Comparative Antennal Morphometry and Sensilla Organization in the Reproductive and Non-Reproductive Castes of the Formosan Subterranean Termite

Abstract

Antennae are the primary sensory organs in insects, where a variety of sensilla are distributed for the perception of the chemical environment. In eusocial insects, colony function is maintained by a division of labor between reproductive and non-reproductive castes, and chemosensation is essential for regulating their specialized social activities. Several social species in Hymenoptera display caste-specific characteristics in antennal morphology and diversity of sensilla, reflecting their differential tasks. In termites, however, little is known about how the division of labor is associated with chemosensory morphology among castes. Using light and scanning electron microscopy, we performed antennal morphometry and characterized the organization of sensilla in reproductive (female and male alates) and non-reproductive (worker and soldier) castes in the Formosan subterranean termite, Coptotermes formosanus Shiraki. Here, we show that the antennal sensilla in alates are twice as abundant as in workers and soldiers, along with the greater number of antennal segments and antennal length in alates. However, all castes exhibit the same types of antennal sensilla, including basiconicum, campaniformium, capitulum, chaeticum I, chaeticum II, chaeticum III, marginal, trichodeum I, and trichodeum I. The quantitative composition of sensilla diverges between reproductive and non-reproductive castes, but not between female and male alates or between worker and soldier castes. The sensilla display spatial-specific distribution, with basiconicum exclusively and capitulum predominantly found on the ventral side of antennae. In addition, the abundance of chemosensilla increases toward the distal end of antennae in each caste. This research provides morphological signatures of chemosensation and their implications for the division of labor, and suggests future neurophysiological and molecular studies to address the mechanisms of chemical communication in termites.

Molecular characterization and expression variation of the odorant receptor co-receptor in the Formosan subterranean termite

“…It is worth noting that the antennal expression of Orco is significantly higher in alates than workers and soldiers ( Fig 4A ). This result is consistent with our previous morphological analysis that alates of both sexes possess longer antennae and higher numbers of antennal sensilla than the non-reproductive castes [ 32 ]. The discrepancy implies differential olfactory sensitivity between reproductive and non-reproductive individuals in C .…”

“…Different from social Hymenoptera, hemimetabolous termite castes are comprised of both females and males [ 6 ]. Our previous study in the Formosan subterranean termite, Coptotermes formosanus , showed that the composition of antennal sensilla varied between reproductive and non-reproductive castes, but not between female and male alates [ 32 ]. In the Japanese subterranean termite, R .…”

Sensory structures on the antenna of soldier and worker castes of the termite species Odontotermes parvidens (Termitidae: Isoptera: Blattaria)

“…Based on our observations, we presume that soldiers with 17 and workers with 19 antennal segments may be the dominant individuals found in colonies of O. parvidens with no or very rare occurrence of soldiers with 16 segmented antenna and workers with 17 segmented antenna. The number of antennal segments gets affected by the age and developmental stages of individuals as maximum number of segments in the individuals with last developmental stages (Castillo, Le, & Sun, 2021). Furthermore, third segment may play an important role in variation of the number of antennal segments as new antennal segments depend on the sub division and growth of the third segment (Roonwal & Chhotani, 1989).…”

“…Sch (II and IV), st (I and II), stc (I and II), sb, scap, and sca I were observed with significant variation in their composition on the antenna of soldiers and workers except sch (I and III) and sca II. In case of the total number of all the sensilla present on the antenna was observed to be more in workers than the soldiers with a significant difference ( (Castillo, Le, & Sun, 2021). Furthermore, third segment may play an important role in variation of the number of antennal segments as new antennal segments depend on the sub division and growth of the third segment (Roonwal & Chhotani, 1989).…”

Conserved orthology in termite chemosensory gene families

“…Like in other insects, antennae are key chemosensory organs in termites (Saran et al, 2007;Du et al, 2019). The antennal flagellum of moniliform antennae is covered by antennal sensilla of various morphological types (Castillo et al, 2021). Irrespective of their different morphologies, all olfactory sensilla house dendrites of olfactory sensory neurons (OSNs), expressing chemosensory receptor proteins that detect the odorants (Hansson and Stensmyr, 2011).…”