Morphology and ultrastructure of the antennal chemoreceptors and mechanoreceptors of worker Coptotermes formosanus Shiraki

Antennal excision reveals disparate olfactory expression patterns within castes in Reticulitermes aculabialis (Isoptera: Rhinotermitidae)

In lower termites, which exhibit a high degree of compound eye degradation or absence, antennae play a pivotal role in information acquisition. This comprehensive study investigates the olfactory system of Reticulitermes aculabialis, spanning five developmental stages and three castes. Initially, we characterize the structures and distribution of antennal sensilla across different developmental stages. Results demonstrate variations in sensilla types and distributions among stages, aligning with caste-specific division of labor and suggesting their involvement in environmental sensitivity detection, signal differentiation, and nestmate recognition. Subsequently, we explore the impact of antennal excision on olfactory gene expression in various caste categories through transcriptomics, homology analysis, and expression profiling. Findings reveal that olfactory genes expression is influenced by antennal excision, with outcomes varying according to caste and the extent of excision. Finally, utilizing fluorescence in situ hybridization, we precisely localize the expression sites of olfactory genes within the antennae. This research reveals the intricate and adaptable nature of the termite olfactory system, highlighting its significance in adapting to diverse ecological roles and demands of social living.

Morphological Characterstics of the Sensilla in a Monophagous Insect: Agasicles hygrophila Selman and Vogt (Coleoptera: Chrysomelidae, Halticinae)

Agasicles hygrophila Selman and Vogt (Coleoptera: Chrysomelidae) is the key natural enemy of Alternanthera philoxeroides (Mart.) Griseb, an invasive weed worldwide. To understand the morphology of A. hygrophila and further explore the specific host localization mechanism, scanning electron microscopy was used to observe and study the morphological characteristics of sensilla on the head appendages, tarsi, and external genital segments of A. hygrophila. Twelve types and forty-six subtypes of sensilla were observed. These contain various types of head appendices, including sensilla chaetica, sensilla trichodea, sensilla basiconca, sensilla coeloconica, sensilla styloconica, Böhm bristles, sensilla campaniform, sensilla terminal, sensilla dome, sensilla digit-like, sensilla aperture, and many subtypes. A new type of sensor was reported for the first time, which may be related to host plant recognition. This sensor was located on the distal segment of the maxillary palps of A. hygrophila and was named as sensilla petal-shaped based on its morphological characteristics. Sensilla chaetica, sensilla trichodea, and sensilla basiconca are also found on the tarsi and external genital segments. In addition, sensilla basiconica 4, sensilla coeloconica 1 and 2, sensilla styloconica 2, Böhm bristles 2, and sensilla campaniform 1 were only found in females. On the contrary, sensilla styloconica 3, sensilla coeloconica 3, and sensilla dome were only found in males. Numbers and sizes of the sensilla were also different between males and females. The potential functions related to structure were discussed in comparison with previous investigations on beetles and other monophagous insects. Our results provide a microscopic morphological basis for further research on the localization and recognition mechanism of A. hygrophila and its obligate host.

Ultrastructure and distribution of sensory receptors on the nonolfactory organs of the soldier caste in subterranean termite (Coptotermes spp.)

The soldier caste of termites uses sensilla to sense pheromonal, tactile, and vibrational cues to communicate inside and outside their nest. Although sensilla with many modalities on the antennae of subterranean termites have been well explored, there remains a lack of information regarding sensillum characteristics and distribution of the nonolfactory organs of the soldier caste in the Coptotermes genus. In this study, the ultrastructure of sensilla from the soldier caste of three Coptotermes spp. (Coptotermes formosanus, Coptotermes curvignathus, and Coptotermes gestroi) was observed by scanning and transmission electron microscopy, and the putative function of each type was deduced. Six total sensillum types were observed, with two mechanoreceptive sensillum types (hair and plate). The long flexible-peg mechanoreceptive sensilla may work as contact-chemoreceptive sensilla due to their elongated dendritic outer segments and uniporous characteristics. There was a significant depletion of mechano-chemoreceptive sensillum numbers in C. gestroi, which was compensated by a high density of short-peg mechanoreceptive sensilla on the pronotum. Finally, cuticular and innervation characteristics of thermo-/hygrosensitive sensilla were observed for the first time on the labrum of the soldier caste of Coptotermes.

Low radiodensity μCT scans to reveal detailed morphology of the termite leg and its subgenual organ

Termites sense tiny substrate-borne vibrations through subgenual organs (SGOs) located within their legs' tibiae. Little is known about the SGOs' structure and physical properties. We applied high-resolution (voxel size 0.45 μm) micro-computed tomography (μCT) to Australian termites, Coptotermes lacteus and Nasutitermes exitiosus (Hill) to test two staining techniques. We compared the effectiveness of a single stain of Lugol's iodine solution (LS) to LS followed by Phosphotungstic acid (PTA) solutions (1% and 2%). We then present results of a soldier of Nasutitermes exitiosus combining μCT with LS + 2%PTS stains and scanning electron microscopy to exemplify the visualisation of their SGOs. The termite's SGO due to its approximately oval shape was shown to have a maximum diameter of 60 μm and a minimum of 48 μm, covering 60 ± 4% of the leg's cross-section and 90.4 ± 5% of the residual haemolymph channel. Additionally, the leg and residual haemolymph channel cross-sectional area decreased around the SGO by 33% and 73%, respectively. We hypothesise that this change in cross-sectional area amplifies the vibrations for the SGO. Since SGOs are directly connected to the cuticle, their mechanical properties and the geometric details identified here may enable new approaches to determine how termites sense micro-vibrations.

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

Odontotermes parvidens is a commonly found important pest species of termite that primarily feed on a range of cellulosic sources. In termites, communication among the nestmates is the basis of all their daily activities in termites and the sensory structures present on the sensory appendages play a crucial role in different social behaviors perceiving and processing various signals of the nestmates and external environments. So, it is essential to understand in detail their sensory structures in order to understand the sensory system of the species O. parvidens. Thus, we have studied the antenna which is one of the primary sensory appendages of both soldier and worker individuals of the species to elucidate various antennal sensory structures and their distribution using scanning electron microscopy. Based on the morphological features of various sensilla, we observed seven types of sensilla as sensilla chaetica (subtypes I, II, III, and IV), sensilla trichodea (subtypes I and II), sensilla trichodea curvata (subtypes I and II), sensilla basiconica, sensilla capitula, Böhm bristles, and sensilla campaniformia (subtypes I and II) along with numerous glandular pores on the antenna of soldier and worker castes. We have also discussed the putative functions of all the sensilla observed based on their external morphology and distributional characteristics on the antenna of soldier and worker castes of the species O. parvidens. Furthermore, the abundance of sensillar types on the antenna of both the castes has also been discussed.

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

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.

Comparative study with scanning electron microscopy on the antennal sensilla of two main castes of Coptotermes formosanus Shiraki (Blattaria:Rhinotermitidae)

Sensilla on antennae of the workers and soldiers of Coptotermes formosanus Shiraki were examined by scanning electron microscopy in this study. As the two castes were allocated totally different tasks in the termite colony, we wondered if there was a big difference between their antennae which were recognized as the main sensory appendages of insects. Therefore, detailed information about the morphology, distribution and abundance of various types of sensilla was described in this report. However, our results showed no obvious caste dimorphism was observed. The morphology of antennae and sensilla as well as the general sensilla distribution pattern did not differ between the workers and soldiers of C. formosanus. In total, seven types of sensilla including sensilla chaetica (I, II, III), Böhm bristles, sensilla campaniformia (I, II, III), sensilla trichodea, sensilla basiconica, sensilla trichodea curvata and sensilla capitula were found on the antennae. Additionally, small apertures were found scattered randomly in the antennal cuticle. Functions of these sensilla or structures were proposed to be mechanoreceptors, chemo-receptors, thermo-hygroreceptors, co₂-receptors etc. which probably play crucial roles in their various social behaviors.

Olfactory Cues from Pathogenic Fungus Affect the Direction of Motion of Termites, Coptotermes formosanus

Formosan subterranean termites, Coptotermes formosanus, tend to avoid pathogen odors when tested in Y-tube olfactometers, but approach and groom exposed nestmates to remove pathogens from their cuticle and maintain a healthy population. To better understand their differential reaction to pathogens and their odors, the relationship between odor cues and direction of motion was examined with the fungus Isaria fumosorosea K3 strain. The results indicate that nestmate odor was strongly attractive only in tests where fungal odors were present in both branches of the olfactometer. Termites generally avoid fungal odors when offered a choice without fungal odor. We also tested termite aversion to 3-octanone and 1-octen-3-ol, major surface chemical compounds of I. fumosorosea K3, and estimated the total mass of these compounds present on the conidial surface by direct extraction method. The total quantity of these chemicals on the surface of fungal conidia was estimated to be approximately 0.01 ng per 10⁷ conidia. This study demonstrates a context dependent behavioral change in termites in response to the odors of pathogenic fungi.

Antennal cropping during colony foundation in termites

The literature on pairing and mating behavior in termites indicates that a number of distal antennal segments in dealates of both sexes are often removed during colony foundation, with terms such as amputation, mutilation and cannibalism typically employed to report the phenomenon. Here we propose the use of the phrase 'antennal cropping' to describe the behavior, and assess naturally occurring levels of its occurrence by comparing the number of antennal segments in museum specimens of alates and dealates in 16 species of Australian termites (four families), supplemented by analyzing published data on Coptotermes gestroi. Dealates had significantly fewer antennal segments than alates in 14 of the 16 termite species, with both exceptions belonging to the family Termitidae. Levels of antennal cropping were not significantly different between the sexes but did vary by family. Dealates in the Kalotermitidae removed the most segments (41.3%) and those in the Termitidae removed the fewest (8.9%). We discuss the biological significance of this phylogenetically widespread termite behavior, and suggest that controlled antennal cropping is not only a normal part of their behavioral repertoire but also a key influence that changes the conduct and physiology of the royal pair during the initial stages of colony foundation.