Not Only Soldiers Have Weapons: Evolution of the Frontal Gland in Imagoes of the Termite Families Rhinotermitidae and Serritermitidae

Abstract

BackgroundThe frontal gland is a unique adaptation of advanced termite families. It has been intensively studied in soldiers with respect to its anatomy and chemistry, with numerous novel compounds being discovered within the tremendous richness of identified products. At the same time, the presence of the frontal gland in non-soldier castes received only negligible attention in the past.Principal FindingsHere, we report on the development of the frontal gland in alate imagoes of 10 genera and 13 species of Rhinotermitidae and Serritermitidae, in order to shed light on the evolution and function of this gland in imagoes. All investigated species possess a frontal gland. In most cases, it is well-developed and equipped with a sac-like reservoir, located in the postero-dorsal part of cranium, but reaching as far as the seventh abdominal segment in some Rhinotermitinae. The only exception is the genus Psammotermes, in which the gland is very small and devoid of the reservoir.ConclusionsOur direct observations and comparisons with soldiers suggest a defensive role of the gland in imagoes of all studied species. This functional analogy, along with the anatomic homology between the frontal gland in soldiers and imagoes, make it likely that the gland appeared once during the early evolution of rhinotermitid ancestors, and remained as a defensive organ of prime importance in both, soldiers and imagoes.

The nature of alarm communication in Constrictotermes cyphergaster (Blattodea: Termitoidea: Termitidae): the integration of chemical and vibroacoustic signals

“…The combination of reduced mandibles and the presence of well-developed mandibular muscles suggests that the pressure leading to the secretion ejection is indeed created by the mandibular muscles ( Holmgren, 1909 ; Quennedey, 1984 ), but without the direct junction between the muscles and the frontal gland reservoir. The tentorial-fontanellar muscle, observed in Nasutitermes by Noirot (1969) , indeed occurs also in C. cyphergaster , and may prevent fontanelle from being plugged up by the frontal gland epithelium under increased pressure, as already discussed by Šobotník et al (2010b) and Kutalová et al (2013) .…”

The clypeal gland: A new exocrine gland in termite imagoes (Isoptera: Serritermitidae, Rhinotermitidae, Termitidae)

“…Unlike other termite castes living in closed gallery systems, which provide suitable environment and protection against desiccation and predators, alate imagoes are directly exposed to predators when they leave the parental nest to found their own colonies (Nutting, 1969). In response, alate imagoes of many termite species have developed defensive glands, such as the frontal gland (well-developed especially in soldiers), which is an active secretory organ producing defensive and repellent secretion (Piskorski et al, 2009;Sobotník et al, 2010b;Kutalov a et al, 2013). Most studies on glands and the secretion they produce were conducted on workers (labial and sternal glands) and soldiers (frontal and labial glands).…”

Functional transformation series and the evolutionary origin of novel forms: evidence from a remarkable termite defensive organ

“…2B). For example, the frontal gland sometimes expands towards the posterior side of the head, and sometimes reaches the abdomen in the mandibulate-soldier caste of Rhinotermitidae and Serritermitidae ( Sobotn ık et al 2010a). 9), appears to have evolved independently from this ancestral state and is not likely the sister group to the fullydeveloped nasute-soldier form ( Fig.…”

“…Furthermore, such a pinching configuration occurs in several mandibulate-soldier lineages that have an enlarged sac-like frontal gland but lack a nasus. For example, the frontal gland sometimes expands towards the posterior side of the head, and sometimes reaches the abdomen in the mandibulate-soldier caste of Rhinotermitidae and Serritermitidae ( Sobotn ık et al 2010a). In these cases the frontal gland attaches to or lies between the left and right mandible closermuscles.…”