The labral gland in termite soldiers

Abstract

The evolutionary success of termites has been driven largely by a complex communication system operated by a rich set of exocrine glands. As many as 20 different exocrine organs are known in termites. While some of these organs are relatively well known, only anecdotal observations exist for others. One of the exocrine organs that has received negligible attention so far is the labral gland. In this study, we examined the structure and ultrastructure of the labrum in soldiers of 28 termite species. We confirm that the labral gland is present in all termite species, and comprises two secretory regions located on the ventral side of the labrum and the dorso-apical part of the hypopharynx. The labrum of Neoisoptera has a hyaline tip, which was secondarily lost in Nasutitermitinae, Microcerotermes and species with snapping soldiers. The epithelium of the gland generally consists of class 1 secretory cells, with an addition of class 3 secretory cells in some species. A common feature of the secretory cells is the abundance of smooth endoplasmic reticulum, an organelle known to produce lipidic and often volatile secretions. Our observations suggest that the labral gland is involved in communication rather than defence as previously suggested. Our study is the first to provide a comprehensive picture of the structure of the labral gland in soldiers across all termite taxa.

Termite exocrine systems: a review of current knowledge

“…Due to the wide occurrence of the labral gland within Isoptera, its defensive role is questionable. Moreover, the abundance of smooth endoplasmic reticulum in its secretory cells indicates a production of lipids, constituents of pheromones that play a role in termite communication (Palma-Onetto et al, 2018).…”

“…The labral gland is present in soldiers, imagoes, and workers, but it is more developed in the former caste (Palma‐Onetto et al, 2018, 2019). This gland was described by Deligne et al (1981) as a secretory structure of epithelial type, located in the ventral part of the labrum in soldiers of Macrotermes bellicosus (Smeathman).…”

“…Nevertheless, at the tip of the labrum of some termite species, this gland can extend towards its dorsal region. The labral gland may be composed solely of class 1 secretory cells or of arrangements of class 1 and 3 cells (Šobotník et al, 2010a; Costa‐Leonardo & Haifig, 2014; Palma‐Onetto et al, 2018, 2019). A glandular reservoir is absent; thus, the labral secretion is accumulated between the secretory epithelium and the cuticle (Costa‐Leonardo & Haifig, 2014) or in the cuticle itself (Palma‐Onetto et al, 2018).…”

Defensive eversible organ in swallowtail larvae: morphology, structure, chemistry and defensive efficacy

“…To super cially characterize the morphology of the cells that make up the osmeterium externally ve larvae were cryoanesthetized at -4°C for 5 min. Osmeteriums were then dissected in a saline solution (0.1 M NaCl + 0.1 M KH 2 PO 4 + 0.1 M Na 2 HPO 4 ) and prepared to their observation under a JEOL® JSM-6380LV microscope (JEOL Ltd., Japan) at the Universidad Austral de Chile (Valdivia, Chile), following the protocol described by Palma-Onetto et al [53].…”

Termite soldiers contribute to social immunity by synthesizing potent oral secretions

“…Soldiers make up 5–20% of a typical insect colony and display defensive adaptations (Thorne et al ., ; Sobotnik et al ., ; de Roode & Lefèvre, ; Tian & Zhou, ; Kaji et al ., ). In termites, soldiers have specialized head and mandibular morphologies and/or chemical secretions that are thought to be synthesized in exocrine glands such as the salivary and the recently described labral gland (Moore, ; Prestwich, ; Sobotnik et al ., ; Palma‐Onetto et al ., ) (Fig. A), which can be employed to physically attack or deter intruders.…”