Tarsomere and distal tibial glands: structure and potential roles in termites (Isoptera: Rhinotermitidae, Termitidae)

Defensive glands in Stylotermitidae (Blattodea, Isoptera)

The large abundance of termites is partially achieved by their defensive abilities. Stylotermitidae represented by a single extant genus, Stylotermes, is a member of a termite group Neoisoptera that encompasses 83% of termite species and 94% of termite genera and is characterized by the presence of the frontal gland. Within Neoisoptera, Stylotermitidae represents a species-poor sister lineage of all other groups. We studied the structure of the frontal, labral and labial glands in soldiers and workers of Stylotermes faveolus, and the composition of the frontal gland secretion in S. faveolus and Stylotermes halumicus. We show that the frontal gland is a small active secretory organ in soldiers and workers. It produces a cocktail of monoterpenes in soldiers, and some of these monoterpenes and unidentified proteins in workers. The labral and labial glands are developed similarly to other termite species and contribute to defensive activities (labral in both castes, labial in soldiers) or to the production of digestive enzymes (labial in workers). Our results support the importance of the frontal gland in the evolution of Neoisoptera. Toxic, irritating and detectable monoterpenes play defensive and pheromonal functions and are likely critical novelties contributing to the ecological success of these termites.

The ultrastructure of the rostral gland in soldiers of Verrucositermes tuberosus (Blattodea: Termitidae: Nasutitermitinae)

The soil-feeding habit is an evolutionary novelty found in some advanced groups of termites. The study of such groups is important to revealing interesting adaptations to this way-of-life. The genus Verrucositermes is one such example, characterized by peculiar outgrowths on the head capsule, antennae and maxillary palps, which are not found in any other termite. These structures have been hypothesized to be linked to the presence of a new exocrine organ, the rostral gland, whose structure has remained unexplored. We have thus studied the ultrastructure of the epidermal layer of the head capsule of Verrucositermes tuberosus soldiers. We describe the ultrastructure of the rostral gland, which consists of class 3 secretory cells only. The dominant secretory organelles comprise rough endoplasmic reticulum and Golgi apparatus, which provide secretions delivered to the surface of the head, likely made of peptide-derived components of unclear function. We discuss a possible role of the rostral gland of soldiers as an adaptation to the frequent encounter with soil pathogens during search for new food resources.

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.

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.

Insects use lubricants to minimize friction and wear in leg joints

A protein-based lubricating substance is discovered in the femoro-tibial joint of the darkling beetle Zophobas morio (Insecta). The substance extrudes to the contacting areas within the joint and appears in a form of filiform flows and short cylindrical fragments. The extruded lubricating substance effectively reduces the coefficient of sliding friction to the value of 0.13 in the tribosystem glass/lubricant/glass. This value is significantly lower than 0.35 in the control tribosystem glass/glass and comparable to the value of 0.14 for the tribosystem glass/dry PTFE (polytetrafluoroethylene or Teflon). The study shows for the first time that the friction-reducing mechanism found in Z. morio femoro-tibial joints is based on the lubricant spreading over the contacting surfaces rolling or moving at low loads and deforming at higher loads, preventing direct contact of joint counterparts. Besides Z. morio, the lubricant has been found in the leg joints of the Argentinian wood roach Blaptica dubia.

The nasus gland: a new gland in soldiers of Angularitermes (Termitidae, Nasutitermitinae)

Termites have developed many exocrine glands, generally dedicated to defence or communication. Although a few of these glands occur in all termite species, or represent synapomorphies of larger clades, others are morphological innovations of a single species, or a few related species. Here, we describe the nasus gland, a new gland occurring at the base of the nasus of Angularitermes soldiers. The nasus gland is composed of class 1, 2, and 3 secretory cells, a rare combination that is only shared by the sternal and tergal glands of some termites and cockroaches. The ultrastructural observations suggest that the secretion is produced by class 2 and 3 secretory cells, and released mostly by class 3 cells. The base of the nasus has a rough appearance due to numerous pits bearing openings of canals conducting the secretion from class 3 secretory cells to the exterior. We tentatively assign a defensive function to the nasus gland, although further research is needed to confirm this function. Although the gland is described only from species of Angularitermes, other genera of Nasutitermitinae also present a rough nasus base, suggesting the presence of a similar, possibly homologous, gland.

Exocrine glands in the legs of the social wasp Vespula vulgaris

This study brings a survey of the exocrine glands in the legs of Vespula vulgaris wasps. We studied workers, males, virgin queens as well as mated queens. A variety of 17 glands is found in the different leg segments. Among these, five glands are novel exocrine structures for social insects (trochanter-femur gland, ventrodistal tibial gland, distal tibial sac gland, ventral tibial gland, and ventral tarsomere gland). Most leg glands are present in the three leg pairs of all castes. This may indicate a mechanical function. This is likely for the numerous glands that occur near the articulation between the various leg segments, where lubricant production may be expected. Other possible functions include antenna cleaning, acting as a hydraulic system, or pheromonal. Further research including leg-related behavioural observations and chemical analyses may help to clarify the functions of these glandular structures in the legs.