Morphology of the Novel Basimandibular Gland in the Ant Genus Strumigenys (Hymenoptera, Formicidae)

Abstract

In 1999, Barry Bolton postulated the presence of a basimandibular gland in the mandibles in all species of the ant genus Strumigenys, solely based on scanning microscopy observations. We now confirm the presence of this putative gland in the proximal outer part of the mandibles of 22 investigated species by histological and ultrastructural examination, including 10 short- and 12 long-mandibulate species. All species have a basimandibular gland, that is formed by 15–25 µm thick epithelial cells and belongs to class-1 following the standard classification of insect exocrine glands. We consider it a novel gland because of its peculiar bowl-shape and special arrangement of the microvilli that are confined to large vacuolar spaces instead of reaching the cuticle. The gland is most pronounced in S. mutica, particularly in the queen. In addition to this gland, we also found scattered class-3 intramandibular gland cells in the mandibles. Queens of S. mutica are peculiar in having a cluster of these cells in the distal tip of their mandibles. As this species is a social parasite, further research is required to determine whether the development of these mandibular glands is related to its parasitic lifestyle.

The basitarsal sulcus gland, a novel exocrine structure in ants

“…Similar cuticular modifications for class-1 glands have also been reported for several other leg glands in ants (Billen, 2009), for the mandibular pit gland in Brachyponera sennaarensis (Billen and Al-Khalifa, 2016) and for the basimandibular gland in Strumigenys (Wang et al, 2021). The cytoplasm is mainly dominated by smooth endoplasmic reticulum and Golgi apparatus, which is indicative for a non-proteinaceous, and hence likely pheromonal secretion (see further).…”

Morphology of the novel ventral scape gland in the ant genus Strumigenys

“…His description of the smooth patch on the ventrodistal scape cuticle was probably based on examination at a rather low magnification, as he did not mention the presence of numerous minute slits around 100 nm in the smooth area. Such extremely small depressions in the cuticular surface have also been observed in the apicofemoral and apicotibial (Billen et al, 2000) and basimandibular gland of Strumigenys (Wang et al, 2021), several other epithelial leg glands of ants (Billen, 2009) and in the mandibular pit gland of Brachyponera sennaarensis (Billen and Al-Khalifa, 2016). These abundant minute slits represent cuticular cracks through which the secretions of class-1 glands can reach the surface, and are totally different from the larger round pores with a characteristic diameter of 0.5-1 µm that are the duct openings of class-3 glands.…”

“…That also workers of the host species Formica cunicularia have similar scape glands can be understood in the context of manipulation of host behaviour (Romani et al, 2006). Among our study species, Strumigenys mutica is also a social parasite (Wang et al, 2021). As this is not a slavemaking species, however, it is only the queen that invades a host colony.…”

“…We moreover found the ventral scape gland in the parasite (S. mutica), the host species (S. formosensis and S. solifontis), as well as in 16 other Strumigenys species, which does not make a connection with social parasitism very likely. It is noteworthy, however, that the novel basimandibular gland is clearly more developed in S. mutica queens than in workers (Wang et al, 2021).…”

“…The reduction in cuticle thickness of the part that overlays the glandular epithelium to half the size it has elsewhere in the scape gives the gland a bowl-shaped appearance. Two additional ultrastructural peculiarities make the ventral scape gland look very similar to the basimandibular gland (Wang et al, 2021) and the apicofemoral and apicotibial glands (Billen et al, 2000;Billen, 2009) that represent other epidermal glands with exclusive occurrence in Strumigenys: microvilli make no direct contact with the overlaying cuticle as is usual for class-1 glands (Noirot and Quennedey, 1974;Billen and Morgan, 1998), but are confined to large vacuolar spaces in the apical part of the cells. As a result, thin subcuticular cytoplasmic processes occur, of which the functional significance as yet remains unclear.…”