Morphologie et fonctionnement du filtre infrabuccal chez une attine AcroṀyrmex octospinosus (Reich) (Hymenoptera : Formicidae): role de la poche infrabuccale

The feeding apparatus of ants: an overview of structure and function

Ants are a dominant family of eusocial terrestrial insects with a diversity of ecologies, lifestyles and morphologies. Ant diet preferences range from strict carnivory through omnivory to almost complete herbivory in species feeding on seeds or exudates of plant-sucking insects. While several studies have investigated ant feeding performance on different substrates, comparatively little is known about the functional morphology of the structures involved in food uptake or their diversification across the ants. To take stock of our current knowledge, we give an overview of how adult ants ingest food, followed by a morphological description of the mouthparts, preoral space and cephalic sucking pump. The mandibles are the most prominent mouthparts and have received considerable attention in the literature, so we focus on the maxillae and labium here. We present current hypotheses for the movement patterns of these parts and discuss morphological differences among ants that may be related to their ecological diversity. Finally, we give short comparisons of the ant condition with some other insects and vertebrates, as well as an outlook summarizing gaps in our knowledge. This sets the stage for future studies elucidating the connections between ant feeding mechanisms and mouthpart evolution. This article is part of the theme issue 'Food processing and nutritional assimilation in animals'.

Morphology and ultrastructure of the infrabuccal pocket in Strumigenys ants

The morphology of the infrabuccal pocket has been studied with light and electron microscopy (SEM, TEM) in 19 species of Strumigenys ants. The structural organization is similar in workers, queens and males, and supports the involvement of the pocket in the filtration of food particles before they can enter the digestive tract. A carpet of posteriorly oriented bristle hairs on the hypopharynx first guide ingested food into the pocket, where large solid particles are compacted into a pellet that will be regurgitated. The remaining products enter the digestive tract through a filtering wall of parallel hair combs lining the pharynx interior that are directed against the food flow. This mechanical filtering allows only liquids and sufficiently small food particles to enter the digestive system. The wall of the infrabuccal pocket is differentiated into a conspicuous glandular epithelium, of which the ultrastructural characteristics can be understood as an adaptation against the frequent shape changes of the pocket. The gland elaborates a non-proteinaceous secretion, although its functional significance still remains unknown.

Effects of Entomopathogenic Fungi on Individuals as Well as Groups of Workers and Immatures of Atta sexdens rubropilosa Leaf-Cutting Ants

Simple Summary

The used active ingredient sulfluramid for toxic baits for the control of leaf-cutting ants has been included in Annex B of the Stockholm Convention on Persistent Organic Pollutants. The use of entomopathogenic fungi to control these insects has shown promising results, Trichoderma harzianum showed high pathogenicity against A. sexdens rubropilosa larvae and pupae, leading to a faster mortality and a decrease in survival rates. Beauveria bassiana was responsible for causing faster worker mortality and lower survival rates. An individual contaminated with B. bassiana or T. harzianum in a group decreases its survival rate, supporting the hypothesis that entomopathogenic fungi are efficient in controlling leaf-cutting ants when contaminated workers are allocated to groups of healthy workers.

Abstract

In 2009, sulfluramid, the main ingredient in toxic baits for leaf-cutting ant control, was included in Annex B of the Stockholm Convention on Persistent Organic Pollutants. This resulted in interest in the use of entomopathogenic fungi such as Beauveria bassiana and Trichoderma harzianum for leaf-cutting ant control. The efficiency of these fungi in controlling these insects and the way that ants react individually or in group to the biological risks posed by these fungi is poorly understood. For this reason, we assessed the effects of B. bassiana and T. harzianum on Atta sexdens rubropilosa larvae, pupae and workers. Moreover, we investigated whether the number of contaminated individuals within a group has an influence in controlling the spread of fungi among workers. We found that the fungus T. harzianum showed high pathogenicity against A. sexdens rubropilosa larvae and pupae, leading to faster mortality and a survival rates. On the other hand, the fungus B. bassiana was responsible for causing faster worker mortality and lower survival rates. In addition, we observed that an increase in individuals contaminated with B. bassiana or T. harzianum in the group decreases its survival rate. The results support the hypothesis that entomopathogenic fungi are efficient in controlling leaf-cutting ants when contaminated workers are allocated to groups of healthy workers.

Fat-Soluble Substance Flow During Symbiotic Fungus Cultivation by Leaf-Cutter Ants

Leaf-cutter ants perform a series of specialized behaviors in preparing plant substrates for their symbiotic fungus. This process may be related to contamination of workers by substances such as insecticides, leading us to hypothesize that substances are spread among workers through behaviors they perform to grow the fungus. To test this hypothesis, we analyzed the behavioral acts of workers during the processing of the pellets by using a fat-soluble tracing dye, since the active ingredient that composes toxic baits, used for control of leaf-cutter ants, is fat-soluble. The frequencies of performed behaviors were recorded and the number of dyed workers was assessed after fungus cultivation. The most frequent behavior is allogrooming and corresponds to 45.87% of the contamination process in workers, followed by holding, licking, and cutting pellets, which account for 40.22% of the process. After pellet processing, the workers had their external and internal morphological structures marked by the tracing dye-93.75% and 79.25%, respectively. These results confirm that behaviors performed during fungus cultivation contribute to dispersing substances such as insecticides, causing the contamination of workers.

Contamination Route of Leaf-Cutting Worker Ants Analyzed Through a Fat-Soluble Tracer Dye in Toxic Bait

Our study seeks to discover contamination routes of leaf-cutting worker ants during chemical control by formicide baits. To do so, toxic baits containing fat-soluble tracer dye were provided to colonies of three subspecies of Acromyrmex under laboratory conditions, in order to assess the proportion of dyed workers by size category, as well as dyed internal morphological structures. Results showed that nearly 50% of the workers come into contact with the active ingredient, since the internal structures of their bodies are dyed by the fat-soluble tracer dye within the first 24 h from contact with the toxic bait. In addition, the three subspecies of leaf-cutting ants present a similarity as to the contamination of their workers, probably due to their specialized behavior performed during the growth of the fungus garden with the baits. We conclude that the workers' pattern of behavior during fungus garden growth was the main means for dispersion of a fat-soluble substance among approximately half of the nest mates in our experiments, serving as a model for further studies on contamination of worker ants with insecticides.

The cephalic anatomy of workers of the ant species Wasmannia affinis (Formicidae, Hymenoptera, Insecta) and its evolutionary implications

Despite the ecological significance of ants and the intensive research attention they have received, thorough treatments of the anatomy and functional morphology are still scarce. In this study we document the head morphology of workers of the myrmicine Wasmannia affinis with optical microscopy, μ-computed tomography, scanning electron microscopy, and 3D reconstruction, providing the first complete anatomical treatment of an ant head with a broad array of modern techniques. We discuss the potential of the applied methods to generate detailed and well-documented morphological data sets with increased efficiency. We also address homology problems, particularly in the context of the cephalic digestive tract. According to our analyses the "pharynx" of previous ant studies is homologous to the prepharynx of other insects. We also discuss the phylogenetic potential and functional significance of the observed characters, with internal features such as tentorium and musculature discussed for the first time. Our investigation underlines that detailed anatomical data for Formicidae are still very fragmentary, which in turn limits our understanding of the major design elements underlying the ant bauplan. We attempt to provide a template for further anatomical studies, which will help to understand the evolution of this fascinating group on the phenotypic level.

Morphology and ultrastructure of the infrabuccal pocket and its lining epithelium in workers of Ectomomyrmex javanus (Hymenoptera: Formicidae)

This paper describes the morphology and ultrastructure of the infrabuccal pocket in workers of the carnivorous ant Ectomomyrmex javanus, using SEM and TEM. The infrabuccal pocket is a flexible bag with a diameter of 330 ± 30 μm. In its anterior part, the pocket wall reaches a thickness of 24.45 ± 3.45 μm, which is thicker than the wall lining the rest of the infrabuccal pocket, where it measures 6.87 ± 1.12 μm. The epidermal cells of the wall form a thickened epithelium with a thickness of 10.18 ± 2.50 μm. There are different kinds of hairs inside the pocket, that help in filtering solid pellets. Literature data on the infrabuccal pocket are limited and the function of the thicker epithelium is not yet known. It may provide mechanical strength as the pocket undergoes daily size changes because of the filling up and spitting out of pellets.

Defending against parasites: fungus-growing ants combine specialized behaviours and microbial symbionts to protect their fungus gardens

Parasites influence host biology and population structure, and thus shape the evolution of their hosts. Parasites often accelerate the evolution of host defences, including direct defences such as evasion and sanitation and indirect defences such as the management of beneficial microbes that aid in the suppression or removal of pathogens. Fungus-growing ants are doubly burdened by parasites, needing to protect their crops as well as themselves from infection. We show that parasite removal from fungus gardens is more complex than previously realized. In response to infection of their fungal gardens by a specialized virulent parasite, ants gather and compress parasitic spores and hyphae in their infrabuccal pockets, then deposit the resulting pellet in piles near their gardens. We reveal that the ants' infrabuccal pocket functions as a specialized sterilization device, killing spores of the garden parasite Escovopsis. This is apparently achieved through a symbiotic association with actinomycetous bacteria in the infrabuccal pocket that produce antibiotics which inhibit Escovopsis. The use of the infrabuccal pocket as a receptacle to sequester Escovopsis, and as a location for antibiotic administration by the ants' bacterial mutualist, illustrates how the combination of behaviour and microbial symbionts can be a successful defence strategy for hosts.

Morphology of a novel glandular epithelium lining the infrabuccal cavity in the ant Monomorium pharaonis (Hymenoptera, Formicidae)

A novel glandular epithelium lining the infrabuccal cavity and anterior pharynx is described in both workers and queens of the pharaoh's ant Monomorium pharaonis. The infrabuccal cavity, connected with the buccal tube, forms a ventral outgrowth of the anterior pharynx, and as such displays the tegumental lining with a cuticle and an epithelial layer. In its dorsal region, the cavity's epithelium reaches a thickness of approx. 11-12mum in both workers and queens, which is considerably thicker than the epithelium lining the rest of the infrabuccal cavity. Also the possible role of the infrabuccal gland is discussed.

Fungal enzymes transferred by leaf-cutting ants in their fungus gardens

Leaf-cutting ants finely fragment the leaf material that they bring to the nest and place faecal droplets on this substrate before incorporating it in the upper part of the fungus-garden. The faecal droplets contain enzymes of which some have been shown to be of fungal origin. Here we explicitly address the enzymatic activity of faecal droplets in the leaf-cutting ants Acromyrmex echinatior and Atta colombica. We used isoelectric focusing and specific staining to show that faecal droplets of both species contain carboxymethylcellulases, laccases, proteases, and pectinases (both pectin esterases and pectin lysases) and we demonstrate that these enzymes originate from the symbiotic fungus and not from the ants themselves. The level of activity of fungal pectin lyase in faecal droplets indicates that fungal enzymes may be protected and possibly concentrated during their passage through the ant gut. This would imply that enzymes that are transferred by ants from mature parts of the fungus gardens may play an important role for the colonization of new substrate by new mycelium of the same fungal clone.