Dual, multilayered somatosensory maps formed by antennal tactile and contact chemosensory afferents in an insect brain

Form-function relationships of the compound eyes and sensory sensilla of a tiny arboreal hemipteran herbivore: Adaptations for close encounters with leaves

Herbivorous insects experience diverse plant stimuli, the relative influence of which depends upon the scale of the interface between both organisms and the insect's life history. Using microCT and SEM, we conducted a whole insect study of the sensory structures of Glycaspis brimblecombei (Hemiptera: Psylloidea; commonly called psyllids or jumping plant lice) to understand this tiny insect's utilisation of the leaves of their tree hosts - especially to reconcile rapid host assessment versus protracted, sinuous searching behaviours. Each compound eye comprises 360 ommatidia of relatively uniform density and facet diameter indicating limited spatial resolution and sensitivity. The areas of highest relative sampling resolution are not directed ventrally towards the surface of leaves but laterally and dorsally. There is a high abundance of chemo- and mechanosensory sensilla on the genal cones (216-240) and fewer on the terminalia (120-150), i.e. body parts regularly in contact with leaf surfaces. There are even fewer such sensilla on the basitarsi (10-16) and only putative olfactory sensilla on the antennae. Leaf surface conformation probably guides females to veins while contact chemoreception likely stimulates probing; the number of eggs deposited is likely determined by the flow and quality of nutrients experienced during feeding. For this psyllid, vision aids movements among leaves and relocation of hosts if dislodged by wind or escaping predators. Walking, as opposed to flying, maintains continuity of exposure to plant stimuli essential to maximising reproductive success. Such a life history is possible on large, evergreen hosts and is facilitated by rapid accept/reject discrimination of ingesta.

Geometry-based navigation in the dark: layout symmetry facilitates spatial learning in the house cricket, Acheta domesticus, in the absence of visual cues

The capacity to navigate by layout geometry has been widely recognized as a robust strategy of place-finding. It has been reported in various species, although most studies were performed with vision-based paradigms. In the presented study, we aimed to investigate layout symmetry-based navigation in the house cricket, Acheta domesticus, in the absence of visual cues. For this purpose, we used a non-visual paradigm modeled on the Tennessee Williams setup. We ensured that the visual cues were indeed inaccessible to insects. In the main experiment, we tested whether crickets are capable of learning to localize the centrally positioned, inconspicuous cool spot in heated arenas of various shapes (i.e., circular, square, triangular, and asymmetric quadrilateral). We found that the symmetry of the arena significantly facilitates crickets’ learning to find the cool spot, indicated by the increased time spent on the cool spot and the decreased latency in locating it in subsequent trials. To investigate mechanisms utilized by crickets, we analyzed their approach paths to the spot. We found that crickets used both heuristic and directed strategies of approaching the target, with the dominance of a semi-directed strategy (i.e., a thigmotactic phase preceding direct navigation to the target). We propose that the poor performance of crickets in the asymmetrical quadrilateral arena may be explained by the difficulty of encoding its layout with cues from a single modality.

Molecular and functional basis of high-salt avoidance in a blood-sucking insect

Salts are essential nutrients required for many physiological processes, and accordingly, their composition and concentration are tightly regulated. Taste is the ultimate sensory modality involved in resource quality assessment, resulting in acceptance or rejection. Here we found that high salt concentrations elicit feeding avoidance in the blood-sucking bug Rhodnius prolixus and elucidate the molecular and neurophysiological mechanisms involved. We found that high-salt avoidance is mediated by a salt-sensitive antennal gustatory receptor neuron (GRN). Using RNAi, we demonstrate that this process requires two amiloride-sensitive pickpocket channels (PPKs; Rpro PPK014276 and Rpro PPK28) expressed within these cells. We found that antennal GRNs project to the insect primary olfactory center, the antennal lobes, revealing these centers as potential sites for the integration of taste and olfactory host-derived cues. Moreover, the identification of the gustatory basis of high-salt detection in a hematophagous insect suggests novel targets for the prevention of biting and feeding.

Description of Chemosensory Genes in Unexplored Tissues of the Moth Spodoptera littoralis

Illumina-based transcriptome sequencing of chemosensory organs has become a standard in deciphering the molecular bases of chemical senses in insects, especially in non-model species. A plethora of antennal transcriptomes is now available in the literature, describing large sets of chemosensory receptors and binding proteins in a diversity of species. However, little is still known on other organs such as mouthparts, legs and ovipositors, which are also known to carry chemosensory sensilla. This is the case of the noctuid Spodoptera littoralis, which has been established as a model insect species in molecular chemical ecology thanks to the description of many—but not all—chemosensory genes. To fulfill this gap, we present here an unprecedented transcriptomic survey of chemosensory tissues in this species. RNAseq from male and female proboscis, labial palps, legs and female ovipositors allowed us to annotate 115 putative chemosensory gene transcripts, including 30 novel genes in this species. Especially, we doubled the number of candidate gustatory receptor transcripts described in this species. We also evidenced ectopic expression of many chemosensory genes. Remarkably, one third of the odorant receptors were found to be expressed in the proboscis. With a total of 196 non-overlapping chemosensory genes annotated, the S. littoralis repertoire is one of the most complete in Lepidoptera. We further evaluated the expression of transcripts between males and females, pinpointing sex-specific transcripts. We identified five female-specific transcripts, including one odorant receptor, one gustatory receptor, one ionotropic receptor and one odorant-binding protein, and one male-specific gustatory receptor. Such sex-biased expression suggests that these transcripts participate in sex-specific behaviors, such as host choice for oviposition in females and/or mating partner recognition in both sexes.

Structure of the pecten neuropil pathway and its innervation by bimodal peg afferents in two scorpion species

The pectines of scorpions are comb-like structures, located ventrally behind the fourth walking legs and consisting of variable numbers of teeth, or pegs, which contain thousands of bimodal peg sensillae. The associated neuropils are situated ventrally in the synganglion, extending between the second and fourth walking leg neuromeres. While the general morphology is consistent among scorpions, taxon-specific differences in pecten and neuropil structure remain elusive but are crucial for a better understanding of chemosensory processing. We analysed two scorpion species (Mesobuthus eupeus and Heterometrus petersii) regarding their pecten neuropil anatomy and the respective peg afferent innervation with anterograde and lipophilic tracing experiments, combined with immunohistochemistry and confocal laser-scanning microscopy. The pecten neuropils consisted of three subcompartments: a posterior pecten neuropil, an anterior pecten neuropil and a hitherto unknown accessory pecten neuropil. These subregions exhibited taxon-specific variations with regard to compartmentalisation and structure. Most notable were structural differences in the anterior pecten neuropils that ranged from ovoid shape and strong fragmentation in Heterometrus petersii to elongated shape with little compartmentalisation in Mesobuthus eupeus. Labelling the afferents of distinct pegs revealed a topographic organisation of the bimodal projections along a medio-lateral axis. At the same time, all subregions along the posterior-anterior axis were innervated by a single peg's afferents. The somatotopic projection pattern of bimodal sensillae appears to be common among arachnids, including scorpions. This includes the structure and organisation of the respective neuropils and the somatotopic projection patterns of chemosensory afferents. Nonetheless, the scorpion pecten pathway exhibits unique features, e.g. glomerular compartmentalisation superimposed on somatotopy, that are assumed to allow high resolution of substrate-borne chemical gradients.

A dynamical model exploring sensory integration in the insect central complex substructures

It is imperative that an animal has the ability to contextually integrate received sensory information to formulate appropriate behavioral responses. Determining a body heading based on a multitude of ego-motion cues and visual landmarks is an example of such a task that requires this context dependent integration. The work presented here simulates a sensory integrator in the insect brain called the central complex (CX). Based on the architecture of the CX, we assembled a dynamical neural simulation of two structures called the protocerebral bridge (PB) and the ellipsoid body (EB). Using non-spiking neuronal dynamics, our simulation was able to recreate in vivo neuronal behavior such as correlating body rotation direction and speed to activity bumps within the EB as well as updating the believed heading with quick secondary system updates. With this model, we performed sensitivity analysis of certain neuronal parameters as a possible means to control multi-system gains during sensory integration. We found that modulation of synapses in the memory network and EB inhibition are two possible mechanisms in which a sensory system could affect the memory stability and gain of another input, respectively. This model serves as an exploration in network design for integrating simultaneous idiothetic and allothetic cues in the task of body tracking and determining contextually dependent behavioral outputs.

Masters of communication: The brain of the banded cleaner shrimp Stenopus hispidus (Olivier, 1811) with an emphasis on sensory processing areas

The pan-tropic cleaner shrimp Stenopus hispidus (Crustacea, Stenopodidea) is famous for its specific cleaning behavior in association with client fish and an exclusively monogamous life-style. Cleaner shrimps feature a broad communicative repertoire, which is considered to depend on superb motor skills and the underlying mechanosensory circuits in combination with sensory organs. Their most prominent head appendages are the two pairs of very long biramous antennules and antennae, which are used both for attracting client fish and for intraspecific communication. Here, we studied the brain anatomy of several specimens of S. hispidus using histological sections, immunohistochemical labeling as well as X-ray microtomography in combination with 3D reconstructions. Furthermore, we investigated the morphology of antennules and antennae using fluorescence and scanning electron microscopy. Our analyses show that in addition to the complex organization of the multimodal processing centers, especially chemomechanosensory neuropils associated with the antennule and antenna are markedly pronounced when compared to the other neuropils of the central brain. We suggest that in their brains, three topographic maps are present corresponding to the sensory appendages. The brain areas which provide the neuronal substrate for these maps share distinct structural similarities to a unique extent in decapods, such as size and characteristic striated and perpendicular layering. We discuss our findings with respect to the sensory landscape within animal's habitat. In an evolutionary perspective, the cleaner shrimp's brain is an excellent example of how sensory potential and functional demands shape the architecture of primary chemomechanosensory processing areas.

A Gustatory Receptor GR8 Tunes Specifically to D-Fructose in the Common Cutworm Spodoptera litura

Gustatory receptors (GRs) are crucial in the peripheral coding of the non-volatile compounds in insects, and thus play important roles in multiple behaviors including feeding, mating, and oviposition. However, little research has been done on GRs in lepidopteran pests. In the current work with Spodoptera litura, an important worldwide crop’s pest, a candidate fructose GR gene (SlitGR8) was cloned in full length, and its spatial and temporal expression profiles were determined by quantitative real-time PCR (qPCR). It revealed that SlitGR8 was highly expressed in antennae of both male and female adults, as well as in larva of first, fifth and sixth instar. Functional analyses were further conducted using the Xenopus oocyte system. SlitGR8 responded specifically to D-fructose among 12 tested sugar compounds. In addition, the behavioral assay demonstrated that both female and male moths could respond with proboscis extension behavior to D-fructose applied onto the antenna, but females showed higher sensitivity than males. The results provide an important base for further elucidation of molecular mechanisms of gustation, and a potential target for development of feeding interfering technique in S. litura.

Transcriptome Analysis of Gene Families Involved in Chemosensory Function in Spodoptera littoralis (Lepidoptera: Noctuidae)

Background

Deciphering the molecular mechanisms mediating the chemical senses, taste, and smell has been of vital importance for understanding the nature of how insects interact with their chemical environment. Several gene families are implicated in the uptake, recognition, and termination of chemical signaling, including binding proteins, chemosensory receptors and degrading enzymes. The cotton leafworm, Spodoptera littoralis, is a phytophagous pest and current focal species for insect chemical ecology and neuroethology.

Results

We produced male and female Illumina-based transcriptomes from chemosensory and non-chemosensory tissues of S. littoralis, including the antennae, proboscis, brain and body carcass. We have annotated 306 gene transcripts from eight gene families with known chemosensory function, including 114 novel candidate genes. Odorant receptors responsive to floral compounds are expressed in the proboscis and may play a role in guiding proboscis probing behavior. In both males and females, expression of gene transcripts with known chemosensory function, including odorant receptors and pheromone-binding proteins, has been observed in brain tissue, suggesting internal, non-sensory function for these genes.

Conclusions

A well-curated set of annotated gene transcripts with putative chemosensory function is provided. This will serve as a resource for future chemosensory and transcriptomic studies in S. littoralis and closely related species. Collectively, our results expand current understanding of the expression patterns of genes with putative chemosensory function in insect sensory and non-sensory tissues. When coupled with functional data, such as the deorphanization of odorant receptors, the gene expression data can facilitate hypothesis generation, serving as a substrate for future studies.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5815-x) contains supplementary material, which is available to authorized users.

Comparative morphology of ultimate and walking legs in the centipede Lithobius forficatus (Myriapoda) with functional implications

BACKGROUND

In the context of evolutionary arthopodial transformations, centipede ultimate legs exhibit a plethora of morphological modifications and behavioral adaptations. Many species possess significantly elongated, thickened, or pincer-like ultimate legs. They are frequently sexually dimorphic, indicating a role in courtship and mating. In addition, glandular pores occur more commonly on ultimate legs than on walking legs, indicating a role in secretion, chemical communication, or predator avoidance. In this framework, this study characterizes the evolutionarily transformed ultimate legs in Lithobius forficatus in comparison with regular walking legs.

RESULTS

A comparative analysis using macro-photography, SEM, μCT, autofluorescence, backfilling, and 3D-reconstruction illustrates that ultimate legs largely resemble walking legs, but also feature a series of distinctions. Substantial differences are found with regard to aspects of the configuration of specific podomeres, musculature, abundance of epidermal glands, typology and distribution of epidermal sensilla, and architecture of associated nervous system structures.

CONCLUSION

In consideration of morphological and behavioral characteristics, ultimate legs in L. forficatus primarily serve a defensive, but also a sensory function. Moreover, morphologically coherent characteristics in the organization of the ultimate leg versus the antenna-associated neuromere point to constructional constraints in the evolution of primary processing neuropils.

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.

Central Projections of Antennal and Labial Palp Sensory Neurons in the Migratory Armyworm Mythimna separata

The oriental armyworm, Mythimna separata (Walker), is a polyphagous, migratory pest relying on olfactory cues to find mates, locate nectar, and guide long-distance flight behavior. In the present study, a combination of neuroanatomical techniques were utilized on this species, including backfills, confocal microscopy, and three-dimensional reconstructions, to trace the central projections of sensory neurons from the antenna and the labial pit organ, respectively. As previously shown, the axons of the labial sensory neurons project via the ipsilateral labial nerve and terminate in three main areas of the central nervous system: (1) the labial-palp pit organ glomerulus of each antennal lobe, (2) the gnathal ganglion, and (3) the prothoracic ganglion of the ventral nerve cord. Similarly, the antennal sensory axons project to multiple areas of the central nervous system. The ipsilateral antennal nerve targets mainly the antennal lobe, the antennal mechanosensory and motor center, and the prothoracic and mesothoracic ganglia. Specific staining experiments including dye application to each of the three antennal segments indicate that the antennal lobe receives input from flagellar olfactory neurons exclusively, while the antennal mechanosensory and motor center is innervated by mechanosensory neurons from the whole antenna, comprising the flagellum, pedicle, and scape. The terminals in the mechanosensory and motor center are organized in segregated zones relating to the origin of neurons. The flagellar mechanosensory axons target anterior zones, while the pedicular and scapal axons terminate in posterior zones. In the ventral nerve cord, the processes from the antennal sensory neurons terminate in the motor area of the thoracic ganglia, suggesting a close connection with motor neurons. Taken together, the numerous neuropils innervated by axons both from the antenna and labial palp indicate the multiple roles these sensory organs serve in insect behavior.

Scorpions pectines - Idiosyncratic chemo- and mechanosensory organs

Scorpions possess specialised chemosensory appendages, the pectines. These comb-shaped limbs are located ventrally behind the walking legs. Like the antennae of mandibulate arthropods, they also serve a mechanosensory function. However, more than 90% of the sometimes well above 100,000 sensory neurons projecting from a pectine to the central nervous system are chemosensory. There are two primary projection neuropils. The posterior one, immediately adjacent to the pectine nerve entrance, has an intriguing substructure reminiscent of the olfactory glomeruli observed in the primary chemosensory neuropils of many arthropods and indeed of most bilaterian animals. There are further similarities, particularly to the antennal lobes of mandibulate arthropods, including dense innervation by a relatively small number of putative serotonergic interneurons and the presence of GABA immunoreactivity, indicative of inhibitory interactions. Scorpion idiosyncrasies include the flattened shape and broad size range of the glomerulus-like neuropil compartments. Further, these compartments are often not clearly delimited and form layers in the neuropil that are arranged like onion peels. In summary, the pectine appendages of scorpions and their central nervous projections appear as promising study subjects, particularly regarding comparative examination of chemosensory representation and processing strategies. The possibility of combined, rather than discrete, representations of chemo- and mechanosensory inputs should merit further study.

Organization of the antennal lobes in the praying mantis (Tenodera aridifolia)

Olfaction in insects plays pivotal roles in searching for food and/or for sexual partners. Although many studies have focused on the olfactory processes of nonpredatory insect species, little is known about those in predatory insects. Here, we investigated the anatomical features of the primary olfactory center (antennal lobes) in an insect predator whose visual system is well developed, the praying mantis Tenodera aridifolia. Both sexes of T. aridifolia were found to possess 54 glomeruli, and each glomerulus was identified based on its location and size. Moreover, we found a sexual dimorphism in three glomeruli (macroglomeruli) located at the entrance of the antennal nerves, which are 15 times bigger in males than their homologs in females. We additionally deduced the target glomeruli of olfactory sensory neurons housed in cognate types of sensilla by degenerating the sensory afferents. The macroglomeruli received sensory inputs from grooved peg sensilla, which are present in a large number at the proximal part of the males' antennae. Furthermore, our findings suggest that glomeruli at the posteriodorsal part of the antennal lobes receive sensory information from putative hygro- and thermosensitive sensilla. The origins of projections connected to the protocerebrum are also discussed. J. Comp. Neurol. 525:1685-1706, 2017. © 2016 Wiley Periodicals, Inc.

Morphological and Transcriptomic Analysis of a Beetle Chemosensory System Reveals a Gnathal Olfactory Center

BACKGROUND

The red flour beetle Tribolium castaneum is an emerging insect model organism representing the largest insect order, Coleoptera, which encompasses several serious agricultural and forest pests. Despite the ecological and economic importance of beetles, most insect olfaction studies have so far focused on dipteran, lepidopteran, or hymenopteran systems.

RESULTS

Here, we present the first detailed morphological description of a coleopteran olfactory pathway in combination with genome-wide expression analysis of the relevant gene families involved in chemoreception. Our study revealed that besides the antennae, also the mouthparts are highly involved in olfaction and that their respective contribution is processed separately. In this beetle, olfactory sensory neurons from the mouthparts project to the lobus glomerulatus, a structure so far only characterized in hemimetabolous insects, as well as to a so far non-described unpaired glomerularly organized olfactory neuropil in the gnathal ganglion, which we term the gnathal olfactory center. The high number of functional odorant receptor genes expressed in the mouthparts also supports the importance of the maxillary and labial palps in olfaction of this beetle. Moreover, gustatory perception seems equally distributed between antenna and mouthparts, since the number of expressed gustatory receptors is similar for both organs.

CONCLUSIONS

Our analysis of the T. castaneum chemosensory system confirms that olfactory and gustatory perception are not organotopically separated to the antennae and mouthparts, respectively. The identification of additional olfactory processing centers, the lobus glomerulatus and the gnathal olfactory center, is in contrast to the current picture that in holometabolous insects all olfactory inputs allegedly converge in the antennal lobe. These findings indicate that Holometabola have evolved a wider variety of solutions to chemoreception than previously assumed.

Central Projection of Antennal Sensory Neurons in the Central Nervous System of the Mirid Bug Apolygus lucorum (Meyer-Dür)

The mirid bug Apolygus lucorum (Meyer-Dür), a polyphagous pest, is dependent on olfactory cues to locate various host plant species and mates. In this study, we traced the projection pathway of the antennal sensory neurons and visualized their projection patterns in the central nervous system of A. lucorum through confocal microscopy and digital reconstructions. We also examined the glomerular organization of the primary olfactory center of the brain, the antennal lobe, and created a three-dimensional model of the glomeruli. We found that the axons of the sensory neurons project into the brain via the ipsilateral antennal nerve, and descend further into the gnathal ganglion, prothoracic ganglion, mesothoracic ganglion, and metathoracic ganglion, and reach as far as to the abdominal ganglion. Such a projection pattern indicates that antennal sensory neurons of A. lucorum may be potentially directly connected to motor neurons. The antennal lobe, however, is the major target area of antennal sensory neurons. The antennal lobe is composed of a large number of glomeruli, i.e. 70–80 glomeruli in one AL of A. lucorum. The results of this study which provide information about the basic anatomical arrangement of the brain olfactory center of A. lucorum, are important for further investigations of chemosensory encoding mechanisms of the mirid bug.

Drosophila Bitter Taste(s)

Most animals possess taste receptors neurons detecting potentially noxious compounds. In humans, the ligands which activate these neurons define a sensory space called “bitter”. By extension, this term has been used in animals and insects to define molecules which induce aversive responses. In this review, based on our observations carried out in Drosophila, we examine how bitter compounds are detected and if bitter-sensitive neurons respond only to molecules bitter to humans. Like most animals, flies detect bitter chemicals through a specific population of taste neurons, distinct from those responding to sugars or to other modalities. Activating bitter-sensitive taste neurons induces aversive reactions and inhibits feeding. Bitter molecules also contribute to the suppression of sugar-neuron responses and can lead to a complete inhibition of the responses to sugar at the periphery. Since some bitter molecules activate bitter-sensitive neurons and some inhibit sugar detection, bitter molecules are represented by two sensory spaces which are only partially congruent. In addition to molecules which impact feeding, we recently discovered that the activation of bitter-sensitive neurons also induces grooming. Bitter-sensitive neurons of the wings and of the legs can sense chemicals from the gram negative bacteria, Escherichia coli, thus adding another biological function to these receptors. Bitter-sensitive neurons of the proboscis also respond to the inhibitory pheromone, 7-tricosene. Activating these neurons by bitter molecules in the context of sexual encounter inhibits courting and sexual reproduction, while activating these neurons with 7-tricosene in a feeding context will inhibit feeding. The picture that emerges from these observations is that the taste system is composed of detectors which monitor different “categories” of ligands, which facilitate or inhibit behaviors depending on the context (feeding, sexual reproduction, hygienic behavior), thus considerably extending the initial definition of “bitter” tasting.

A Computational Model of a Descending Mechanosensory Pathway Involved in Active Tactile Sensing

Many animals, including humans, rely on active tactile sensing to explore the environment and negotiate obstacles, especially in the dark. Here, we model a descending neural pathway that mediates short-latency proprioceptive information from a tactile sensor on the head to thoracic neural networks. We studied the nocturnal stick insect Carausius morosus, a model organism for the study of adaptive locomotion, including tactually mediated reaching movements. Like mammals, insects need to move their tactile sensors for probing the environment. Cues about sensor position and motion are therefore crucial for the spatial localization of tactile contacts and the coordination of fast, adaptive motor responses. Our model explains how proprioceptive information about motion and position of the antennae, the main tactile sensors in insects, can be encoded by a single type of mechanosensory afferents. Moreover, it explains how this information is integrated and mediated to thoracic neural networks by a diverse population of descending interneurons (DINs). First, we quantified responses of a DIN population to changes in antennal position, motion and direction of movement. Using principal component (PC) analysis, we find that only two PCs account for a large fraction of the variance in the DIN response properties. We call the two-dimensional space spanned by these PCs ‘coding-space’ because it captures essential features of the entire DIN population. Second, we model the mechanoreceptive input elements of this descending pathway, a population of proprioceptive mechanosensory hairs monitoring deflection of the antennal joints. Finally, we propose a computational framework that can model the response properties of all important DIN types, using the hair field model as its only input. This DIN model is validated by comparison of tuning characteristics, and by mapping the modelled neurons into the two-dimensional coding-space of the real DIN population. This reveals the versatility of the framework for modelling a complete descending neural pathway.

Many nocturnal and burrowing animals rely on their tactile sense to explore the surrounding space, and tactile cues are often used to adapt locomotion to a structurally complex environment. Most mammals use facial whiskers for active tactile exploration, while most insects use their antennae. Since whiskers and antennae are long, thin, cylindrical structures, they must be moved to probe the surrounding space. The nervous system therefore has to keep track of tactile sensor movement by encoding sensor position and motion in order to locate tactile contacts. Here, we model a descending neural pathway of the stick insect, which transfers information about tactile sensor movement to thoracic neural networks with short latency. We show that information about sensor position and motion can be derived from a single class of proprioceptors at the antennal joints, and present a computational model that explains the activity of four previously described groups of descending interneurons during antennal stimulation. Our model is validated against electrophysiological data on antennal mechanoreceptors and descending interneurons.

Coarse topographic organization of pheromone-sensitive afferents from different antennal surfaces in the American cockroach

In contrast to visual, auditory, taste, and mechanosensory neuropils, in which sensory afferents are topographically organized on the basis of their peripheral soma locations, axons of cognate sensory neurons from different locations of the olfactory sense organ converge onto a small spherical neuropil (glomerulus) in the first-order olfactory center. In the cockroach Periplaneta americana, sex pheromone-sensitive afferents with somata in the antero-dorsal and postero-ventral surfaces of a long whip-like antenna are biased toward the anterior and posterior regions of a macroglomerulus, respectively. In each region, afferents with somata in the more proximal antenna project to more proximal region, relative to the axonal entry points. However, precise topography of afferents in the macroglomerulus has remained unknown. Using single and multiple neuronal stainings, we showed that afferents arising from anterior, dorsal, ventral and posterior surfaces of the proximal regions of an antenna were biased progressively from the anterior to posterior region of the macroglomerulus, reflecting chiasmatic axonal re-arrangements that occur immediately before entering the antennal lobe. Morphologies of individual afferents originating from the proximal antenna matched results of mass neuronal stainings, but their three-dimensional origins in the antenna were hardly predictable on the basis of the projection patterns. Such projection biases made by neuronal populations differ from strict somatotopic projections of antennal mechanosensory neurons in the same species, suggesting a unique sensory mechanism to process information about odor location and direction on a single antenna.

Sensory processing within antenna enables rapid implementation of feedback control for high-speed running maneuvers

Animals are remarkably stable during high-speed maneuvers. As the speed of locomotion increases, neural bandwidth and processing delays can limit the ability to achieve and maintain stable control. Processing the information of sensory stimuli into a control signal within the sensor itself could enable rapid implementation of whole-body feedback control during high-speed locomotion. Here, we show that processing in antennal afferents is sufficient to act as control signal for a fast sensorimotor loop. American cockroaches Periplaneta americana use their antennae to mediate escape running by tracking vertical surfaces such as walls. A control theoretic model of wall following predicts that stable control is possible if the animal can compute wall position (P) and velocity, its derivative, (D). Previous whole-nerve recordings from the antenna during simulated turning experiments demonstrated a population response consistent with P and D encoding, and suggested that the response was synchronized with the timing of a turn executed while wall following. Here, we record extracellularly from individual mechanoreceptors distributed along the antenna and show that these receptors encode D and have distinct latencies and filtering properties. When summed, receptors transform the stimulus into a control signal that could control rapid steering maneuvers. The D encoding within the antenna in addition to the temporal filtering properties and P dependence of the population of afferents support a sensory encoding hypothesis from control theory. Our findings support the hypothesis that peripheral sensory processing can enable rapid implementation of whole-body feedback control during rapid running maneuvers.

Cockchafer larvae smell host root scents in soil

In many insect species olfaction is a key sensory modality. However, examination of the chemical ecology of insects has focussed up to now on insects living above ground. Evidence for behavioral responses to chemical cues in the soil other than CO(2) is scarce and the role played by olfaction in the process of finding host roots below ground is not yet understood. The question of whether soil-dwelling beetle larvae can smell their host plant roots has been under debate, but proof is as yet lacking that olfactory perception of volatile compounds released by damaged host plants, as is known for insects living above ground, occurs. Here we show that soil-dwelling larvae of Melolontha hippocastani are well equipped for olfactory perception and respond electrophysiologically and behaviorally to volatiles released by damaged host-plant roots. An olfactory apparatus consisting of pore plates at the antennae and about 70 glomeruli as primary olfactory processing units indicates a highly developed olfactory system. Damage induced host plant volatiles released by oak roots such as eucalyptol and anisol are detected by larval antennae down to 5 ppbv in soil air and elicit directed movement of the larvae in natural soil towards the odor source. Our results demonstrate that plant-root volatiles are likely to be perceived by the larval olfactory system and to guide soil-dwelling white grubs through the dark below ground to their host plants. Thus, to find below-ground host plants cockchafer larvae employ mechanisms that are similar to those employed by the adult beetles flying above ground, despite strikingly different physicochemical conditions in the soil.

Sensillum-specific, topographic projection patterns of olfactory receptor neurons in the antennal lobe of the cockroach Periplaneta americana

In vertebrates and many invertebrates, olfactory signals detected by peripheral olfactory receptor neurons (ORNs) are conveyed to a primary olfactory center with glomerular organization in which odor-specific activity patterns are generated. In the cockroach, Periplaneta americana, ORNs in antennal olfactory sensilla project to 205 unambiguously identifiable antennal lobe (AL) glomeruli that are classified into 10 glomerular clusters (T1-T10 glomeruli) innervated by distinct sensory tracts. In this study we employed single sensillum staining techniques and investigated the topographic projection patterns of individual ORNs to elucidate the relationship between sensillum types and glomerular organization in the AL. Axons of almost all ORNs projected to individual glomeruli. Axons of ORNs in perforated basiconic sensilla selectively innervated the anterodorsal T1-T4 glomeruli, whereas those in trichoid and grooved basiconic sensilla innervated the posteroventral T5-T9 glomeruli. About 90% of stained ORNs in trichoid sensilla sent axons to the T5 glomeruli and more than 90% of ORNs in grooved basiconic sensilla innervated the T6, T8, and T9 glomeruli. The T5 and T9 glomeruli exclusively receive sensory inputs from the trichoid and grooved basiconic sensilla, respectively. All investigated glomeruli received convergent input from a single type of sensillum except F11 glomerulus in the T6 glomeruli, which was innervated from both trichoid and grooved basiconic sensilla. These results suggest that ORNs in distinct sensillum types project to glomeruli in distinct glomerular clusters. Since ORNs in distinct sensillum types are each tuned to distinct subsets of odorant molecules, the AL is functionally compartmentalized into groups of glomeruli.

Active tactile sampling by an insect in a step-climbing paradigm

Many insects actively explore their near-range environment with their antennae. Stick insects (Carausius morosus) rhythmically move their antennae during walking and respond to antennal touch by repetitive tactile sampling of the object. Despite its relevance for spatial orientation, neither the spatial sampling patterns nor the kinematics of antennation behavior in insects are understood. Here we investigate unrestrained bilateral sampling movements during climbing of steps. The main objectives are: (1) How does the antennal contact pattern relate to particular object features? (2) How are the antennal joints coordinated during bilateral tactile sampling? We conducted motion capture experiments on freely climbing insects, using steps of different height. Tactile sampling was analyzed at the level of antennal joint angles. Moreover, we analyzed contact patterns on the surfaces of both the obstacle and the antenna itself. Before the first contact, both antennae move in a broad, mostly elliptical exploratory pattern. After touching the obstacle, the pattern switches to a narrower and faster movement, caused by higher cycle frequencies and lower cycle amplitudes in all joints. Contact events were divided into wall- and edge-contacts. Wall contacts occurred mostly with the distal third of the flagellum, which is flexible, whereas edge contacts often occurred proximally, where the flagellum is stiff. The movement of both antennae was found to be coordinated, exhibiting bilateral coupling of functionally analogous joints [e.g., left head-scape (HS) joint with right scape-pedicel (SP) joint] throughout tactile sampling. In comparison, bilateral coupling between homologous joints (e.g., both HS joints) was significantly weaker. Moreover, inter-joint coupling was significantly weaker during the contact episode than before. In summary, stick insects show contact-induced changes in frequency, amplitude and inter-joint coordination during tactile sampling of climbed obstacles.

Comparative analysis of deutocerebral neuropils in Chilopoda (Myriapoda): implications for the evolution of the arthropod olfactory system and support for the Mandibulata concept

Background

Originating from a marine ancestor, the myriapods most likely invaded land independently of the hexapods. As these two evolutionary lineages conquered land in parallel but separately, we are interested in comparing the myriapod chemosensory system to that of hexapods to gain insights into possible adaptations for olfaction in air. Our study connects to a previous analysis of the brain and behavior of the chilopod (centipede) Scutigera coleoptrata in which we demonstrated that these animals do respond to volatile substances and analyzed the structure of their central olfactory pathway.

Results

Here, we examined the architecture of the deutocerebral brain areas (which process input from the antennae) in seven additional representatives of the Chilopoda, covering all major subtaxa, by histology, confocal laser-scan microscopy, and 3D reconstruction. We found that in all species that we studied the majority of antennal afferents target two separate neuropils, the olfactory lobe (chemosensory, composed of glomerular neuropil compartments) and the corpus lamellosum (mechanosensory). The numbers of olfactory glomeruli in the different chilopod taxa ranged from ca. 35 up to ca. 90 and the shape of the glomeruli ranged from spheroid across ovoid or drop-shape to elongate.

Conclusion

A split of the afferents from the (first) pair of antennae into separate chemosensory and mechanosensory components is also typical for Crustacea and Hexapoda, but this set of characters is absent in Chelicerata. We suggest that this character set strongly supports the Mandibulata hypothesis (Myriapoda + (Crustacea + Hexapoda)) as opposed to the Myriochelata concept (Myriapoda + Chelicerata). The evolutionary implications of our findings, particularly the plasticity of glomerular shape, are discussed.

Anatomical organization of antennal-lobe glomeruli in males and females of the scarab beetle Holotrichia diomphalia (Coleoptera: Melolonthidae)

The glomerular organization of the primary olfactory brain center, the antennal lobe, was studied in males and females of Holotrichia diomphalia adults using serial histological sections labeled by the reduced silver-stain technique. The results revealed an apparent sexual dimorphism. Whereas an enlarged cap-shaped glomerulus was found at the antennal nerve entrance into the antennal lobe in males, no such unit was present in females. Also the size of the antennal lobe differed between the sexes, the antennal lobe of males being larger than that of females. We estimated the total number of glomeruli at approximately 60 units in the female antennal lobe. In males, we could discriminate only those glomeruli that were located in the anterior area of the antennal lobe.

A new view of insect-crustacean relationships I. Inferences from neural cladistics and comparative neuroanatomy

Traditional hypotheses regarding the relationships of the major arthropod lineages focus on suites of comparable characters, often those that address features of the exoskeleton. However, because of the enormous morphological variety among arthropods, external characters may lead to ambiguities of interpretation and definition, particularly when species have undergone evolutionary simplification and reversal. Here we present the results of a cladistic analysis using morphological characters associated with brains and central nervous systems, based on the evidence that cerebral organization is generally robust over geological time. Well-resolved, strongly supported phylogenies were obtained from a neuromorphological character set representing a variety of discrete neuroanatomical traits. Phylogenetic hypotheses from this analysis support many accepted relationships, including monophyletic Chelicerata, Myriapoda, and Hexapoda, paraphyletic Crustacea and the union of Hexapoda and Crustacea (Tetraconata). They also support Mandibulata (Myriapoda + Tetraconata). One problematic result, which can be explained by symplesiomorphies that are likely to have evolved in deep time, is the inability to resolve Onychophora as a taxon distinct from Arthropoda. Crucially, neuronal cladistics supports the heterodox conclusion that both Hexapoda and Malacostraca are derived from a common ancestor that possessed a suite of discrete neural centers comprising an elaborate brain. Remipedes and copepods, both resolved as basal to Branchiopoda share a neural ground pattern with Malacostraca. These findings distinguish Hexapoda (Insecta) from Branchiopoda, which is the sister group of the clade Malacostraca + Hexapoda. The present study resolves branchiopod crustaceans as descendents of an ancestor with a complex brain, which means that they have evolved secondary simplification and the loss or reduction of numerous neural systems.

Neural architecture of the primary gustatory center of Drosophila melanogaster visualized with GAL4 and LexA enhancer-trap systems

Gustatory information is essential for animals to select edible foods and avoid poisons. Whereas mammals detect tastants with their taste receptor cells, which convey gustatory signals to the brain indirectly via the taste sensory neurons, insect gustatory receptor neurons (GRNs) send their axons directly to the primary gustatory center in the suboesophageal ganglion (SOG). In spite of this relatively simple architecture, the precise structure of the insect primary gustatory center has not been revealed in enough detail. To obtain comprehensive anatomical knowledge about this brain area, we screened the Drosophila melanogaster GAL4 enhancer-trap strains that visualize specific subsets of the gustatory neurons as well as putative mechanosensory neurons associated with the taste pegs. Terminals of these neurons form three branches in the SOG. To map the positions of their arborization areas precisely, we screened newly established LexA::VP16 enhancer-trap strains and obtained a driver line that labels a large subset of peripheral sensory neurons. By double-labeling specific and landmark neurons with GAL4 and LexA strains, we were able to distinguish 11 zones in the primary gustatory center, among which 5 zones were identified newly in this study. Arborization areas of various known GRNs on the labellum, oesophagus, and legs were also mapped in this framework. The putative mechanosensory neurons terminate exclusively in three zones of these areas, supporting the notion of segregated primary centers that are specialized for chemosensory and mechanosensory signals associated with gustatory sensation.

Complete mapping of glomeruli based on sensory nerve branching pattern in the primary olfactory center of the cockroach Periplaneta americana

Glomeruli are structural and functional units in the primary olfactory center in vertebrates and insects. In the cockroach Periplaneta americana, axons of different types of sensory neurons housed in sensilla on antennae form dorsal and ventral antennal nerves and then project to a number of glomeruli. In this study, we identified all antennal lobe (AL) glomeruli based on detailed innervation patterns of sensory tracts in addition to the shape, size, and locations in the cockroach. The number of glomeruli is approximately 205, and no sex-specific difference is observed. Anterograde dye injections into the antennal nerves revealed that axons supplying the AL are divided into 10 sensory tracts (T1-T10). Each of T1-T3 innervates small, oval glomeruli in the anteroventral region of the AL, with sensory afferents invading each glomerulus from multiple directions, whereas each of T4-T10 innervates large glomeruli with various shapes in the posterodorsal region, with a bundle of sensory afferents invading each glomerulus from one direction. The topographic branching patterns of all these tracts are conserved among individuals. Sensory afferents in a sub-tract of T10 had axon terminals in the dorsal margin of the AL and the protocerebrum, where they form numerous small glomerular structures. Sensory nerve branching pattern should reflect developmental processes to determine spatial arrangement of glomeruli, and thus the complete map of glomeruli based on sensory nerve branching pattern should provide a basis for studying the functional significance of spatial arrangement of glomeruli and its developmental basis.

Critical roles of mecamylamine-sensitive mushroom body neurons in insect olfactory learning

In insects, cholinergic neurons are thought to transmit olfactory conditioned stimulus (CS) to the sites for associating the CS with unconditioned stimulus (US), but the types of acetylcholine (ACh) receptor used by neurons participating in the association have not been determined. In cockroaches, a type of nicotinic ACh receptor specifically antagonized by mecamylamine (MEC) has been characterized. Here we investigated the roles of neurons possessing MEC-sensitive ACh receptors (MEC-sensitive neurons) in olfactory conditioning of salivation, monitored by changes in activities of salivary neurons, in cockroaches. Local and bilateral microinjection of MEC into each of the three olfactory centers, antennal lobes, calyces of the mushroom bodies and lateral protocerebra, impaired olfactory responses of salivary neurons, indicating that MEC-sensitive neurons in all olfactory centers participate in pathways mediating olfactory responses of salivary neurons. Conditioning of olfactory CS with sucrose US was impaired by injection of MEC into the antennal lobes or calyces, i.e., conditioned responses were absent even after recovery from MEC injection, suggesting that the CS-US association occurs in MEC-sensitive neurons in calyces (most probably Kenyon cells) or in neurons in downstream pathways. In contrast, conditioned responses appeared after recovery from MEC injection into the lateral protocerebra, suggesting that MEC-sensitive neurons in the lateral protocerebra are downstream of the association sites. Since lateral protocerebra are major termination areas of mushroom body efferent neurons, we suggest that input synapses of MEC-sensitive Kenyon cells, or their output synapses upon mushroom body efferent neurons, are the sites for CS-US association for conditioning of salivation.

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.

Ground plan of the insect mushroom body: functional and evolutionary implications

In most insects with olfactory glomeruli, each side of the brain possesses a mushroom body equipped with calyces supplied by olfactory projection neurons. Kenyon cells providing dendrites to the calyces supply a pedunculus and lobes divided into subdivisions supplying outputs to other brain areas. It is with reference to these components that most functional studies are interpreted. However, mushroom body structures are diverse, adapted to different ecologies, and likely to serve various functions. In insects whose derived life styles preclude the detection of airborne odorants, there is a loss of the antennal lobes and attenuation or loss of the calyces. Such taxa retain mushroom body lobes that are as elaborate as those of mushroom bodies equipped with calyces. Antennal lobe loss and calycal regression also typify taxa with short nonfeeding adults, in which olfaction is redundant. Examples are cicadas and mayflies, the latter representing the most basal lineage of winged insects. Mushroom bodies of another basal taxon, the Odonata, possess a remnant calyx that may reflect the visual ecology of this group. That mushroom bodies persist in brains of secondarily anosmic insects suggests that they play roles in higher functions other than olfaction. Mushroom bodies are not ubiquitous: the most basal living insects, the wingless Archaeognatha, possess glomerular antennal lobes but lack mushroom bodies, suggesting that the ability to process airborne odorants preceded the acquisition of mushroom bodies. Archaeognathan brains are like those of higher malacostracans, which lack mushroom bodies but have elaborate olfactory centers laterally in the brain.

Tritocerebral tract input to the insect mushroom bodies

Insect mushroom bodies, best known for their role in olfactory processing, also receive sensory input from other modalities. In crickets and grasshoppers, a tritocerebral tract containing afferents from palp mechanosensory and gustatory centers innervates the accessory calyx. The accessory calyx is uniquely composed of Class III Kenyon cells, and was shown by immunohistochemistry to be present sporadically across several insect orders. Neuronal tracers applied to the source of tritocerebral tract axons in several species of insects demonstrated that tritocerebral tract innervation of the mushroom bodies targeted the accessory calyx when present, the primary calyces when an accessory calyx was not present, or both. These results suggest that tritocerebral tract input to the mushroom bodies is likely ubiquitous, reflecting the importance of gustation for insect behavior. The scattered phylogenetic distribution of Class III Kenyon cells is also proposed to represent an example of generative homology, in which the developmental program for forming a structure is retained in all members of a lineage, but the program is not "run" in all branches.

Hedonic taste in Drosophila revealed by olfactory receptors expressed in taste neurons

Taste and olfaction are each tuned to a unique set of chemicals in the outside world, and their corresponding sensory spaces are mapped in different areas in the brain. This dichotomy matches categories of receptors detecting molecules either in the gaseous or in the liquid phase in terrestrial animals. However, in Drosophila olfactory and gustatory neurons express receptors which belong to the same family of 7-transmembrane domain proteins. Striking overlaps exist in their sequence structure and in their expression pattern, suggesting that there might be some functional commonalities between them. In this work, we tested the assumption that Drosophila olfactory receptor proteins are compatible with taste neurons by ectopically expressing an olfactory receptor (OR22a and OR83b) for which ligands are known. Using electrophysiological recordings, we show that the transformed taste neurons are excited by odor ligands as by their cognate tastants. The wiring of these neurons to the brain seems unchanged and no additional connections to the antennal lobe were detected. The odor ligands detected by the olfactory receptor acquire a new hedonic value, inducing appetitive or aversive behaviors depending on the categories of taste neurons in which they are expressed i.e. sugar- or bitter-sensing cells expressing either Gr5a or Gr66a receptors. Taste neurons expressing ectopic olfactory receptors can sense odors at close range either in the aerial phase or by contact, in a lipophilic phase. The responses of the transformed taste neurons to the odorant are similar to those obtained with tastants. The hedonic value attributed to tastants is directly linked to the taste neurons in which their receptors are expressed.

The olfactory pathway mediates sheltering behavior of Caribbean spiny lobsters, Panulirus argus, to conspecific urine signals

The "noses" of diverse taxa are organized into different subsystems whose functions are often not well understood. The "nose" of decapod crustaceans is organized into two parallel pathways that originate in different populations of antennular sensilla and project to specific neuropils in the brain-the aesthetasc/olfactory lobe pathway and the non-aesthetasc/lateral antennular neuropil pathway. In this study, we investigated the role of these pathways in mediating shelter selection of Caribbean spiny lobsters, Panulirus argus, in response to conspecific urine signals. We compared the behavior of ablated animals and intact controls. Our results show that control and non-aesthetasc ablated lobsters have a significant overall preference for shelters emanating urine over control shelters. Thus the non-aesthetasc pathway does not play a critical role in shelter selection. In contrast, spiny lobsters with aesthetascs ablated did not show a preference for either shelter, suggesting that the aesthetasc/olfactory pathway is important for processing social odors. Our results show a difference in the function of these dual chemosensory pathways in responding to social cues, with the aesthetasc/olfactory lobe pathway playing a major role. We discuss our results in the context of why the noses of many animals contain multiple parallel chemosensory systems.

The antennal lobe of the African malaria mosquito, Anopheles gambiae - innervation and three-dimensional reconstruction

Antibody labelling and subsequent three-dimensional reconstructions of the primary olfactory centres, the antennal lobes, of male and female African malaria mosquitoes, Anopheles gambiae, revealed 61 and 60 glomerular neuropils respectively. In addition to the small difference in number of glomeruli, sexual dimorphism was observed in both the size of the antennal lobe and of individual glomeruli. Furthermore, sexual specificity was observed within the array. Anterograde staining of afferents from peripheral olfactory organs support the reconstruction of the glomerular array. Although anterograde stainings support an organotopic organization of the antennal lobe, convergence of afferents originating from different organs into single glomeruli is observed. This finding, in both A. gambiae and A. aegypti, may shed new light upon the development and function of the olfactory system.

Termination profiles of insect chemosensory afferents in the antennal lobe are dependent on their origin on the flagellum

In cockroach antennae, sensory afferents from sensilla on the anterodorsal surface of the flagellum form the anterior antennal nerve, while afferents from the posteroventral surface form the posterior nerve. Anterograde staining was used to investigate afferent termination profiles in the glomeruli of the antennal lobe. The densities of terminal arborizations from the two nerves differed between glomeruli, with groupings of similar glomeruli evident. Individual glomeruli showed heterogeneous distribution of afferent terminals, with posterior nerve afferent terminals occurring near the nerve/glomeruli interface, and anterior nerve afferent terminals occurring on the opposite side. This study demonstrates, for the first time, a correlation between the distribution of primary afferent terminals in the individual glomeruli, and their origin on the surface of the flagellum.