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Article

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

by
Paula Castillo
,
Nathan Le
and
Qian Sun
*
Department of Entomology, Louisiana State University Agricultural Center, Baton Rouge, LA 70803, USA
*
Author to whom correspondence should be addressed.
Insects 2021, 12(7), 576; https://doi.org/10.3390/insects12070576
Submission received: 18 May 2021 / Revised: 17 June 2021 / Accepted: 18 June 2021 / Published: 24 June 2021
(This article belongs to the Special Issue Behavior and Ecology of Social Insects)
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Abstract

:

Simple Summary

Insects use antennae to perceive the chemical environment, and olfaction (the sense of smell) is essential for a variety of behavioral responses. Termites are social insects that display a division of labor based on an elaborate caste system consisting of reproductive (queen and king) and non-reproductive individuals (workers and soldiers). Whether these castes have different senses of smell is poorly understood. In this study, we characterized the morphology of antennae in alates (winged reproductives), workers, and soldiers in the Formosan subterranean termite, Coptotermes formosanus, and further analyzed the diversity and abundance of the antennal sensilla (sensory receptors) in each caste. We found that both female and male alates had longer antennae and greater numbers of sensilla than workers and soldiers, but all castes possessed the same nine types of antennal sensilla. Each type of sensilla had a specific spatial distribution along the antenna. The quantitative composition of sensilla differed between the reproductive and non-reproductive castes, but few differences were found between female and male alates or between worker and soldier castes. These results suggest that the olfactory morphology is associated with the reproductive division of labor in subterranean termites.

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.

1. Introduction

Antennae are essential peripheral sensory structures in insects. The antennae carry multiple cuticular structures called sensilla, which vary in morphology and harbor different types of neurons for perceiving volatile and non-volatile chemicals, humidity, temperature, and tactile stimuli [1,2]. The majority of insects rely on chemicals for the regulation of fundamental behaviors such as foraging and mating [3]. The morphology of the chemosensory system evolved with the chemical ecology of insect species, resulting in enormously diverse shapes and sizes of antennae and different compositions of antennal sensilla [2,3].
In eusocial insects, chemosensation is crucial for communication among colony members and the modulation of social activities. Eusocial colonies consist of individuals that belong to different castes, with a limited number of individuals dominating reproduction and workers (and soldiers in some species) acting as helpers [4]. The antennal morphology and organization of sensilla often exhibit sex- and caste-specificity, reflecting differential sensory capabilities underlying the division of labor [5,6,7,8,9,10,11]. Sexual dimorphism is common in eusocial Hymenoptera (ants, wasps, and bees), where females are diploid, but males are haploid individuals that develop from unfertilized eggs. In ants, male antennae typically have a shorter scape (i.e., the proximal antennal segment) compared with the female [12]. In addition, male ants in a few species lack basiconicum sensilla, a type of sensilla for perception of cuticular hydrocarbons used for nestmate recognition [5,6,13,14,15]. In the honey bee, Apis mellifera Linnaeus, males possess a greater number of sensilla placodea than workers, presumably enhancing their olfactory sensitivity for mate seeking [10]. Differences in antennal morphology and organization sensilla are also associated with the division of labor between non-reproductive castes in several species [8,11]. For example, the guards of a stingless bee, Tetragonisca angustula Latreille, have larger antennal surface areas and greater numbers of sensilla, which allow them to detect heterospecific intruders more effectively than other soldiers [11]. Similarly, in the weaver ant, Oecophylla smaragdina (Fabricius), major workers that leave the nest and forage possess increased numbers of antennal sensilla than minor workers that remain inside or close to the nest [8].
Compared with eusocial Hymenoptera, termites (Blattodea) are among the few hemimetabolous eusocial insects with diplodiploid sex determination [4]. Unlike hymenopteran colonies that are dominated by females, termite colonies are composed both female and male individuals in each caste. In subterranean termites (Rhinotermitidae), colonies are usually founded by a pair of dispersed alates after a nuptial flight. The alates shed their wings (i.e., dealation), and pair up by tandem running, with a male following a female that releases a sex pheromone; the pair then cooperatively construct a nesting chamber, where they mate and rear the first cohort of brood [16,17,18]. Developing workers then take over the task of brood care, construct foraging tunnels to search for food, and perform hygienic activities. Soldiers companion workers while they forage, protect the colony through aggressive behavior toward intruders, and provide a social buffering effect by alleviating competitor- or predator-induced stress [19,20]. While alates develop compound eyes and ocelli, worker and soldiers are blind [21]. Colony members live in dark underground nests and heavily rely on chemical cues and vibrational communication to organize social behavior [22]. Most previous research on termite chemical communication aimed to characterize the semiochemicals, leading to identification of a number of active compounds, such as the trail pheromones for nestmate recruitment [23], reproductive pheromones that regulate caste differentiation and royal recognition [24,25], sex pheromones for courtship behavior [26,27], and death cues that mediate undertaking behavior [28]. While vibrational behavior is performed in various social contexts such as policing behavior [29], colony defense [20], and reproductive recognition [25], this type of substrate-borne signal is likely perceived by the subgenual organ in the legs [30]. The antennae of termites have been demonstrated to play important roles in chemical-mediated nestmate recognition [31], mating behavior [32], and pathogen detection [33]. However, little is known about the neurophysiological process of chemosensation, mainly due to the lack of morphologically characterized chemosensory system in most termite species.
Termites have moniliform antennae, and each antenna is divided into three basic regions including the scape, the pedicel, and the flagellum composed of varying numbers of flagellomeres (Figure 1A). In termites, antennal segment proliferation occurs at the proximal end of the flagellum upon molting, resulting in an increase in the number of flagellomeres as individuals develop [21]. In the Formosan subterranean termite, Coptotermes formosanus Shiraki, the number of antennal segments (including scape and pedicel) varies from 9 in the first instar larvae to 21 in alates in a mature colony [34]. The caste differentiation in termites is primarily a result of developmental plasticity of individuals in response to social and external environments [4], but it remains unknown how organization of antennal sensilla changes as individuals develop into different castes. To date, a few morphological studies on termite antennal sensilla have been conducted [35,36,37], but full characterization of antennal sensilla has been limited to the non-reproductive individuals of C. formosanus [38,39,40,41]. Tarumingkeng et al. [38] examined the antennal sensilla of workers in C. formosanus, and classified them into chemo- and mechanoreceptors based on the presence or absence of pores in the sensilla. Yanagawa et al. [40] further provided more detailed morphological descriptions of all sensilla in worker antennae, as well as their predicted functions and distribution in each antennal segment. Two studies compared the antennal sensilla between worker and soldier castes, revealing no caste dimorphism in their morphology, distribution and abundance [39,41]. However, no information was available for the antennal morphology and organization of sensilla in the reproductive caste, and the sensory bases of differential behavioral responses among castes were also poorly understood in termites.
The reproductive caste is an integral part for understanding chemosensation and its relationship with the division of labor in termite societies, and a comparison of chemosensory morphology between reproductive and non-reproductive individuals warrants further investigation. In this study, we hypothesized that the worker, soldier, and reproductive castes of termites display differences in antennal morphology and organization of antennal sensilla, which may contribute to their differential sensory capabilities. This hypothesis was tested in C. formosanus, one of the most invasive pest species in the world [42]. Specifically, we examined the morphological characteristics of antennae using light microscopy, investigated the composition and spatial distribution of antennal sensilla by scanning electron microscopy, and comparatively analyzed the data in workers, soldiers, female and male alates.

2. Materials and Methods

2.1. Insects

Female and male alates of C. formosanus were collected using ultraviolet light traps (BioQuip, Rancho Dominguez, CA, USA) during the swarm season (May and June) from three populations in Baton Rouge, Louisiana (population A: 30°18′36″ N, 91°05′15″ W; population B: 30°22′14″ N, 91°06′39″ W; population C: 30°21′26″ N, 91°05’27″ W). Upon collection, the alates were processed immediately for analyses of antennal morphology and sensilla. Workers and soldiers were collected from three colonies in New Orleans, Louisiana (colony D: 30°01′26.7″ N, 90°01′01.3″ W; colony E: 29°59′41.3″ N, 90°05′18.4″ W; colony F: 29°54′32″ N, 90°00′32″ W). These colonies were kept at 25 ± 1 °C in clear acrylic containers (38.48 × 45.72 × 22.86 cm), provided with an approximately 4.0 cm layer of organic soil at the bottom and moistened pine wood logs as the food source. The relatively humidity (RH) in each container was monitored weekly, and water was added to maintain 80–99% RH.

2.2. Morphometric Analyses

Workers and soldiers with both intact antennae were selected at random from three freshly collected field colonies (n = 30 with 10 samples per colony). A total of 29 female and 25 male alates with both intact antennae were selected at random from three populations (female: 9, 10, and 10 individuals from population A, B and C, respectively; male: 5, 10, and 10 individuals from population A, B and C, respectively). To prepare the antennae samples, the termites were anesthetized on ice for at least 5 min and placed in a Petri dish (10 cm in diameter) under a Nikon SMZ1270 microscope coupled with a Nikon DS-Ri2 camera (Nikon instruments Inc., Melville, NY, USA). The antennae were carefully removed from the head of each termite using fine forceps (Dumont #5SF, Fine Science Tools, Foster City, CA, USA) and placed in the Petri dish. Pictures of both antennae (left and right) were taken immediately after dissection with the NIS-Elements software version 4.51 (Nikon instruments Inc., Melville, NY, USA) for further morphometric measurements using ImageJ version 1.52p (National Institutes of Health, Bethesda, MD, USA). The measurements included the total number of flagellomeres, total antennal length, lengths of scape and pedicel, widths of scape and pedicel, and widths of proximal, central, and distal flagellomere. The body lengths were also measured, and the average body length of each caste or sex from pooled samples (n = 20 for female and male alates, and n = 30 for workers and soldiers) was used to normalize the antennal length of individuals from each caste or sex group.

2.3. Antennal Sensilla Analyses

Individuals of each caste with intact antennae were selected for analysis of sensilla using scanning electron microscopy (SEM). Each individual was used for imaging either the ventral or dorsal side of an antenna. A total of 18 female and 18 male alates were analyzed, with n = 9 per antennal side each sex (6 and 3 samples from population B and C, respectively). A total of 23 soldiers (n = 11 for dorsal side: 5, 3, and 3 samples from colony D, E and F, respectively; n = 12 for ventral side: 6, 3 and 3 samples from colony D, E and F, respectively) and 24 workers (n = 12 with 6, 3, and 3 samples from colony D, E and F, respectively, for both dorsal and ventral sides) were analyzed. Individuals were anesthetized on ice for at least 5 min. Then, the heads were dissected using fine forceps (Dumont #5SF, Fine Sciences Tools, Foster City, CA, USA), and fixed for 48 h at 4 °C in a mixture of fixatives that contains 4% paraformaldehyde, and 2.5% glutaraldehyde in 0.1 M sodium cacodylate. After fixation, the samples were dehydrated using a gradient of acetone (0, 10%, 25%, 50%, 75%, 100% acetone: water, v/v) to finally air dry, and then mounted onto an aluminum stud. The samples were metal-coated with platinum-palladium in a EMS550X Sputter coater (Electron Microscopy Sciences, Hatfield, PA, USA), and each sample was mounted either for dorsal or ventral view. Pictures were taken in a JSM-6610LV SEM (JEOL Ltd., Peabody, MA, USA) and utilized for the identification and quantification of antennal sensilla. Types of sensilla were classified based on morphology, and putative functions were further determined based on their location and previously published descriptions [40,41].

2.4. Statistical Analyses

All statistical analyses were carried out using the R software version 3.6.3 (The R Foundation, Vienna, Austria) [43], with the exception of principal component analysis (PCA), which was performed using JMP Pro 15 (SAS Institute, Cary, NC, USA) [44]. To check the homogeneity of variances and normality of data distributions, Levene’s tests and Shapiro–Wilk tests were performed, respectively. Generalized linear mixed models (GLMMs) and least square means were utilized to analyze the morphometric data with collection location (i.e., population or colony) coded as a random factor, and caste coded as the fixed factor. Because GLMMs did not fit the antennal sensilla data, the data were pooled from multiple collection locations and analyzed using different methods. Specifically, Kruskal–Wallis and Wilcoxon rank sum tests were performed to evaluate the percentages of sensilla per functional category and the dorsal-ventral distribution of antennal sensilla. In order to compare the relative abundance of each type of sensilla among castes, the proportion of each sensillar type to total sensilla was calculated for each sample. The data were then analyzed by one-way analysis of variance (ANOVA) followed by Tukey’s honestly significant difference (HSD) test. An alpha level of 0.05 was chosen for all tests performed. The relative abundances of nine types of sensilla were also used for PCA to examine the overall patterns across castes.

3. Results

3.1. Morphometric Analyses

A gross comparison of the antennae from a soldier, worker, and female alate is shown in Figure 1A. Female and male alates did not show difference in their antennal morphology. The antennal cuticle of alates and soldiers showed heavier sclerotization than workers (Figure 1A). Female and male alates possessed significantly more flagellomeres than workers and soldiers (Figure 1B), with the number of flagellomeres per antenna ranging from 17 to 19 in female and male alates, 12 to 13 in soldiers, and 11 to 13 in workers. The most frequent numbers of flagellomeres were 18 in both female and male alates (62.06% and 60.0%, with n = 29 and n = 25, respectively); 13 in soldiers (66.66%, n = 30), and 12 in workers (70.0%, n = 30). Female and male alates had significantly longer antennae than workers and soldiers, and soldiers had longer antennae than workers; however, soldiers had the longest antennae in relation to body length among all castes (Table 1). Soldiers and alates did not significantly differ in lengths of scape and pedicel, which in workers were significantly shorter (Table 1). Alates of both sexes had significantly wider scape, pedicel, proximal and central flagellomeres, compared with soldiers and workers; the width of the distal flagellomere, however, did not differ significantly across castes in the measurements performed on left antennae (Table 1). In addition, soldiers had significantly narrower pedicel, proximal and central flagellomeres, compared with other castes (Table 1).
No significant differences were found between the left and right antennae in most of the measurements performed in each caste, but a few exceptions were detected (Table 1). These asymmetries included significantly longer left scape in workers (paired t-test; t = 2.703, df = 29, p = 0.0114), wider right central flagellomere in workers (paired t-test; t = −2.4393, df = 29, p = 0.0211), wider right scape in female alates (paired t-test; t = −2.9006, df = 28, p = 0.0072), and wider right central flagellomere in male alates (paired t-test; t = −2.6622, df = 24, p = 0.0136).

3.2. Types of Antennal Sensilla

Nine types of sensilla were identified in all castes, including sensilla basiconicum, campaniformium, capitulum, chaeticum I, chaeticum II, chaeticum III, marginal, trichodeum I, and trichodeum II (Figure 2A). These sensilla showed different morphological characteristics, presumably associated with different functions (Table 2). Among them, sensilla basiconicum, chaeticum II, trichodeum I, and trichodeum II have a putative chemosensory function; while chaeticum I, chaeticum III, campaniformium, and marginal have a putative mechanosensory function. Sensillum capitulum is the only type of sensilla found with a putative hygro/thermoreceptive function.
Chemosensilla were the most abundant category of sensilla that constituted approximately 90% of total sensilla in each caste (Figure 2B; 90.10 ± 0.47% in soldiers, n = 23; 89.93 ± 0.37% in workers, n = 24; 92.16 ± 0.32% in female alates, n = 18; 92.09 ± 0.22% in male alates, n = 18; dorsal and ventral sides pooled). Alates had significantly higher percentages of chemosensilla than workers and soldiers (soldier-female alate p = 0.0014; worker-female alate p = 0.0002; soldier-male alate p = 0.0009; worker-male alate p < 0.0001; Kruskal–Wallis followed by pairwise Wilcoxon rank sum tests with Bonferroni correction) (Figure 2B). The putative mechanosensory sensilla were less abundant and constituted less than 10% of total sensilla in each caste (Figure 2B; 9.56 ± 0.40% in soldiers, n = 23; 9.88 ± 0.40% in workers, n = 24; 7.77 ± 0.33% in female alates, n = 18; 7.87 ± 0.22% in male alates; n = 18; dorsal and ventral sides pooled). Compared with alates, significantly higher percentages of mechanosensilla were found in non-reproductives (soldier-female alate p = 0.0043; worker-female alate p = 0.0013; soldier-male alate p = 0.0043; worker-male alate p = 0.0013; Kruskal-Wallis followed by pairwise Wilcoxon rank sum tests with Bonferroni correction) (Figure 2B). Sensilla capitulum with a putative hygro/thermoreceptive function did not exceed 0.17% in any caste (Figure 2B). The proportion of this sensillum was significantly reduced in male alates compared with workers and soldiers (p = 0.034 in both comparisons; Kruskal–Wallis followed by pairwise Wilcoxon rank sum tests with Bonferroni correction).

3.3. Spatial Organization of Antennal Sensilla

The nine types of sensilla showed different patterns of distribution and abundance between the dorsal and ventral side of antennae. In soldiers, female, and male alates, the total number of antennal sensilla did not differ significantly between the dorsal and ventral side of their antennae (Figure 3A; p > 0.05; Kruskal–Wallis followed by pairwise Wilcoxon rank sum tests with Bonferroni correction). However, workers had significantly more sensilla on the ventral than dorsal side (Figure 3A; p = 0.0069 by Kruskal–Wallis followed by pairwise Wilcoxon rank sum tests with Bonferroni correction). Interestingly, basiconicum sensilla were present exclusively on the ventral side of all castes (Figure 3B), and capitulum sensilla were distributed predominantly on the ventral side in workers, soldiers, and female alates (Figure 3C). A few other types of sensilla also exhibited significant differences between dorsal and ventral distribution in certain castes (Supplementary Materials Figure S1). For example, chaeticum II sensillum was significantly more abundant on the ventral side of soldiers and workers (for both castes: W = 9, p = 0.0003; Wilcoxon rank sum test), while its abundance did not significantly differ between the two sides in alates (p > 0.05; Wilcoxon rank sum test) (Supplementary Materials Figure S1B). On the contrary, chaeticum III sensillum was significantly more abundant on the dorsal side in female (W = 63.5, p = 0.0411; Wilcoxon rank sum test) and male alates (W = 67, p = 0.0190; Wilcoxon rank sum test), but no significant dorsal-ventral asymmetry was found in soldiers and workers (p > 0.05; Wilcoxon rank sum test) (Supplementary Materials Figure S1C). In addition, marginal sensilla were more abundant on the dorsal than ventral antennae of soldiers (W = 123.5, p = 0.0025; Wilcoxon rank sum test) (Supplementary Materials Figure S1F).
The abundance of total sensilla increased towards the distal end of antennae in all castes (Figure 4). However, not all types of sensilla were ubiquitously present along the antennae. In all castes, sensilla basiconicum, chaeticum I, chaeticum II, trichodeum I, and trichodeum II were distributed in all flagellomeres with increased abundance towards the distal end (Figure 5A–E). However, sensillum capitulum was mostly distributed between the scape and the eighth flagellomere, with decreased abundance towards the distal end (Figure 5F). Sensilla chaeticum III and campaniformium were predominantly found on scape and pedicel (Figure 5G,H), while marginal sensilla were mostly present on the scape and pedicel of all castes, with a distribution between flagellomeres 4 to 7 in workers (Figure 5I).
Trichodeum I was the most abundant type of sensilla found in all castes, corresponding to 49.55 ± 0.65%, 49.44 ± 1.12%, 46.83 ± 0.36% and 46.63 ± 0.42% (mean ± SE) of the total antennal sensilla in soldiers, workers, female and male alates, respectively, on the ventral side (Figure 6B). The second most abundant type of sensilla was chaeticum II, which ranged from 34.22 ± 1.17% in workers up to 39.76 ± 0.40% in female alates (Figure 6B). Sensillum chaeticum I and trichodeum II were the third and fourth most abundant types of sensilla, respectively. Sensillum chaeticum I ranged from 6.76 ± 0.26% in female alates up to 8.20 ± 0.34% in workers; sensillum trichodeum II ranged from 4.00 ± 0.20% in male alates up to 5.10 ± 0.28% in workers (Figure 6B). Sensillum basiconicum, capitulum, chaeticum III, campaniformium, and marginal were the least abundant types of sensilla, which together did not exceed 3.0% of the total antennal sensilla (Figure 6B). The rank for the abundance of the nine types of sensilla on the dorsal side followed the same order (Figure 6A).

3.4. Composition of Antennal Sensilla in Different Castes

The total number of sensilla was significantly higher in female and male alates than workers and soldiers (Figure 3A). When normalized to total sensilla on the dorsal side, no significant differences across castes were found for sensilla campaniformium, capitulum, chaeticum III, and trichodeum II (Figure 6A). However, workers and soldier had significantly higher proportions of sensilla chaeticum I than alates (soldier-female p = 0.031; worker-female p = 0.01; soldier-male p = 0.015; worker-male p = 0.004; one-way ANOVA followed by Tukey’s HSD), while increased percentages of sensilla chaeticum II were found in female and male alates (soldier-female p = 0.007; worker-female p = 0.0002; soldier-male p = 0.005; worker-male p = 0.0002; one-way ANOVA followed by Tukey’s HSD) (Figure 6A). In addition, soldiers had a significantly higher proportion of marginal sensilla than female alates (p = 0.006; one-way ANOVA followed by Tukey’s HSD), and sensilla trichodeum I were detected in a significantly higher proportion in workers than both female and male alates (worker-female p = 0.009; worker-male p = 0.009; one-way ANOVA followed by Tukey’s HSD) (Figure 6A). For the ventral sensilla, no significant differences across castes were found for sensilla campaniformium, chaeticum I, II, and III, marginal sensillum, and sensillum trichodeum I (Figure 6B). However, a significantly lower proportion of sensillum basiconicum was found in soldiers compared with male alates (p = 0.0111; one-way ANOVA followed by Tukey’s HSD) (Figure 6B). Workers possessed a higher proportion of sensillum trichodeum II than other castes (worker-female alate: p = 0.0022; worker-male alate: p = 0.0031; worker-soldier: p = 0.0019; one-way ANOVA followed by Tukey’s HSD), and sensillum capitulum was found in significantly higher proportions in soldiers and workers than female and male alates (soldier-female alate: p = 0.0300; soldier-male alate: p = 0.0022; worker-female alate: p = 0.0040; worker-male alate: p = 0.00023; one-way ANOVA followed by Tukey’s HSD) (Figure 6B). When the proportions of all nine types of sensilla were included, PCA revealed a divergence between the reproductive (female and male alate) and non-reproductive (worker and soldier) castes; however, female and male alates were not distinguished from each other, and there was no separation between worker and soldier castes (Figure 7; the first two principal components explained 58.8% and 50.5% of the total variance for dorsal and ventral sensilla, respectively).

4. Discussion

4.1. Antennal Morphology

Our results revealed caste-specific differences in antennal morphology, but no major sex-specific differences were found in alates. The varying number of flagellomeres reflects the age and developmental status of these castes, as flagellomere number increases with the development of termites [21,34]. In subterranean termites, soldiers differentiate from workers (and additionally the second instar larvae in incipient colonies of C. formosanus), and both workers and soldiers are immature; alates are the only mature individuals that undergo more molts than most other individuals [4,34]. The caste development and age composition of individuals is also dependent on colony age [34], and the colonies used for non-reproductive samples in our study were likely mature colonies (>5 years), based on their antennal flagellomere ranges (11–13 in workers and 12, 13 in soldiers) and previously published information in C. formosanus [34].
It is worth noting that soldiers had longer antennae in relation to body length than workers and alates, and more heavily sclerotized antennae compared with workers, which highlight the defensive role of the soldier caste [19,45]. The longer antennae may facilitate perception of environmental information in a relatively wider range, which allows soldiers to efficiently detect threatening cues from predators and competitors. The elongation in antennae is among several specialized morphological characteristics in C. formosanus soldiers, such as enlarged and heavily sclerotized heads and mandibles for physical defense, and the presence of frontal gland for chemical defense [46]. Such a modification in soldier antennal morphology may be widespread in termites, which awaits further investigation across taxonomic groups.
Bilateral (left/right) asymmetry in antennal length was not found in any caste examined in this study; however, asymmetries were detected in a few morphometric measurements, such as the length of scape in workers, the width of scape in female alates, and the width of central flagellomere in workers and male alates (Table 1). The proximate causes and functions are unknown for these bilateral asymmetries. Our observations are possibly a case of fluctuating asymmetry (i.e., small, random deviations from perfect bilateral symmetry), which is a result of gene–environment interaction and an indicator of environmental stress during development [47,48]. The hemimetabolism has predisposed termites to display developmental plasticity in response to social and external environments, which are important for caste development [4]; however, additional empirical evidence is needed to determine the genetic and environmental influences on antennal development in termites.

4.2. Antennal Sensillar Types and Spatial Organization

We classified the antennal sensilla into nine morphological types, corroborating previously published descriptions by Yanagawa et al. [40] (but different nomenclatures are used by Deng et al. [39] and Fu et al. [41]). These sensilla belong to three functional groups, and the abundance analysis (Figure 2B) indicates that the antennae of C. formosanus are mainly chemosensory organs, but also sensitive to other sensory information such as mechanical stimuli, humidity, and temperature.
A few types of sensilla were differentially distributed on the dorsal and ventral surfaces of antennae. In particular, basiconicum sensilla were located only on the ventral side in each caste. A number of electrophysiological studies have shown that basiconicum sensilla in ants are responsive to cuticular hydrocarbons [14,15,49], an important class of chemicals for communication of species, colony, caste, and reproductive status in social insects [50,51,52]. Sexual dimorphism and spatial-specific distribution of basiconicum sensilla have been reported in ants. These sensilla are absent from males in the red imported fire ant, Solenopsis invicta Buren, the Japanese carpenter ant, Camponotus japonicus Mayr, and the clonal raider ant, Ooceraea biroi (Forel) [5,6,13]. In S. invicta, most basiconicum sensilla are clustered on the ventral and medial surface of the distal region of antennal club [13]; in C. japonicus, the abundance of basiconicum sensilla increases towards the distal antennal segments [6]; and in O. biroi, these sensilla are restricted to the ventral side of the antennal club [5]. Such a sex- and spatial-specific distribution corresponds to the limited social communication in male ants, as well as the social behavior by workers that use restricted antennal club regions to contact nestmates and brood. Termite colonies are composed of female and male individuals both extensively participating in social interactions. In C. formosanus, the ventral-biased distribution in each caste is consistent with our observation that individuals often use the ventral surface of antennae to contact the body of their nestmates, suggesting that basiconicum sensilla play a role in social communication in this species. However, the odorant receptors housed in and the function of basiconicum sensilla are yet to be determined in termites.
The capitulum is a putative hygro/thermo sensillum with a ventral-biased distribution in workers, soldiers, and female alates. In subterranean termites, environmental temperature, humidity, and substrate moisture are important factors influencing survival and foraging behavior of workers and soldiers [53,54], and electrophysiological evidence shows that C. formosanus worker antennae are responsive to humidity changes [55]. The ventral-biased distribution of capitulum sensilla suggests termites may use them for moisture and/or temperature detection by contacting the substrate, but it is puzzling that the distribution pattern was different in male alates.
In all C. formosanus castes examined in this study, antennal sensilla were unevenly distributed across the flagellum with an increase in the abundance of chemosensilla towards the distal end (Figure 5), indicating the distal flagellomeres are more sensitive to environmental odorants. Although intact antennae were analyzed in this study, we have observed that field-collected workers and soldiers often had partially damaged antennae, presumably due to aggressive interactions with other termite colonies or predators. Although the abundance varies along flagellum, all four putative chemosensilla (i.e., basiconicum, chaeticum II, trichodeum I, and trichodeum II) were found in nearly every flagellomere (Figure 5). This suggests that partial damage to antennae may lead to impaired sensitivity but not functional loss of chemosensation. An interesting and widely documented behavior is the antennal cropping by both female and male dealates (i.e., wingless primary reproductives) after colony foundation [56,57,58]. In the congeneric species C. gestroi and C. lacteus, an average of 7.3 and 5.1 flagellomeres, respectively, has been documented to be cropped in dealates [56]. Such a behavior was proposed to reduce pheromonal sensitivity upon the transition from nuptial flight in the open environment to reproduction in the enclosed nest [56], and this “sensitivity hypothesis” is supported by the results of our sensilla abundance analysis. Additional research on pheromone perception in the antennal sensilla and pheromone processing in the brain is required to provide insights into the neural mechanisms underlying the behavioral transition in reproductives.

4.3. Comparison of Antennal Sensilla among Castes

Overall, we found no difference in the diversity of antennal sensilla among the worker, soldier, and reproductive castes in C. formosanus; in addition, our results show quantitative differences in sensilla between the non-reproductive and reproductive castes, but not between female and male alates (Figure 3, Figure 6 and Figure 7). The discrepancy in the quantitative composition of antennal sensilla reflects differential sensitivity to environmental stimuli, which may underlie the division of labor among castes in C. formosanus colonies.
In subterranean termites, alates are the only individuals that are exposed to the open environment, and these individuals have an expanded behavioral repertoire compared with other castes [16,17,18]. The greater antennal sensilla abundance in alates may allow them to perceive a greater range of information. Before colony foundation, these individuals must detect suitable environmental conditions for dispersal [59], sex pheromones for mate seeking [27,32], and available food and moisture for nest construction [16]. During the incipient stage of a colony, the primary reproductives are expected to recognize chemical and/or tactile signals from the offspring for brood care [60], and cuticular hydrocarbons for nestmate recognition [61]. By contrast, workers and soldiers live in enclosed underground nests and perform a suite of collective behavior related to foraging, colony hygiene, and defense [4,19]. The smaller number of antennal sensilla in the non-reproductive individuals, as a result of shorter antennae due to developmental immaturity, may reflect an individual-level reduction in sensitivity to environmental cues related to dispersal and mating.
When the proportions of each sensillar type to total sensilla were compared among castes, alates displayed an enriched dorsal distribution of chaeticum II (Figure 6A), a putative chemosensillum most abundant on the distal flagellomere (Figure 5). This finding, together with the commonly observed antennal cropping behavior [56], suggest that chaeticum II may be involved in mating seeking and nesting site searching in the dispersal reproductives. In termites, the female displays calling behavior by releasing a sex pheromone, and leads the search for a suitable nesting location, while the male follows in tandem upon detection of the female [17,27]. Therefore, sexual dimorphism in chemosensation is expected in alates. However, in our study, no sex-specificity was found in the organization of antennal sensilla. The chemoreceptors and other chemosensory organs such as mouthparts should be examined for a better understanding of the sex-specific behavior.
Compared with the alates, chaeticum I was enriched on the dorsal side and capitulum was enriched on the ventral side of workers and soldiers (Figure 6). Chaeticum I sensillum has a predicted mechanosensory function and capitulum is likely responsive to humidity/moisture and thermal changes [40]. Such an organization in the non-reproductive individuals highlights their sensitivity to mechanical stimuli, which include tactile information for nestmate interactions and physical disturbance to the nest caused by biotic and abiotic factors [62,63], as well as environmental humidity and moisture, which are crucial for the survival of subterranean termites [53,54]. Despite many behavioral differences between workers and soldiers, these two castes share similar quantitative composition of antennal sensilla (Figure 6 and Figure 7), which is consistent with previously reported findings by Fu et al. [41]. Compared with soldiers, workers only exhibited a greater proportion of trichodeum II sensilla on the ventral but not dorsal side. Trichodeum II is a putative chemosensillum, but its biological function is yet to be determined. The results imply that, other than structural bases, molecular and neural level differences in chemosensation may contribute to the division of labor among C. formosanus castes.

5. Future Directions

Our results provide an increased understanding of the morphological basis of chemosensation and its relationship with the division of labor in subterranean termites. We suggest additional studies across termite taxa for comparative analysis of chemosensory morphology. Although C. formosanus castes exhibit several differences in the morphology of antennae and the abundance of sensilla, the structural signatures are among many other chemosensory characteristics that may show more prominent caste- and/or sex-specificity. To elucidate the chemosensory bases of the division of labor in termites, future investigations are required to address the neurophysiological and molecular mechanisms governing chemical perception in the peripheral sensory system and information processing in the brain. We propose immediate next steps that include functional tests of various types of sensilla using single sensillum recording (SSR), and transcriptomic analysis of chemosensory gene repertoires in multiple castes. In addition, morphological and molecular characterization of other sensory organs, particularly the mouthparts, may offer further information on chemical communication in termites. Compared with current knowledge in social Hymenoptera [64,65], electrophysiological studies and functional genetics in termites are still in their infancy. However, recent utilization of neurophysiological approaches such as SSR [66] and immunohistochemistry [67], the development of next-generation sequencing tools [68,69], and the successful application of the RNA interference technique [70,71], offer important avenues for the future exploration of chemosensation and division of labor in termites.

Supplementary Materials

The following are available online at https://www.mdpi.com/article/10.3390/insects12070576/s1, Figure S1: Dorsal and ventral distribution of sensilla chaeticum I (A), chaeticum II (B), chaeticum III (C), trichodeum I (D), trichodeum II sensilla (E) marginal (F), and campaniformium (G), Table S1: Original data.

Author Contributions

P.C. and Q.S. conceived the study; P.C. and N.L. performed the experiments; P.C. analyzed the data; P.C. and Q.S. wrote the manuscript. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by a Hatch fund (Accession Number: 1019492; Project Number: LAB94437) from the USDA National Institute of Food and Agriculture and the Structural Pest Research grant from the Louisiana Department of Agriculture and Forestry to QS, and the College of Agriculture Undergraduate Research Grant from Louisiana State University to NL. In addition, PC was supported by the Economic Development Assistantship from Louisiana State University. The funding agencies have no role in experimental design, data collection and analysis, or preparation of the manuscript.

Data Availability Statement

The data presented in this study are available in the Supplementary Material.

Acknowledgments

We thank Claudia Husseneder, Timothy Schowalter, Junyan Chen, and Joseph McCarthy (Louisiana State University), as well as Carrie Cottone and Shaun Broadley (New Orleans Mosquito, Termite and Rodent Control Board) for assistance with termite collection. We also thank Daniel Swale for allowing access to light microscope and imaging system, the LSU Shared Instrument Facility for assistance with scanning electron microscopy, and Thanos Gentimis for discussion and comments on statistical analyses.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Zacharuk, R.Y. Ultrastructure and function of insect chemosensilla. Annu. Rev. Entomol 1980, 25, 27–47. [Google Scholar] [CrossRef]
  2. Elgar, M.A.; Zhang, D.; Wang, Q.; Wittwer, B.; Pham, H.T.; Johnson, T.L.; Freelance, C.B.; Coquilleau, M. Focus: Ecology and evolution: Insect antennal morphology: The evolution of diverse solutions to odorant perception. Yale J. Biol. Med. 2018, 91, 457. [Google Scholar]
  3. Hansson, B.S.; Stensmyr, M.C. Evolution of insect olfaction. Neuron 2011, 72, 698–711. [Google Scholar] [CrossRef] [Green Version]
  4. Korb, J.; Hartfelder, K. Life history and development-a framework for understanding developmental plasticity in lower termites. Biol. Rev. 2008, 83, 295–313. [Google Scholar] [CrossRef]
  5. McKenzie, S.K.; Fetter-Pruneda, I.; Ruta, V.; Kronauer, D.J.C. Transcriptomics and neuroanatomy of the clonal raider ant implicate an expanded clade of odorant receptors in chemical communication. Proc. Natl. Acad. Sci. USA 2016, 113, 14091–14096. [Google Scholar] [CrossRef] [Green Version]
  6. Nakanishi, A.; Nishino, H.; Watanabe, H.; Yokohari, F.; Nishikawa, M. Sex-specific antennal sensory system in the ant Camponotus japonicus: Structure and distribution of sensilla on the flagellum. Cell Tissue Res. 2009, 338, 79–97. [Google Scholar] [CrossRef]
  7. Ravaiano, S.V.; de Paiva Ferreira, R.; de Oliveira Campos, L.A.; Martins, G.F. The antennal sensilla of Melipona quadrifasciata (Hymenoptera: Apidae: Meliponini): A study of different sexes and castes. Naturwissenschaften 2014, 101, 603–611. [Google Scholar] [CrossRef]
  8. Babu, M.J.; Ankolekar, S.M.; Rajashekhar, K.P. Castes of the weaver ant Oecophylla smaragdina (Fabricius) differ in the organization of sensilla on their antennae and mouthparts. Curr. Sci. 2011, 101, 755–764. [Google Scholar]
  9. Month-Juris, E.; Ravaiano, S.V.; Lopes, D.M.; Fernandes Salomão, T.M.; Martins, G.F. Morphological assessment of the sensilla of the antennal flagellum in different castes of the stingless bee Tetragonisca fiebrigi. J. Zool. 2020, 310, 110–125. [Google Scholar] [CrossRef]
  10. Stort, A.C.; Moraes-Alves, M.M.B. Differences in the number of antennal sensory structures of males of three honey bee types. Rev. Bras. Biol. 1999, 59, 161–166. [Google Scholar] [CrossRef]
  11. Grüter, C.; Segers, F.H.I.D.; Santos, L.L.G.; Hammel, B.; Zimmermann, U.; Nascimento, F.S. Enemy recognition is linked to soldier size in a polymorphic stingless bee. Biol. Lett. 2017, 13, 20170511. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  12. Hölldobler, B.; Wilson, E.O. The Ants; Harvard University Press: Cambridge, MA, USA, 1990. [Google Scholar]
  13. Renthal, R.; Velasquez, D.; Olmos, D.; Hampton, J.; Wergin, W.P. Structure and distribution of antennal sensilla of the red imported fire ant. Micron 2003, 34, 405–413. [Google Scholar] [CrossRef]
  14. Kidokoro-Kobayashi, M.; Iwakura, M.; Fujiwara-Tsujii, N.; Fujiwara, S.; Sakura, M.; Sakamoto, H.; Higashi, S.; Hefetz, A.; Ozaki, M. Chemical discrimination and aggressiveness via cuticular hydrocarbons in a supercolony-forming ant, Formica yessensis. PLoS ONE 2012, 7, e46840. [Google Scholar] [CrossRef] [Green Version]
  15. Sharma, K.R.; Enzmann, B.L.; Schmidt, Y.; Moore, D.; Jones, G.R.; Parker, J.; Berger, S.L.; Reinberg, D.; Zwiebel, L.J.; Breit, B. Cuticular hydrocarbon pheromones for social behavior and their coding in the ant antenna. Cell Rep. 2015, 12, 1261–1271. [Google Scholar] [CrossRef] [PubMed]
  16. Nutting, W.L. Flight and colony foundation. In Biology of Termites; Krishna, K., Weesner, F.M., Eds.; Academic Press: Cambridge, MA, USA, 1969; Volume 1, pp. 233–282. [Google Scholar]
  17. Hartke, T.R.; Baer, B. The mating biology of termites: A comparative review. Anim. Behav. 2011, 82, 927–936. [Google Scholar] [CrossRef]
  18. Bordereau, C.; Pasteels, J.M. Pheromones and Chemical Ecology of Dispersal and Foraging in Termites. In Biology of Termites: A Modern Synthesis; Bignell, D.E., Roisin, Y., Lo, N., Eds.; Springer: Dordrecht, The Netherlands, 2011; pp. 279–320. [Google Scholar]
  19. Tian, L.; Zhou, X. The soldiers in societies: Defense, regulation, and evolution. Int. J. Biol. Sci. 2014, 10, 296. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  20. Tian, L.; Preisser, E.L.; Haynes, K.F.; Zhou, X. Social buffering in a eusocial invertebrate: Termite soldiers reduce the lethal impact of competitor cues on workers. Ecology 2017, 98, 952–960. [Google Scholar] [CrossRef] [Green Version]
  21. Krishna, K.; Weesner, F.M. Biology of Termites; Academic Press: New York, NY, USA, 1969; Volume 1. [Google Scholar]
  22. Hager, F.A.; Krausa, K.; Kirchner, W.H. Vibrational Behavior in Termites (Isoptera). In Biotremology: Studying Vibrational Behavior; Hill, P.S.M., Lakes-Harlan, R., Mazzoni, V., Narins, P.M., Virant-Doberlet, M., Wessel, A., Eds.; Springer International Publishing: Cham, Switzerland, 2019; pp. 309–327. [Google Scholar]
  23. Tokoro, M.; Takahashi, M.; Tsunoda, K.; Yamaoka, R. Isolation and Primary Structure of Trail Pheromone of the Termite, Coptotermes formosanus Shiraki (Isoptera: Rhinotermitidae). Wood Res. Bull. Wood Res. Inst. Kyoto Univ. 1989, 76, 29–38. [Google Scholar]
  24. Matsuura, K.; Himuro, C.; Yokoi, T.; Yamamoto, Y.; Vargo, E.L.; Keller, L. Identification of a pheromone regulating caste differentiation in termites. Proc. Natl. Acad. Sci. USA 2010, 107, 12963–12968. [Google Scholar] [CrossRef] [Green Version]
  25. Funaro, C.F.; Böröczky, K.; Vargo, E.L.; Schal, C. Identification of a queen and king recognition pheromone in the subterranean termite Reticulitermes flavipes. Proc. Natl. Acad. Sci. USA 2018, 115, 3888–3893. [Google Scholar] [CrossRef] [Green Version]
  26. Hanus, R.; Luxová, A.; Šobotník, J.; KalinovÁ, B.; Jiroš, P.; Křeček, J.; Bourguignon, T.; Bordereau, C. Sexual communication in the termite Prorhinotermes simplex (Isoptera, Rhinotermitidae) mediated by a pheromone from female tergal glands. Insectes Soc. 2009, 56, 111–118. [Google Scholar] [CrossRef]
  27. Chouvenc, T.; Sillam-Dussès, D.; Robert, A. Courtship behavior confusion in two subterranean termite species that evolved in allopatry (Blattodea, Rhinotermitidae, Coptotermes). J. Chem. Ecol. 2020, 46, 461–474. [Google Scholar] [CrossRef] [PubMed]
  28. Sun, Q.; Haynes, K.F.; Zhou, X. Dynamic changes in death cues modulate risks and rewards of corpse management in a social insect. Funct. Ecol. 2017, 31, 697–706. [Google Scholar] [CrossRef]
  29. Sun, Q.; Hampton, J.D.; Merchant, A.; Haynes, K.F.; Zhou, X. Cooperative policing behaviour regulates reproductive division of labour in a termite. Proc. R. Soc. B Biol. Sci. 2020, 287, 20200780. [Google Scholar] [CrossRef]
  30. Howse, P.E. An investigation into the mode of action of the subgenual organ in the termite, Zootermopsis angusticollis Emerson, and in the cockroach, Periplaneta americana L. J. Insect Physiol. 1964, 10, 409–424. [Google Scholar] [CrossRef]
  31. Huang, Q. Aggressive behavior and the role of antennal sensillae in the termite Reticulitermes chinensis (Isoptera: Rhinotermitidae). Sociobiology 2014, 59, 1239–1251. [Google Scholar]
  32. Raina, A.K.; Bland, J.M.; Dickens, J.C.; Park, Y.I.; Hollister, B. Premating behavior of dealates of the Formosan subterranean termite and evidence for the presence of a contact sex pheromone. J. Insect Behav. 2003, 16, 233–245. [Google Scholar] [CrossRef]
  33. Yanagawa, A.; Yokohari, F.; Shimizu, S. The role of antennae in removing entomopathogenic fungi from cuticle of the termite, Coptotermes formosanus. J. Insect Sci. 2009, 9. [Google Scholar] [CrossRef] [Green Version]
  34. Chouvenc, T.; Su, N.Y. Colony age-dependent pathway in caste development of Coptotermes formosanus Shiraki. Insectes Soc. 2014, 61, 171–182. [Google Scholar] [CrossRef]
  35. Prestage, J.J.; Slifer, E.H.; Stephens, L.B. Thin-walled sensory pegs on the antenna of the termite worker, Reticulitermes flavipes. Ann. Entomol. Soc. Am. 1963, 56, 874–878. [Google Scholar] [CrossRef]
  36. Costa-Leonardo, A.M.; Soares, H.X. Morphological aspects of neotropical termite antenna under scanning microscopy. Rev. Bras. Entomol. 1997, 41, 47–52. [Google Scholar]
  37. Ishikawa, Y.; Koshikawa, S.; Miura, T. Differences in mechanosensory hairs among castes of the damp-wood termite Hodotermopsis sjostedti (Isoptera: Termopsidae). Sociobiology 2007, 50, 895–907. [Google Scholar]
  38. Tarumingkeng, R.C.; Coppel, H.C.; Matsumura, F. Morphology and ultrastructure of the antennal chemoreceptors and mechanoreceptors of worker Coptotermes formosanus Shiraki. Cell Tissue Res. 1976, 173, 173–178. [Google Scholar] [CrossRef] [PubMed]
  39. Deng, T.F.; Mo, J.C.; He, H.Y.; Pan, C.Y.; Cheng, J.A. Differences of morphology of antennae between soldiers and workers in Coptotermes formosanus (Isoptera: Rhinotermitidae). Sociobiology 2006, 48, 689–700. [Google Scholar]
  40. Yanagawa, A.; Shimizu, S.; Noma, K.; Nishikawa, M.; Kazumasa, O.; Yokohari, F. Classification and distribution of antennal sensilla of the termite Coptotermes formosanus (Isoptera: Rhinotermitidae). Sociobiology 2009, 54, 327. [Google Scholar]
  41. Fu, B.X.; Rong, N.H.; Hong, J.; Zhu, Z.R.; Mo, J.C.; Zhang, D. Comparative study with scanning electron microscopy on the antennal sensilla of two main castes of Coptotermes formosanus Shiraki (Blattaria: Rhinotermitidae). Micron 2020, 129, 102777. [Google Scholar] [CrossRef]
  42. Evans, T.A.; Forschler, B.T.; Grace, J.K. Biology of invasive termites: A worldwide review. Annu. Rev. Entomol. 2013, 58, 455–474. [Google Scholar] [CrossRef] [Green Version]
  43. Ross, I.; Gentleman, R. R: A language for data analysis and graphics. J. Comput. Graph. Stat. 1996, 5, 299–314. [Google Scholar]
  44. Sall, J.; Lehman, A.; Stephens, M.; Loring, S. JMP Start Statistics: A Guide to Statistics and Data Analysis Using JMP; SAS Institute: Cary, NC, USA, 2017. [Google Scholar]
  45. Scholtz, O.I.; Macleod, N.; Eggleton, P. Termite soldier defence strategies: A reassessment of Prestwich’s classification and an examination of the evolution of defence morphology using extended eigenshape analyses of head morphology. Zool. J. Linn. Soc. 2008, 153, 631–650. [Google Scholar] [CrossRef] [Green Version]
  46. Chen, J.; Henderson, G.; Laine, R.A. Lignoceric acid and hexacosanoic acid: Major components of soldier frontal gland secretions of the Formosan subterranean termite (Coptotermes formosanus). J. Chem. Ecol. 1999, 25, 817–824. [Google Scholar] [CrossRef]
  47. Parsons, P.A. Fluctuating asymmetry: An epigenetic measure of stress. Biol. Rev. Camb. Philos. Soc. 1990, 65, 131–145. [Google Scholar] [CrossRef] [PubMed]
  48. De Coster, G.; Van Dongen, S.; Malaki, P.; Muchane, M.; Alcántara-Exposito, A.; Matheve, H.; Lens, L. Fluctuating asymmetry and environmental stress: Understanding the role of trait history. PLoS ONE 2013, 8, e57966. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  49. Ozaki, M.; Wada-Katsumata, A.; Fujikawa, K.; Iwasaki, M.; Yokohari, F.; Satoji, Y.; Nisimura, T.; Yamaoka, R. Ant nestmate and non-nestmate discrimination by a chemosensory sensillum. Science 2005, 309, 311–314. [Google Scholar] [CrossRef] [Green Version]
  50. Martin, S.; Drijfhout, F. A review of ant cuticular hydrocarbons. J. Chem. Ecol. 2009, 35, 1151–1161. [Google Scholar] [CrossRef]
  51. Van Oystaeyen, A.; Oliveira, R.C.; Holman, L.; van Zweden, J.S.; Romero, C.; Oi, C.A.; d’Ettorre, P.; Khalesi, M.; Billen, J.; Wäckers, F. Conserved class of queen pheromones stops social insect workers from reproducing. Science 2014, 343, 287–290. [Google Scholar] [CrossRef]
  52. Sun, Q.; Haynes, K.F.; Zhou, X. Temporal changes in cuticular hydrocarbons during worker-reproductive transition in the eastern subterranean termite (Blattodea: Rhinotermitidae). Ann. Entomol. Soc. Am. 2020. [Google Scholar] [CrossRef]
  53. Gautam, B.K.; Henderson, G. Effects of sand moisture level on food consumption and distribution of Formosan subterranean termites (Isoptera: Rhinotermitidae) with different soldier proportions. Entomol. Sci. 2011, 46, 1–13. [Google Scholar] [CrossRef]
  54. Gautam, B.K.; Henderson, G. Relative humidity preference and survival of starved Formosan subterranean termites (Isoptera: Rhinotermitidae) at various temperature and relative humidity conditions. Environ. Entomol. 2011, 40, 1232–1238. [Google Scholar] [CrossRef] [Green Version]
  55. Yanagawa, A.; Yoshimura, T.; Yanagawa, T.; Yokohari, F. Detection of a humidity difference by antennae in the termite Coptotermes formosanus (Isoptera: Rhinotermitidae). Sociobiology 2010, 56, 255–270. [Google Scholar]
  56. Nalepa, C.A.; Evans, T.A.; Lenz, M. Antennal cropping during colony foundation in termites. ZooKeys 2011, 148, 185–196. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  57. Miyaguni, Y.; Sugio, K.; Tsuji, K. Antennal cropping in the Asian dry-wood termite, Neotermes koshunensis. Insectes Soc. 2013, 60, 223–229. [Google Scholar] [CrossRef] [Green Version]
  58. Chiu, C.-I.; Neoh, K.-B.; Li, H.-F. Colony-founding success of pleometrosis in a fungus-growing termite Odontotermes formosanus. Behav. Ecol. Sociobiol. 2018, 72, 13. [Google Scholar] [CrossRef]
  59. Chouvenc, T.; Scheffrahn, R.H.; Mullins, A.J.; Su, N.Y. Flight phenology of two Coptotermes species (Isoptera: Rhinotermitidae) in southeastern Florida. J. Econ. Entomol. 2017, 110, 1693–1704. [Google Scholar] [CrossRef]
  60. Chouvenc, T.; Su, N.Y. Irreversible transfer of brood care duties and insights into the burden of caregiving in incipient subterranean termite colonies. Ecol. Entomol. 2017, 42, 777–784. [Google Scholar] [CrossRef]
  61. Gordon, J.M.; Šobotník, J.; Chouvenc, T. Colony-age-dependent variation in cuticular hydrocarbon profiles in subterranean termite colonies. Ecol. Evol. 2020, 10, 10095–10104. [Google Scholar] [CrossRef]
  62. Bardunias, P.M.; Su, N.Y. Queue size determines the width of tunnels in the Formosan subterranean termite (Isoptera: Rhinotermitidae). J. Insect Behav. 2010, 23, 189–204. [Google Scholar] [CrossRef]
  63. Crosland, M.W.J.; Traniello, J.F.A. Behavioral plasticity in division of labor in the lower termite Reticulitermes fukienensis. Naturwissenschaften 1997, 84, 208–211. [Google Scholar] [CrossRef]
  64. Ferguson, S.T.; Bakis, I.; Zwiebel, L.J. Advances in the study of olfaction in eusocial ants. Insects 2021, 12, 252. [Google Scholar] [CrossRef]
  65. Yan, H.; Liebig, J. Genetic basis of chemical communication in eusocial insects. Genes Dev. 2021, 35, 470–482. [Google Scholar] [CrossRef]
  66. Yanagawa, A.; Yokohari, F.; Shimizu, S. Influence of fungal odor on grooming behavior of the termite, Coptotermes formosanus. J. Insect Sci. 2010, 10, 141. [Google Scholar] [CrossRef] [Green Version]
  67. Ishikawa, Y.; Aonuma, H.; Sasaki, K.; Miura, T. Tyraminergic and octopaminergic modulation of defensive behavior in termite soldier. PLoS ONE 2016, 11, e0154230. [Google Scholar] [CrossRef] [PubMed]
  68. Mitaka, Y.; Kobayashi, K.; Mikheyev, A.; Tin, M.M.Y.; Watanabe, Y.; Matsuura, K. Caste-specific and sex-specific expression of chemoreceptor genes in a termite. PLoS ONE 2016, 11, e0146125. [Google Scholar] [CrossRef]
  69. Harrison, M.C.; Jongepier, E.; Robertson, H.M.; Arning, N.; Bitard-Feildel, T.; Chao, H.; Childers, C.P.; Dinh, H.; Doddapaneni, H.; Dugan, S. Hemimetabolous genomes reveal molecular basis of termite eusociality. Nat. Ecol. Evol. 2018, 2, 557–566. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  70. Zhou, X.; Oi, F.M.; Scharf, M.E. Social exploitation of hexamerin: RNAi reveals a major caste-regulatory factor in termites. Proc. Natl. Acad. Sci. USA 2006, 103, 4499–4504. [Google Scholar] [CrossRef] [Green Version]
  71. Sun, P.; Yu, S.; Merchant, A.; Lei, C.; Zhou, X.; Huang, Q. Downregulation of Orco and 5-HTT alters nestmate discrimination in the subterranean termite Odontotermes formosanus (Shiraki). Front. Physiol. 2019, 10, 714. [Google Scholar] [CrossRef]
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Figure 1. Overall antennal morphology in C. formosanus. (A) Representative images of antennae from a female alate, a soldier, and a worker. The antenna is divided into three main regions: scape, pedicel, and flagellum, which is composed of flagellomeres numbered from the proximal to distal end, as exemplified in the alate. (B) Numbers of flagellomeres on the left and right antennae in soldiers, workers, female and male alates. Bars represent mean ± standard error (SE). Groups denoted with the same letter are not significantly different (generalized linear mixed model (GLMM), p > 0.05; n = 30 for soldiers and workers, n = 29 for female alates, n = 25 for male alates).
Figure 1. Overall antennal morphology in C. formosanus. (A) Representative images of antennae from a female alate, a soldier, and a worker. The antenna is divided into three main regions: scape, pedicel, and flagellum, which is composed of flagellomeres numbered from the proximal to distal end, as exemplified in the alate. (B) Numbers of flagellomeres on the left and right antennae in soldiers, workers, female and male alates. Bars represent mean ± standard error (SE). Groups denoted with the same letter are not significantly different (generalized linear mixed model (GLMM), p > 0.05; n = 30 for soldiers and workers, n = 29 for female alates, n = 25 for male alates).
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Insects 12 00576 g002 550
Figure 2. Morphological types of sensilla and functional categories. (A) Nine types of sensilla identified in all castes C putative chemosensory sensilla; H/T: putative hygro/thermoreceptive sensilla; M: putative mechanosensory sensilla). (B) Proportions of sensilla per functional category. Bars represent percentage of the different types of sensilla combined per functional category (mean ± SE; n = 24 for workers, n = 23 for soldiers; and n = 18 for both female and male alates). For each functional category, groups denoted with the same letter are not significantly different (p > 0.05, Kruskal–Wallis followed by pairwise Wilcoxon rank sum tests with Bonferroni correction).
Figure 2. Morphological types of sensilla and functional categories. (A) Nine types of sensilla identified in all castes C putative chemosensory sensilla; H/T: putative hygro/thermoreceptive sensilla; M: putative mechanosensory sensilla). (B) Proportions of sensilla per functional category. Bars represent percentage of the different types of sensilla combined per functional category (mean ± SE; n = 24 for workers, n = 23 for soldiers; and n = 18 for both female and male alates). For each functional category, groups denoted with the same letter are not significantly different (p > 0.05, Kruskal–Wallis followed by pairwise Wilcoxon rank sum tests with Bonferroni correction).
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Insects 12 00576 g003 550
Figure 3. Dorsal and ventral distribution of antennal sensilla in C. formosanus castes. (A) Total number of sensilla on dorsal and ventral sides per antenna (mean ± SE). Groups denoted with the same letter are not significantly different (p > 0.05, Kruskal–Wallis followed by pairwise Wilcoxon rank sum tests with Bonferroni correction). (B,C) show number of basiconicum and capitulum sensilla per antenna, respectively, distributed on each side. Means are presented above bars. Bars represent mean ± SE (*, p < 0.05; n.s., not significant, p > 0.05; Wilcoxon rank sum test). Dorsal side: n = 11, 12, 9, and 9 for soldier, worker, female and male alate, respectively; Ventral side: n = 12, 12, 9, and 9 for soldier, worker, female and male alate, respectively.
Figure 3. Dorsal and ventral distribution of antennal sensilla in C. formosanus castes. (A) Total number of sensilla on dorsal and ventral sides per antenna (mean ± SE). Groups denoted with the same letter are not significantly different (p > 0.05, Kruskal–Wallis followed by pairwise Wilcoxon rank sum tests with Bonferroni correction). (B,C) show number of basiconicum and capitulum sensilla per antenna, respectively, distributed on each side. Means are presented above bars. Bars represent mean ± SE (*, p < 0.05; n.s., not significant, p > 0.05; Wilcoxon rank sum test). Dorsal side: n = 11, 12, 9, and 9 for soldier, worker, female and male alate, respectively; Ventral side: n = 12, 12, 9, and 9 for soldier, worker, female and male alate, respectively.
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Figure 4. Distribution of total sensilla along the antennae in C. formosanus castes. (A) Ventral side abundance of sensilla per antennal segment. F1-F18 denote flagellomeres numbered from proximal to distal end. All alate samples analyzed here had 18 flagellomeres, while worker and soldier samples had 12 flagellomeres. Data are presented as mean ± SE (n = 12 for soldiers and workers, n = 9 for female and male alates). (B) A worker antenna representing the distribution of sensilla at the distal and proximal ends.
Figure 4. Distribution of total sensilla along the antennae in C. formosanus castes. (A) Ventral side abundance of sensilla per antennal segment. F1-F18 denote flagellomeres numbered from proximal to distal end. All alate samples analyzed here had 18 flagellomeres, while worker and soldier samples had 12 flagellomeres. Data are presented as mean ± SE (n = 12 for soldiers and workers, n = 9 for female and male alates). (B) A worker antenna representing the distribution of sensilla at the distal and proximal ends.
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Figure 5. Distribution of nine types of sensilla along the ventral side of the antennae in C. formosanus castes. The numbers of sensilla basiconicum (A), chaeticum I (B), chaetucum II (C), trichodeum I (D), trichodeum II (E), capitulum (F), chaeticum III (G), campaniformium (H), and marginal (I) located on the scape, pedicel, and each flagellomere are shown (mean ± SE, n = 12 for soldiers and workers, n = 9 for female and male alates).
Figure 5. Distribution of nine types of sensilla along the ventral side of the antennae in C. formosanus castes. The numbers of sensilla basiconicum (A), chaeticum I (B), chaetucum II (C), trichodeum I (D), trichodeum II (E), capitulum (F), chaeticum III (G), campaniformium (H), and marginal (I) located on the scape, pedicel, and each flagellomere are shown (mean ± SE, n = 12 for soldiers and workers, n = 9 for female and male alates).
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Figure 6. Comparison of relative abundance of nine sensillar types among castes. The plot shows percentage of each type of sensilla to total sensilla on the dorsal (A) and ventral (B) side. Bars represent mean ± SE. For each type of sensilla, groups denoted with the same letter are not significantly different (p > 0.05, one-way analysis of variance (ANOVA) followed by Tukey’s honestly significant difference (HSD); Dorsal side: n = 11, 12, 9, and 9 for soldier, worker, female and male alate, respectively; Ventral side: n = 12, 12, 9, and 9 for soldier, worker, female and male alate, respectively; NA: not available).
Figure 6. Comparison of relative abundance of nine sensillar types among castes. The plot shows percentage of each type of sensilla to total sensilla on the dorsal (A) and ventral (B) side. Bars represent mean ± SE. For each type of sensilla, groups denoted with the same letter are not significantly different (p > 0.05, one-way analysis of variance (ANOVA) followed by Tukey’s honestly significant difference (HSD); Dorsal side: n = 11, 12, 9, and 9 for soldier, worker, female and male alate, respectively; Ventral side: n = 12, 12, 9, and 9 for soldier, worker, female and male alate, respectively; NA: not available).
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Figure 7. Principal component analysis (PCA) of dorsal (A) and ventral (B) sensilla in different castes. All nine types of sensilla were included, and percentages of each sensillar type to total sensilla were used in the analysis. The two gray ellipses in each panel encircle approximate 90% samples in the reproductive (female and male alate) and non-reproductive (soldier and worker) castes, respectively. (Dorsal side: n = 11, 12, 9, and 9 for soldier, worker, female and male alate, respectively; Ventral side: n = 12, 12, 9, and 9 for soldier, worker, female and male alate, respectively).
Figure 7. Principal component analysis (PCA) of dorsal (A) and ventral (B) sensilla in different castes. All nine types of sensilla were included, and percentages of each sensillar type to total sensilla were used in the analysis. The two gray ellipses in each panel encircle approximate 90% samples in the reproductive (female and male alate) and non-reproductive (soldier and worker) castes, respectively. (Dorsal side: n = 11, 12, 9, and 9 for soldier, worker, female and male alate, respectively; Ventral side: n = 12, 12, 9, and 9 for soldier, worker, female and male alate, respectively).
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Table
Table 1. Morphometric comparison of antennae among C. formosanus castes. Measurements (mean ± SE) from left and right antennae were analyzed separately to compare soldier, worker, female and male alate samples. Groups denoted with the same letter in each row are not significantly different (GLMM, p > 0.05; n = 30 for soldiers and workers, n = 29 for female alates, n = 25 for male alates).
Table 1. Morphometric comparison of antennae among C. formosanus castes. Measurements (mean ± SE) from left and right antennae were analyzed separately to compare soldier, worker, female and male alate samples. Groups denoted with the same letter in each row are not significantly different (GLMM, p > 0.05; n = 30 for soldiers and workers, n = 29 for female alates, n = 25 for male alates).
SideSoldierWorkerFemale AlateMale Alate
Total antennal length (mm)Left1.643 ± 0.019 (b)1.277 ± 0.017 (a)2.309 ± 0.014 (c)2.284 ± 0.020 (c)
Right1.648 ± 0.019 (b)1.265 ± 0.020 (a)2.335 ± 0.016 (c)2.284 ± 0.026 (c)
Antennal length normalized to body length (%)Left45.78 ± 0.540 (b)36.42 ± 0.555 (a)34.45 ± 0.215 (a)35.61 ± 0.310 (a)
Right45.93 ± 0.540 (b)36.11 ± 0.660 (a)34.84 ± 0.251 (a)35.60 ± 0.409 (a)
Length of scape (µm)Left163.50 ± 3.053 (ab)159.13 ± 2.750 (a)173.16 ± 4.160 (c)179.66 ± 2.360 (bc)
Right165.43 ± 2.906 (b)148.27 ± 3.635 (a)179.03 ± 2.202 (b)173.08 ± 2.974 (b)
Length of pedicel (µm)Left90.23 ± 1.506 (b)84.50 ± 0.816 (a)91.97 ± 0.131 (b)92.52 ± 1.372 (b)
Right89.40 ± 1.252 (b)82.47 ± 1.462 (a)95.38 ± 1.285 (b)94.72 ± 1.285 (b)
Width of scape (µm)Left95.60 ± 0.601 (a)94.90 ± 0.929 (a)120.38 ± 1.097 (b)124.00 ± 1.350 (c)
Right96.20 ± 0.904 (a)93.03 ± 1.287 (a)124.55 ± 1.344 (b)121.40 ± 1.316 (b)
Width of pedicel (µm)Left72.90 ± 0.597 (a)78.10 ± 0.422 (b)94.36 ± 0.984 (c)93.55 ± 0.930 (c)
Right72.90 ± 0.524 (a)77.23 ± 0.644 (b)95.45 ± 0.628 (c)93.76 ± 0.722 (c)
Width of proximal flagellomere (µm)Left64.03 ± 0.885 (a)71.20 ± 0.643 (b)86.80 ± 1.235 (c)87.41 ± 1.604 (c)
Right65.83 ± 1.092 (a)72.13 ± 0.847 (b)88.97 ± 1.283 (c)86.40 ± 1.786 (c)
Width of central flagellomere (µm)Left90.83 ± 0.924 (a)93.90 ± 1.004 (a)115.45 ± 1.294 (c)109.40 ± 1.094 (b)
Right91.63 ± 0.931 (a)96.27 ± 0.953 (b)115.55 ± 1.110 (c)112.76 ± 1.482 (c)
Width of distal flagellomere (µm)Left80.17 ± 0.803 (a)83.07 ± 0.694 (a)83.34 ± 1.073 (a)83.68 ± 1.174 (a)
Right80.60 ± 0.592 (a)83.50 ± 0.805 (b)82.41 ± 0.873 (ab)83.48 ± 0.818 (ab)
Table
Table 2. Characteristics of sensilla. Morphological measurement data (mean ± SE, n = 3) were obtained from workers. Putative functions were determined based on morphology and location of sensilla and previously published descriptions [40,41].
Table 2. Characteristics of sensilla. Morphological measurement data (mean ± SE, n = 3) were obtained from workers. Putative functions were determined based on morphology and location of sensilla and previously published descriptions [40,41].
Sensillar TypeMorphological CharacteristicsPutative FunctionTotal Length (µm)Basal Diameter (µm)Tip Diameter (µm)Socket Diameter (µm)Central Diameter (µm)
BasiconicumShort, blunt-tipped, cylindrical-shaped with a narrow tipChemosensory12.916 ± 0.6852.129 ± 0.0670.504 ± 0.0683.559 ± 0.070-
CampaniformiumSmooth-surfaced oval-shaped poreMechanosensory4.472 ± 0.127---1.702 ± 0.088
CapitulumCone-shaped, wider at the base and narrowed towards the tipHygro/thermoreceptive6.579 ± 0.1622.483 ± 0.0870.608 ± 0.0323.970 ± 0.226-
Chaeticum ILongest sensilla; the wide base gives support to its cylindrical shape that ends in a thin tipMechanosensory114.649 ± 7.5954.209 ± 0.3100.618 ± 0.0075.523 ± 0.226-
Chaeticum IIWider at the base and narrower towards the tip; slightly curved towards the surface of the antennaeChemosensory18.790 ± 2.4051.986 ± 0.0990.315 ± 0.0062.593 ± 0.106-
Chaeticum IIIShort, cone-shaped sensilla located in a socket-like structureMechanosensory5.860 ± 0.5001.636 ± 0.0290.397 ± 0.0452.776 ± 0.150-
MarginalSmooth-surfaced dome in a socketMechanosensory---3.360 ± 0.6971.899 ± 0.464
Trichodeum ISimilar in shape to Chaeticum II but straight, without curving towards the surface of the antennaeChemosensory49.093 ± 1.0692.473 ± 0.0610.455 ± 0.0153.546 ± 0.302-
Trichodeum IIWider than trichodeum I, slightly curved towards the surface of the antennaeChemosensory11.520 ± 0.2071.840 ± 0.1620.291 ± 0.0612.487 ± 0.235-
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Castillo, P.; Le, N.; Sun, Q. Comparative Antennal Morphometry and Sensilla Organization in the Reproductive and Non-Reproductive Castes of the Formosan Subterranean Termite. Insects 2021, 12, 576. https://doi.org/10.3390/insects12070576

AMA Style

Castillo P, Le N, Sun Q. Comparative Antennal Morphometry and Sensilla Organization in the Reproductive and Non-Reproductive Castes of the Formosan Subterranean Termite. Insects. 2021; 12(7):576. https://doi.org/10.3390/insects12070576

Chicago/Turabian Style

Castillo, Paula, Nathan Le, and Qian Sun. 2021. "Comparative Antennal Morphometry and Sensilla Organization in the Reproductive and Non-Reproductive Castes of the Formosan Subterranean Termite" Insects 12, no. 7: 576. https://doi.org/10.3390/insects12070576

APA Style

Castillo, P., Le, N., & Sun, Q. (2021). Comparative Antennal Morphometry and Sensilla Organization in the Reproductive and Non-Reproductive Castes of the Formosan Subterranean Termite. Insects, 12(7), 576. https://doi.org/10.3390/insects12070576

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