of the ants on the trail was classified as inspection, threatening and biting. Gland volatiles were obtained by headspace solid phase microextraction, and PPG by solid sample analyses; and chemical analyses were performed using gas chromatography-mass spectrometry. Workers performed agonistic behaviors more frequently towards non-nestmate whole bodies, body parts and gland extracts, primarily those from the post-pharyngeal glands. Nestmate thoraces odorized with alien extracts were also recognized from aliens. Nestmate recognition ability persists for 12 h and 24 h after isolation, but decreases after 48 h, suggesting that the chemical signal changes over time. Inter-colonial chemical analyses showed no differences in the PPG hydrocarbon profiles, while the relative proportions of MG compounds showed significant differences between colonies. These results showed that nestmate recognition was achieved by MG volatiles. Possibly, the PPG secretion works as a matrix that absorbs MG volatiles. We report, for the first time, some mandibular gland compounds for the genus Acromyrmex and for the tribe Attini group.

Toxicological and histopathological effects of hydramethylnon on Atta sexdens rubropilosa (Hymenoptera: Formicidae) workers

The leaf-cut ants are important agricultural pest, because they can cause intense defoliation in plants and destroy large areas cultivated. Although there are several works for the control of these insects by examining the toxicity of natural chemical compounds on various species of ants, few are focused on analyses of morphological changes caused in the affected organs. The aim of this study was to evaluate the effects of hydramethylnon on Atta sexdens rubropilosa workers through toxicological bioassays and morphological analysis of the post-pharyngeal glands, midgut, and Malpighian tubules of these ants. Hydramethylnon dissolved either in acetone (HA) or in a mixture of acetone and soy oil (HAO) was added to the artificial diet at a concentration of 200μg/mL. The workers fed daily with the diet containing hydramethylnon showed higher mortality than the controls, especially when HAO was used. Moreover, light and electron microscopy revealed morphological alterations in the midgut and Malpighian tubules of workers treated with HA, whereas alterations of the post-pharyngeal glands were observed in the HAO-treated group. These results indicated that the presence of soy oil provided an alternate route for the ingestion of the formicide's active ingredient and corroborated previous studies that suggested a role for the post-pharyngeal glands in lipid metabolism. Our findings suggest that the oil may carry hydramethylnon to the gland lumen, resulting in lower quantity of the active ingredient in the intestinal lumen and Malpighian tubules that explains the lower degree of morphological alterations in these structures in the workers treated with HAO. These results may provide insight into the toxicological effects of hydramethylnon on leaf-cutting ants and the use of vegetable oil as an adjuvant in baits to control ants.

Effect of gland extracts on digging and residing preferences of red imported fire ant workers (Hymenoptera: Formicidae)

There is evidence that ant-derived chemical stimuli are involved in regulating the digging behavior in Solenopsis invicta Buren. However, the source gland(s) and chemistry of such stimuli have never been revealed. In this study, extracts of mandibular, Dufour's, postpharyngeal, and poison glands were evaluated for their effect on ant digging and residing preferences of S. invicta workers from three colonies. In the intracolonial bioassays, workers showed significant digging preferences to mandibular gland extracts in 2 of 3 colonies and significant residing preferences in 1 of 3 colonies; significant digging preferences to Dufour's gland extracts in 1 of 3 colonies and significant residing preferences in 2 of 3 colonies. No digging and residing preferences were found for postpharyngeal and poison gland extracts. In intercolonial bioassays, significant digging and residing preferences were found for mandibular gland extracts in 3 of 6 colony combinations. Significant digging preferences to Dufour's gland extracts were found in 4 of 6 colony combinations and significant residing preferences in all 6 colony combinations. For postpharyngeal gland extracts, significant digging preferences were found only in 1 of 6 colonial combinations and no significant residing preferences were found. For poison gland extracts, no significant digging preferences were found; significant residing preferences were found in 1 of 6 colony combinations. However, a significant residing deterrence (negative residing preference index) was found for 2 of 6 colony combinations. Statistical analyses using data pooled from all colonies showed that mandibular and Dufour's gland extracts caused significant digging and residing preferences in both intracolonial and intercolonial bioassays but not postpharyngeal and poison gland extracts. By analyzing the data pooled from the same three colonies used for gland extract bioassays, it was found that, in no cases, workers showed significant digging and residing preferences to 2-ethyl-3,6-dimethylpyrazine, an alarm pheromone component from mandibular gland.

Structure, chemical composition and putative function of the postpharyngeal gland of the emerald cockroach wasp, Ampulex compressa (Hymenoptera, Ampulicidae)

The postpharyngeal gland (PPG) plays a major role in the social integration of ant colonies. It had been thought to be restricted to ants but was recently also described for a solitary wasp, the European beewolf ( Philanthus triangulum). This finding posed the question whether the gland has evolved independently in the two taxa or has been inherited from a common ancestor and is hence homologous. The latter alternative would be supported if a PPG was found in more basal taxa. Therefore, we examined a species at the base of the Apoidea, the solitary ampulicid wasp Ampulex compressa, for the existence of a PPG. Both sexes of this species possess a cephalic gland that branches off the posterior part of the pharynx, is lined by a cuticular intima and surrounded by a monolayered epithelium with the epithelial cells bearing long hairs. Most of these morphological characteristics conform to those of the PPG of ants and beewolves. Chemical analysis of the gland content revealed that it contains mainly hydrocarbons and that there is a congruence of the pattern of hydrocarbons in the gland, on the cuticle, and in the hemolymph, as has also been reported for both ants and beewolves. Based on these morphological and chemical results we propose that the newly described cephalic gland is a PPG and discuss its possible function in A. compressa. The present study supports the view of a homologous origin of the PPG in the aculeate Hymenoptera.

Toxicological and Histopathological Effects of Boric Acid on <I>Atta sexdens rubropilosa</I> (Hymenoptera: Formicidae) Workers

The current study compared the toxicity of different concentrations of boric acid in adult workers of Atta sexdens rubropilosa Forel (Hymenoptera: Formicidae), with toxicological bioassays, and examining the dose-dependent and time-dependent histopathological changes, of the midgut, Malpighian tubules, and postpharyngeal glands. Our results revealed the importance of conducting toxicological bioassays combined with morphological analyses of the organs of ants chronically exposed to insecticides used in commercial ant baits. In vitro bioassays showed that boric acid significantly decreases the survivorship of workers regardless of concentration, whereas the morphological data suggested progressive dose-dependent and time-dependent changes in the organs examined, which were evident in the midgut. The midgut is the first organ to be affected, followed by the postpharyngeal gland and Malpighian tubules. This sequence is in agreement with the absorption pathway of this chemical compound in the midgut, its transference to the hemolymph, possibly reaching the postpharyngeal glands, and excretion by the Malpighian tubules. These progressive changes might be due to the cumulative and delayed effect of boric acid. Our findings provide important information for the understanding of the action of boric acid in ant baits in direct and indirect target organs.

Is the postpharyngeal gland of a solitary digger wasp homologous to ants? Evidence from chemistry and physiology

The postpharyngeal gland (PPG) was thought to be restricted to ants where it serves a crucial function in the generation of the colony odour. Recently, head glands that closely resemble the PPG of ants were discovered in females of a solitary digger wasp, the European beewolf. The function of this gland necessarily differs from ants: beewolf females apply the secretion of their PPG onto the bodies of paralysed honeybees that serve as larval provisions in order to delay fungus growth. Since ants and digger wasps are not closely related, the occurrence of this gland in these two taxa might either be due to convergent evolution or it is a homologous organ inherited from a common ancestor. Here we test the hypothesis that the PPGs of both taxa are homologous by comparing characteristics of chemical composition and physiology of the PPG of beewolves and ants. Based on reported characteristics of the PPG content of ants, we tested three predictions that were all met. First, the PPG of beewolves contained mainly long-chain hydrocarbons and very few compounds with functional groups. Second, the composition of hydrocarbons in the beewolf PPG was similar to that of the hemolymph. Taking the structure of the gland epithelium and the huge requirements of beewolf females for gland secretion into account this result suggests that the content of the PPG is also sequestered from the hemolymph in beewolves. Third, the chemical composition of the PPG and the cuticle was similar in beewolves since cuticular hydrocarbons derive either from the hemolymph or the PPG. Taking the considerable morphological similarities into account, our results support the hypothesis of a homologous origin of the PPG in beewolves and ants.