The functions of antennal mechanoreceptors and antennal joints in tactile discrimination of the honeybee (Apis mellifera L.)

Flexible tactile sensors inspired by bio-mechanoreceptors

Mechanoreceptors in animals and plants play a crucial role in sensing mechanical stimuli such as touch, motion, stretch, and vibration. Learning from the mechanisms of mechanoreceptors may facilitate the development of bionic tactile sensors, leading to higher sensitivity, spatial resolution, and dynamic ranges. However, very little literature has comprehensively discussed the relevance of biological tactile sensing systems and machine-learning-based bionic tactile sensors. This review first introduces the structural features, signal acquisition and transmission mechanisms, and feedback processes of both plant and animal mechanoreceptors, and then summarizes the efforts to develop bionic tactile sensors by mimicking the morphologies and structures of mechanoreceptors in plants and animals. Additionally, the integration of artificial intelligence approaches with these sensors for data processing and analysis are demonstrated, followed by the perspectives on current challenges and future trends in bionic tactile sensors. This review addresses the challenges in developing high-performance tactile sensors by focusing on surface microstructures and biological mechanoreceptors, serving as a valuable reference for developing bionic tactile sensors with enhanced sensitivity and multimodal sensing capabilities. Furthermore, it may benefit the future development of smart sensing systems integrated with artificial intelligence for more precise object and texture recognition.

Sensory Organ Investment Varies with Body Size and Sex in the Butterfly Pieris napi

In solitary insect pollinators such as butterflies, sensory systems must be adapted for multiple tasks, including nectar foraging, mate-finding, and locating host-plants. As a result, the energetic investments between sensory organs can vary at the intraspecific level and even among sexes. To date, little is known about how these investments are distributed between sensory systems and how it varies among individuals of different sex. We performed a comprehensive allometric study on males and females of the butterfly Pieris napi where we measured the sizes and other parameters of sensory traits including eyes, antennae, proboscis, and wings. Our findings show that among all the sensory traits measured, only antenna and wing size have an allometric relationship with body size and that the energetic investment in different sensory systems varies between males and females. Moreover, males had absolutely larger antennae and eyes, indicating that they invest more energy in these organs than females of the same body size. Overall, the findings of this study reveal that the size of sensory traits in P. napi are not necessarily related to body size and raises questions about other factors that drive sensory trait investment in this species and in other insect pollinators in general.

Peripheral taste detection in honey bees: What do taste receptors respond to?

Understanding the neural principles governing taste perception in species that bear economic importance or serve as research models for other sensory modalities constitutes a strategic goal. Such is the case of the honey bee (Apis mellifera), which is environmentally and socioeconomically important, given its crucial role as pollinator agent in agricultural landscapes and which has served as a traditional model for visual and olfactory neurosciences and for research on communication, navigation, and learning and memory. Here we review the current knowledge on honey bee gustatory receptors to provide an integrative view of peripheral taste detection in this insect, highlighting specificities and commonalities with other insect species. We describe behavioral and electrophysiological responses to several tastant categories and relate these responses, whenever possible, to known molecular receptor mechanisms. Overall, we adopted an evolutionary and comparative perspective to understand the neural principles of honey bee taste and define key questions that should be answered in future gustatory research centered on this insect.

Comparative morphology of antennal surface structures in pleurostict scarab beetles (Coleoptera)

The diverse pleurostict (phytophagous) scarab beetles with characteristically clubbed antennae exhibit striking morphological variation and a variety of different antennal sensilla. Here we compare the morphology of the antennal surface between major pleurostict lineages, including Cetoniinae, Dynastinae, Melolonthinae, Rutelinae, and a few outgroups, including Scarabaeinae and Hybosoridae. We identified various types of antennal sensilla morphologically and searched for phylogenetic patterns of sensilla within the Scarabaeidae. Sensilla were examined using SEM micrographs of 36 species and the occurrence of the different types of antennal sensilla was studied for each species. We observed a high diversity of sensilla, including multiple transitional forms. There were also a number of other interesting structures on the antennal surface with adaptive value, such as elongate elevations, serial bags, and fields of setae. Our results confirm earlier findings that within pleurostict scarabs there has occurred a clear differentiation of sensilla composition and patterns.

Electroantennogram reveals a strong correlation between the passion of honeybee and the properties of the volatile

INTRODUCTION

Insects use their antennae to detect food, mates, and predators, mainly via olfactory recognition of specific volatile compounds. Honeybees also communicate, learn complex tasks, and show adaptable behavior by recognizing and responding to specific odors. However, the relationship between the electroantennogram and the passion of honeybee has not been determined.

METHODS

We established a four-channel maze system to detect the degree of sensitivity of the honeybee's antenna to different odors. In addition, electroantennography (EAG) signal was recorded from the right antennae of the honeybees in our experiments to explore electrophysiological responses to different volatiles.

RESULTS

The olfactory sensilla on the antennae of honeybees engender distinct electrophysiological responses to different volatiles. The bees were exposed to honey, 1-hexanol and formic acid, and EAG parameters like depolarization time, falling slope, and amplitude were measured. The EAG indicators varied significantly between honey and formic acid, indicating either "happy" or "anxious" moods.

CONCLUSIONS

Honeybee can express its passion by the characteristic changes of EAG parameters. We defined a preference factor (F) to quantify the preference of bees to varying concentrations of different compounds, where greater positive values indicate an increased passion. Our findings provide novel insights into the understanding of odor recognition in insects.

Functional Consequences of Keratan Sulfate Sulfation in Electrosensory Tissues and in Neuronal Regulation

Keratan sulfate (KS) is a functional electrosensory and neuro-instructive molecule. Recent studies have identified novel low sulfation KS in auditory and sensory tissues such as the tectorial membrane of the organ of Corti and the Ampullae of Lorenzini in elasmobranch fish. These are extremely sensitive proton gradient detection systems that send signals to neural interfaces to facilitate audition and electrolocation. High and low sulfation KS have differential functional roles in song learning in the immature male zebra song-finch with high charge density KS in song nuclei promoting brain development and cognitive learning. The conductive properties of KS are relevant to the excitable neural phenotype. High sulfation KS interacts with a large number of guidance and neuroregulatory proteins. The KS proteoglycan microtubule associated protein-1B (MAP1B) stabilizes actin and tubulin cytoskeletal development during neuritogenesis. A second 12 span transmembrane synaptic vesicle associated KS proteoglycan (SV2) provides a smart gel storage matrix for the storage of neurotransmitters. MAP1B and SV2 have prominent roles to play in neuroregulation. Aggrecan and phosphacan have roles in perineuronal net formation and in neuroregulation. A greater understanding of the biology of KS may be insightful as to how neural repair might be improved.

Proprioceptive input to a descending pathway conveying antennal postural information: Terminal organisation of antennal hair field afferents

Like several other arthropod species, stick insects use their antennae for tactile exploration of the near-range environment and for spatial localisation of touched objects. More specifically, Carausius morosus continuously moves its antennae during locomotion and reliably responds to antennal contact events with directed movements of a front leg. Here we investigate the afferent projection patterns of antennal hair fields (aHF), proprioceptors known to encode antennal posture and movement, and to be involved in antennal movement control. We show that afferents of all seven aHF of C. morosus have terminal arborisations in the dorsal lobe (DL) of the cerebral (=supraoesophageal) ganglion, and descending collaterals that terminate in a characteristic part of the gnathal (=suboesophageal) ganglion. Despite differences of functional roles among aHF, terminal arborisation patterns show no topological arrangement according to segment specificity or direction of movement. In the DL, antennal motoneuron neurites show arborizations in proximity to aHF afferent terminals. Despite the morphological similarity of single mechanoreceptors of aHF and adjacent tactile hairs on the pedicel and flagellum, we find a clear separation of proprioceptive and exteroceptive mechanosensory neuropils in the cerebral ganglion. Moreover, we also find this functional separation in the gnathal ganglion.

Separation of different pollen types by chemotactile sensing in Bombus terrestris

When tasting food, animals rely on chemical and tactile cues, which determine the animal’s decision on whether or not to eat food. As food nutritional composition has enormous consequences for the survival of animals, food items should generally be tasted before they are eaten or collected for later consumption. Even though recent studies confirmed the importance of e.g. gustatory cues, compared to olfaction only little is known about the representation of chemotactile stimuli at the receptor level (let alone higher brain centers) in animals other than vertebrates. To better understand how invertebrates may process chemotactile cues, we used bumblebees as a model species and combined electroantennographical (EAG) recordings with a novel technique for chemotactile antennal stimulation in bees. The recorded EAG responses to chemotactile stimulation clearly separated volatile compounds by both compound identity and concentration, and could be successfully applied to test the receptor activity evoked by different types of pollen. We found that two different pollen types (apple and almond) (which were readily distinguished by bumblebees in a classical conditioning task) evoked significantly distinct neural activity already at the antennal receptor level. Our novel stimulation technique therefore enables investigation of chemotactile sensing which is highly important for assessing food nutritional quality while foraging. It can further be applied to test other chemosensory behaviors, such as mate or nest mate recognition, or to investigate whether toxic substances, e.g. in pollen, affect neuronal separation of different food types.

The final moments of landing in bumblebees, Bombus terrestris

In comparison to other insects, like honeybees, bumblebees are very effective pollinators. Even though landing is a crucial part of pollination, little is known about how bumblebees orchestrate the final, critical moments of landing. Here, we use high-speed recordings to capture the fine details of the landing behaviour of free-flying bumblebees (Bombus terrestris), while landing on a flat platform with different orientations. We find that the bees have a fairly constant body and head orientation at the moment of leg extension, irrespective of platform tilt. At the same moment in time, the distance to the platform is held constant at around 8 mm (with the exception of low platform tilts). The orientation of the antennae and the first appendage that touches the platform vary between platform orientations, while the duration of the hover phase does not. Overall, the final moments of landing in bumblebees and their close relatives, the honeybees, are similar. However, the distance to the platform at the moment of leg extension and the duration of the hover phase are different in bumblebees and honeybees, suggesting that they are primarily adapted to land on surfaces with different orientations.

The neural mechanisms of antennal positioning in flying moths

In diverse insects, the forward positioning of the antenna is often among the first behavioral indicators of the onset of flight. This behavior may be important for the proper acquisition of the mechanosensory and olfactory inputs by the antennae during flight. Here, we describe the neural mechanisms of antennal positioning in hawk moths from behavioral, neuroanatomical and neurophysiological perspectives. The behavioral experiments indicated that a set of sensory bristles called Böhm's bristles (or hair plates) mediate antennal positioning during flight. When these sensory structures were ablated from the basal segments of their antenna, moths were unable to bring their antennae in flight position causing frequent collisions with the flapping wing. Fluorescent dye-fills of the underlying sensory and motor neurons revealed that the axonal arbors of the mechanosensory bristle neurons spatially overlapped with the dendritic arbors of the antennal motor neurons. Moreover, the latency between the activation of antennal muscles following stimulation of sensory bristles was also very short (< 10 ms), indicating that the sensory-motor connections may be direct. Together, these data show that Böhm's bristles control antennal positioning in moths via a reflex mechanism. Because the sensory structures and motor organization is conserved across most Neoptera, the mechanisms underlying antennal positioning, as described here, is likely to be conserved in these diverse insects.

Effect of fipronil on side-specific antennal tactile learning in the honeybee

In the honeybee, the conditioning of the proboscis extension response using tactile antennal stimulations is well suited for studying the side-specificity of learning including the possible bilateral transfer of memory traces in the brain, and the role of inhibitory networks. A tactile stimulus was presented to one antenna in association with a sucrose reward to the proboscis. The other antenna was either not stimulated (A+/0 training), stimulated with a non-reinforced tactile stimulus B (A+/B- training) or stimulated with B reinforced with sucrose to the proboscis (A+/B+ training). Memory tests performed 3 and 24h after training showed in all situations that a tactile stimulus learnt on one side was only retrieved ipsilaterally, indicating no bilateral transfer of information. In all these groups, we investigated the effect of the phenylpyrazole insecticide fipronil by applying a sublethal dose (0.5 ng/bee) on the thorax 15 min before training. This treatment decreased acquisition success and the subsequent memory performances were lowered but the distribution of responses to the tactile stimuli between sides was not affected. These results underline the role of the inhibitory networks targeted by fipronil on tactile learning and memory processes.

Functional and topographic segregation of glomeruli revealed by local staining of antennal sensory neurons in the honeybee Apis mellifera

In the primary olfactory center of animals, glomeruli are the relay stations where sensory neurons expressing cognate odorant receptors converge onto interneurons. In cockroaches, moths, and honeybees, sensory afferents from sensilla on the anterodorsal surface and the posteroventral surface of the flagellum form two nerves of almost equal thicknesses. In this study, double labeling of the two nerves, or proximal/distal regions of the nerves, with fluorescent dyes was used to investigate topographic organization of sensory afferents in the honeybee. The sensory neurons of ampullaceal sensilla responsive to CO2, coelocapitular sensilla responsive to hygrosensory, and thermosensory stimuli and coeloconic sensilla of unknown function were characterized with large somata and supplied thick axons exclusively to the ventral nerve. Correspondingly, all glomeruli innervated by sensory tract (T) 4 received thick axonal processes exclusively from the ventral nerve. Almost all T1-3 glomeruli received a similar number of sensory afferents from the two nerves. In the macroglomerular complexes of the drone, termination fields of afferents from the two nerves almost completely overlapped; this differs from moths and cockroaches, which show heterogeneous terminations in the glomerular complex. In T1-3 glomeruli, sensory neurons originating from more distal flagellar segments tended to terminate within the inner regions of the cortical layer. These results suggest that some degree of somatotopic organization of sensory afferents exist in T1-3 glomeruli, and part of T4 glomeruli serve for processing of hygro- and thermosensory signals.

Does substrate coarseness matter for foraging ants? An experiment with Lasius niger (Hymenoptera; Formicidae)

We investigated whether workers of the ant species Lasius niger are able to sense and discriminate the coarseness of the substrate on which they walk. First, we studied the way in which substrate coarseness affects the ants' locomotory behaviour. Second, we investigated the spontaneous preference of ants for substrates of different coarseness. And third, we tested with a differential conditioning procedure the ants' capacity to learn to associate a given coarseness with a food reward. The locomotory behaviour of ants differed according to substrate coarseness: ants moved significantly faster and had more sinuous trajectories on a fine than on a coarse substrate. No spontaneous preference for a substrate of a given coarseness was observed and, even after 20 successive conditioning trials, there was little evidence of the effect of experience on substrate coarseness discrimination. Overall however, ants trained on fine sand made significantly more correct choice than those trained on coarse sand. We discuss these results and argue that in L. niger substrate coarseness may be more important at the collective level, by interacting with the chemical properties of the pheromone trail used in mass recruitment to food source, than at the individual level.

Antennal and locomotor responses to attractive and aversive odors in the searching cockroach

The behavioral responses to attractive and aversive odors were examined in blinded adult male cockroaches under tethered-walking conditions. A sex pheromone-like stimulant derived from adult virgin females and artificially synthesized limonene were used as attractive and aversive odor sources, respectively. When a searching animal was stimulated with the attractive female-derived odor, the horizontal deflections of both the antennae were increased, and in most cases the vertical antennal positions were shifted downward. The stimulation also significantly decreased the walking speed of the animal. These behavioral changes imply a careful search in the immediate surroundings. The aftereffect of the sex pheromone was more pronounced on locomotion than on antennal movement. On the other hand, stimulation with the aversive odor (limonene) tended to suppress active antennal movement, and also increased the walking speed. Immediately after the withdrawal of the aversive odor, the active movement of the antennae was resumed, and the walking speed rapidly decreased to a level approximately the same as that of the control period. These results indicate that the responses to the qualitatively opposite types of odor are reciprocal to each other with regard to both antennal movement and locomotion.

Central projections of sensory systems involved in honey bee dance language communication

Honey bee dance language is a unique and complex form of animal communication used to inform nest mates in the colony about the specific location of food sources or new nest sites. Five different sensory systems have been implicated in acquiring and communicating the information necessary for dance language communication. We present results from neuronal tracer studies identifying the central projections from four of the five. Sensory neurons of the dorsal rim area of the compound eyes, involved in acquiring sun-compass based information, project to the dorsal-most part of the medulla. Sensory neurons of the neck hair plates, required to transpose sun-compass based information to gravity-based information in the dark hive, project to the dorsal labial neuromere of the subesophageal ganglion. Sensory neurons from the antennal joint hair sensilla and the Johnston's organ, which perceive information on dance direction and distance from mechanostimuli generated by abdomen waggling and wing vibration, project to the deutocerebral dorsal lobe and the subesophageal ganglion, and the posterior protocerebrum, respectively. We found no 'dance-specific' projections relative to those previously described for drone and queen honey bees and other insect species that do not exhibit dance communication. We suggest that the evolution of dance language communication was likely based on the modification of central neural pathways associated with path integration, the capability to calculate distance, and directional information during flight.

Central gustatory projections and side-specificity of operant antennal muscle conditioning in the honeybee

Gustatory stimuli to the antennae, especially sucrose, are important for bees and are employed in learning paradigms as unconditioned stimulus. The present study identified primary antennal gustatory projections in the bee brain and determined the impact of stimulation of the antennal tip on antennal muscle activity and its plasticity. Central projections of antennal taste hairs contained axons of two morphologies projecting into the dorsal lobe, which is also the antennal motor centre. Putative mechanosensory axons arborised in a dorso-lateral area. Putative gustatory axons projected to a ventro-medial area. Bees scan gustatory and mechanical stimuli with their antennae using variable strategies but sensory input to the motor system has not been investigated in detail. Mechanical, gustatory, and electrical stimulation of the ipsilateral antennal tip were found to evoke short-latency responses in an antennal muscle, the fast flagellum flexor. Contralateral gustatory stimulation induced smaller responses with longer latency. The activity of the fast flagellum flexor was conditioned operantly by pairing high muscle activity with ipsilateral antennal sucrose stimulation. A proboscis reward was unnecessary for learning. With contralateral antennal sucrose stimulation, conditioning was unsuccessful. Thus, muscle activity induced by gustatory stimulation was important for learning success and conditioning was side-specific.

Active tactile sensing for localization of objects by the cockroach antenna

Antennal movement during tactile orientation behavior was examined three-dimensionally in American cockroaches during tethered walking. When a wooden rod was presented to the tip of one antenna in an upright orientation at one of the three different horizontal positions (30 degrees , 60 degrees , or 90 degrees from the center of the head), the animal touched it repeatedly with the antenna, and tried to approach it (positive thigmotaxis). Positional shifts were also observed for the contralateral unstimulated antenna. The ipsilateral antenna tended to touch the object during inward movement (adduction) at all three test angles. The cumulative turn angle made during a continuous test period of 24 s clearly depended on the object's position; however, the contact frequencies were almost the same regardless of the position. The relationships between contact frequency and some locomotion parameters were also investigated on a shorter time scale of 3 s. The contact frequency positively correlated with the turn angle, with the accuracy of orientation at all three test angles, and with the translation velocity at test angles of 30 degrees and 60 degrees . It is concluded that the performance during tactile orientation can be represented effectively by the frequency with which the antennae touch the attractive objects.