Worker Defensive Behavior Associated with Toxins in the Neotropical Termite Neocapritermes braziliensis (Blattaria, Isoptera, Termitidae, Termitinae)

Morphology of the head-associated exocrine glands in Cornitermes cumulans with the description of a novel gland for the worker caste

Exocrine glands are important mediators of communication in eusocial insects and the description of novel glands reflects the complex context in which these animals live. Here we revisit the head-associated glands in workers of the Neotropical termite Cornitermes cumulans through histological analysis and describe a novel gland for this caste, the intramandibular glands. This structure is located underneath the cuticle of the dorsodistal part of each mandible. The glands showed an epithelial arrangement, but the cytological morphology is complex, comprising classes I and III of secretory cells. The present data highlight the importance of the intramandibular glands in the worker caste and demonstrate different morphology of these glands in Isoptera, probably related to the specialized function of the castes. Features of active glandular activity were also observed in the mandibular, labral, and salivary glands of these workers. Despite the intramandibular glands being also found in workers of other social insects, their occurrence in termites was restricted to the soldier caste of Machadotermes. Even so, their cytological structure differs from those of C. cumulans workers. The likely function of the worker intramandibular glands is discussed considering the separate-nest life type present by C. cumulans and tasks performed by workers. The chemical nature of the secretion and the occurrence of the glands in other termite taxa still require further investigation.

Differentiation of workers into soldiers is associated with a size reduction of higher-order brain centers in the neotropical termite Procornitermes araujoi

Comparing the size of functionally distinct brain regions across individuals with remarkable differences in sensory processing and cognitive demands provides important insights into the selective forces shaping animal nervous systems. We took advantage of the complex system of worker-to-soldier differentiation in the termitid Procornitermes araujoi, to investigate how a profound modification of body morphology followed by an irreversible shift in task performance are translated in terms of brain structure and size. This behavioural shift is characterised by a reduction of the once wide and complex behavioural repertoire of workers to one exclusively dedicated to nest defence (soldiers). In accordance with soldier's reduced cognitive and sensory demands, we show here that differentiation of workers into soldiers is associated with a size reduction of the mushroom body (MB) compartments, higher-order brain regions responsible for multimodal processing and integration of sensory information, as well as learning, memory, and decision-making. Moreover, in soldiers, we found an apparent fusion of the medial and lateral MB calyces likely associated with its volume reduction. These results illustrate a functional neuroplasticity of the MB associated with division of labour, supporting the link between MB size and behavioural flexibility in social insect workers.

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.

Involvement of the Salivary Glands in the Suicidal Defensive Behavior of Workers in Neocapritermes opacus (Blattaria, Isoptera, Termitidae)

Suicidal behavior in termite workers is an extreme defensive strategy, probably a consequence of having a low number of soldiers available in the colony and there being high predation from enemies. We investigated the suicidal mechanism in workers of the Neotropical termite Neocapritermes opacus, which involves salivary gland autothysis followed by body cuticle rupture and the release of a defensive secretion. Autothysis was triggered by a physical stimulus such as a soldier bite that causes the protrusion of the salivary acini, burst reservoirs, and foregut. Histochemical and ultrastructural analyses showed salivary acini composed of peripheral parietal cells and two types of central cells, types I and II. Type I cells are filled with large electron-lucent secretory vesicles, which reacted positively to bromophenol blue and xylidine-Ponceau tests, indicating the occurrence of proteins. Type II cells are elongated and display smaller apical secretory vesicles. Parietal cells present an intracellular canaliculus with dense microvilli and cytoplasm rich in mitochondria and large electron-dense vesicles, which may participate in the self-destructive mechanism. Worker suicidal behavior was previously reported for N. taracua and N. braziliensis. N. opacus is a new species in which a salivary weapon has been developed and factors contributing to this altruistic response are discussed.

Proteomic-components provide insights into the defensive secretion in termite workers of the soldierless genus Ruptitermes

Termite soldiers constitute the defensive frontline of the colonies, despite workers also perform such tasks, especially within the Neotropical Apicotermitinae, in which all species are soldierless. Workers of the genus Ruptitermes display an extreme form of defense, characterized by body rupture and release of a sticky secretion. Previous observations suggested that such behavior may be advantageous against enemies, but the chemical composition of this secretion has been neglected. Here we firstly provide the proteomic profile of the defensive secretion of Ruptitermes reconditus and Ruptitermes pitan workers. Additionally, the mechanisms of action of this behavior was evaluated through different bioassays. A total of 446 proteins were identified in R. reconditus and 391 proteins in R. pitan, which were classified into: toxins, defensins and proteolytic enzymes; sticky components/ alarm communication; proteins related to detoxification processes; proteins involved in folding/conformation and post-translational modifications; housekeeping proteins; and uncharacterized/hypothetical proteins. According to the bioassays, the self-sacrifice is triggered by a physical stimulus, and the defensive secretion may cause immobility and death of the opponents. Assuming that termites are abundant in the tropics and therefore exposed to predators, suicidal behaviors seem to be advantageous, since the loss of an individual benefit the whole colony. SIGNIFICANCE: Although recent studies have reported the biochemical composition of different weapons in soldiered species of termites, such efforts had not been applied to sordierless taxa up until now. Thus, this is the first report of the defensive mechanisms in soldierless termite species based on proteomic analysis. The diversity of compounds, which included toxin-like and mucin-like proteins, reflect the mechanisms of action of the defensive secretion released by termite workers, which may cause immobility and death of the opponents. Our findings may contribute to the knowledge regarding the development of defensive strategies in termites, especially in groups which lost the soldier caste during the evolution.