Neuropeptide complexity in the crustacean central olfactory pathway: immunolocalization of A-type allatostatins and RFamide-like peptides in the brain of a terrestrial hermit crab

Immunocytochemical Localization of Enzymes Involved in Dopamine, Serotonin, and Acetylcholine Synthesis in the Optic Neuropils and Neuroendocrine System of Eyestalks of Paralithodes camtschaticus

Identifying the neurotransmitters secreted by specific neurons in crustacean eyestalks is crucial to understanding their physiological roles. Here, we combined immunocytochemistry with confocal microscopy and identified the neurotransmitters dopamine (DA), serotonin (5-HT), and acetylcholine (ACh) in the optic neuropils and X-organ sinus gland (XO-SG) complex of the eyestalks of Paralithodes camtschaticus (red king crab). The distribution of Ach neurons was studied by choline acetyltransferase (ChAT) immunohistochemistry and compared with that of DA neurons examined in the same or adjacent sections by tyrosine hydroxylase (TH) immunohistochemistry. We detected 5-HT, TH, and ChAT in columnar, amacrine, and tangential neurons in the optic neuropils and established the presence of immunoreactive fibers and neurons in the terminal medulla in the XO region of the lateral protocerebrum. Additionally, we detected ChAT and 5-HT in the endogenous cells of the SG of P. camtschaticus for the first time. Furthermore, localization of 5-HT- and ChAT-positive cells in the SG indicated that these neurotransmitters locally modulate the secretion of neurohormones that are synthesized in the XO. These findings establish the presence of several neurotransmitters in the XO-SG complex of P. camtschaticus.

Anatomy of the Nervous System in Chelifer cancroides (Arachnida: Pseudoscorpiones) with a Distinct Sensory Pathway Associated with the Pedipalps

Simple Summary

Most arthropods (uniting animals such as the chelicerates, e.g., spiders and their kin, as well as millipedes, centipedes, crustaceans, and insects) have distinct sensory appendages at the second head segment, the so-called antennae. The Arachnida (e.g., spiders and scorpions) do not possess antennae, but have evolved highly specialized sensory organs on different body regions. However, very limited information is available concerning pseudoscorpions (false scorpions). These animals do not seem to possess such specialized structures, but show dominant, multifunctional appendages prior to the first walking leg, called pedipalps. Here, we investigate the neuronal pathway of these structures as well as general aspects of the nervous system. We describe new details of typical arthropod brain compartments, such as the arcuate body and a comparatively small mushroom body. Neurons associated with the pedipalps terminate in two regions in the central nervous system of characteristic arrangement: a glomerular and a layered center, which we interpret as a chemo- and a mechanosensory center, respectively. The centers, which fulfill the same function in other animals, show a similar arrangement. These similarities in the sensory systems of different evolutionary origin have to be interpreted as functional prerequisites. Identifying these similarities helps to understand the general functionality of sensory systems, not only within arthropods.

Abstract

Many arachnid taxa have evolved unique, highly specialized sensory structures such as antenniform legs in Amblypygi (whip spiders), for instance, or mesosomal pectines in scorpions. Knowledge of the neuroanatomy as well as functional aspects of these sensory organs is rather scarce, especially in comparison to other arthropod clades. In pseudoscorpions, no special sensory structures have been discovered so far. Nevertheless, these animals possess dominant, multifunctional pedipalps, which are good candidates for being the primary sensory appendages. However, only little is known about the anatomy of the nervous system and the projection pattern of pedipalpal afferents in this taxon. By using immunofluorescent labeling of neuronal structures as well as lipophilic dye labeling of pedipalpal pathways, we identified the arcuate body, as well as a comparatively small mushroom body, the latter showing some similarities to that of Solifugae (sun spiders and camel spiders). Furthermore, afferents from the pedipalps terminate in a glomerular and a layered neuropil. Due to the innervation pattern and structural appearance, we conclude that these neuropils are the first integration centers of the chemosensory and mechanosensory afferents. Within Arthropoda, but also other invertebrates or even vertebrates, sensory structures show rather similar neuronal arrangement. Thus, these similarities in the sensory systems of different evolutionary origin have to be interpreted as functional prerequisites of the respective modality.

Local olfactory interneurons provide the basis for neurochemical regionalization of olfactory glomeruli in crustaceans

The primary olfactory centers of metazoans as diverse as arthropods and mammals consist of an array of fields of dense synaptic neuropil, the olfactory glomeruli. However, the neurochemical structure of crustacean olfactory glomeruli is largely understudied when compared to the insects. We analyzed the glomerular architecture in selected species of hermit crabs using immunohistochemistry against presynaptic proteins, the neuropeptides orcokinin, RFamide and allatostatin, and the biogenic amine serotonin. Our study reveals an unexpected level of structural complexity, unmatched by what is found in the insect olfactory glomeruli. Peptidergic and aminergic interneurons provide the structural basis for a regionalization of the crustacean glomeruli into longitudinal and concentric compartments. Our data suggest that local olfactory interneurons take a central computational role in modulating the information transfer from olfactory sensory neurons to projection neurons within the glomeruli. Furthermore, we found yet unknown neuronal elements mediating lateral inhibitory interactions across the glomerular array that may play a central role in modulating the transfer of sensory input to the output neurons through presynaptic inhibition. Our study is another step in understanding the function of crustacean olfactory glomeruli as highly complex units of local olfactory processing.

Comparative neuroanatomical distribution and expression levels of neuropeptide F in the central nervous system of the female freshwater prawn, Macrobrachium rosenbergii, during the ovarian cycle

Neuropeptide F (NPF) plays critical roles in controlling the feeding and reproduction of prawns. In the present study, we investigated changes in the expression levels of Macrobrachium rosenbergii neuropeptide F (MrNPF), and its neuroanatomical distribution in eyestalk (ES), brain (BR), subesophageal ganglion (SEG), thoracic ganglia (TG), and abdominal ganglia (AG), during the ovarian cycle of female prawn. By qRT-PCR, the amount of MrNPF transcripts exhibited a gradual increase in the ES, BR, and combined SEG and TG from stages I and II, to reach a maximum level at stage III, and slightly declined at stage IV, respectively. The highest to lowest expression levels were detected in combined SEG and TG, BR, ES, and AG, respectively. MrNPF immunolabeling was observed in several neuronal clusters, associated fibers, and neuropils of these central nervous system (CNS) tissues. MrNPF-ir was more intense in neurons and neuropils of SEG and TG than those found in other parts of the CNS. The number of MrNPF-ir neurons and intensity of MrNPF-ir were higher in the ES, BR, SEG, and TG at the late stages than those at the early stages of the ovarian cycle, while those in AG exhibited insignificant change. Taken together, there is a correlation between changes in the neuroanatomical distribution of MrNPF and stages of the ovarian cycle, implying that MrNPF may be an important neuropeptide that integrates sensory stimuli, including photo-, chemo-, and gustatory receptions, to control feeding and reproduction, particularly ovarian development, of this female prawn, M. rosenbergii.

More than one way to smell ashore - Evolution of the olfactory pathway in terrestrial malacostracan crustaceans

Crustaceans provide a fascinating opportunity for studying adaptations to a terrestrial lifestyle because within this group, the conquest of land has occurred at least ten times convergently. The evolutionary transition from water to land demands various morphological and physiological adaptations of tissues and organs including the sensory and nervous system. In this review, we aim to compare the brain architecture between selected terrestrial and closely related marine representatives of the crustacean taxa Amphipoda, Isopoda, Brachyura, and Anomala with an emphasis on the elements of the olfactory pathway including receptor molecules. Our comparison of neuroanatomical structures between terrestrial members and their close aquatic relatives suggests that during the convergent evolution of terrestrial life-styles, the elements of the olfactory pathway were subject to different morphological transformations. In terrestrial anomalans (Coenobitidae), the elements of the primary olfactory pathway (antennules and olfactory lobes) are in general considerably enlarged whereas they are smaller in terrestrial brachyurans compared to their aquatic relatives. Studies on the repertoire of receptor molecules in Coenobitidae do not point to specific terrestrial adaptations but suggest that perireceptor events - processes in the receptor environment before the stimuli bind - may play an important role for aerial olfaction in this group. In terrestrial members of amphipods (Amphipoda: Talitridae) as well as of isopods (Isopoda: Oniscidea), however, the antennules and olfactory sensilla (aesthetascs) are largely reduced and miniaturized. Consequently, their primary olfactory processing centers are suggested to have been lost during the evolution of a life on land. Nevertheless, in terrestrial Peracarida, the (second) antennae as well as their associated tritocerebral processing structures are presumed to compensate for this loss or rather considerable reduction of the (deutocerebral) primary olfactory pathway. We conclude that after the evolutionary transition from water to land, it is not trivial for arthropods to establish aerial olfaction. If we consider insects as an ingroup of Crustacea, then the Coenobitidae and Insecta may be seen as the most successful crustacean representatives in this respect.

Localization of neurons expressing choline acetyltransferase, serotonin and/or FMRFamide in the central nervous system of the decapod shore crab Hemigrapsus sanguineus

Although it is now established that neurons in crustacea contain multiple transmitter substances, little is know about patterns of expression and co-expression or about the functional effects of such co-transmission. The present study was designed to characterize the distributions and potential colocalization of choline acetyltransferase (ChAT), serotonin (5-HT) and neuropeptide H-Phe-Met-Arg-Phe-NH2 (FMRFamide) in the central nervous system (CNS) of the Asian shore crab, Hemigrapsus sanguineus using immunohistochemical analyses in combination with laser scanning confocal microscopy. ChAT was found to be expressed by small, medium-sized, and large neurons in all regions of the brain and ventral nerve cord (VNC). For the most part, ChAT, FMRFamide, and 5-HT are expressed in different neurons, although some colocalization of ChAT- with FMRFamide- or 5-HT-LIR is observed in small and medium-sized cells, mostly neurons that immunostain only weakly. In the brain, such double immunolabeling is observed primarily in neurons of the protocerebrum and, to a particularly great extent, in local olfactory interneurons of the deutocerebrum. The clusters of neurons in the VNC that stain most intensely for ChAT, FMRFamide, and 5-HT, with colocalization in some cases, are located in the subesophageal ganglia. This colocalization appears to be related to function, since it is present in regions of the CNS characterized by multiple afferent projections and outputs to a variety of functionally related centers involved in various physiological and behavioral processes. Further elucidation of the functional significance of these neurons and of the widespread process of co-transmission in the crustaceans should provide fascinating new insights.

Functional morphology of the primary olfactory centers in the brain of the hermit crab Coenobita clypeatus (Anomala, Coenobitidae)

Terrestrial hermit crabs of the genus Coenobita display strong behavioral responses to volatile odors and are attracted by chemical cues of various potential food sources. Several aspects of their sense of aerial olfaction have been explored in recent years including behavioral aspects and structure of their peripheral and central olfactory pathway. Here, we use classical histological methods and immunohistochemistry against the neuropeptides orcokinin and allatostatin as well as synaptic proteins and serotonin to provide insights into the functional organization of their primary olfactory centers in the brain, the paired olfactory lobes. Our results show that orcokinin is present in the axons of olfactory sensory neurons, which target the olfactory lobe. Orcokinin is also present in a population of local olfactory interneurons, which may relay lateral inhibition across the array of olfactory glomeruli within the lobes. Extensive lateral connections of the glomeruli were also visualized using the histological silver impregnation method according to Holmes-Blest. This technique also revealed the structural organization of the output pathway of the olfactory system, the olfactory projection neurons, the axons of which target the lateral protocerebrum. Within the lobes, the course of their axons seems to be reorganized in an axon-sorting zone before they exit the system. Together with previous results, we combine our findings into a model on the functional organization of the olfactory system in these animals.

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.

Neuroanatomy of a hydrothermal vent shrimp provides insights into the evolution of crustacean integrative brain centers

Alvinocaridid shrimps are emblematic representatives of the deep hydrothermal vent fauna at the Mid-Atlantic Ridge. They are adapted to a mostly aphotic habitat with extreme physicochemical conditions in the vicinity of the hydrothermal fluid emissions. Here, we investigated the brain architecture of the vent shrimp Rimicaris exoculata to understand possible adaptations of its nervous system to the hydrothermal sensory landscape. Its brain is modified from the crustacean brain ground pattern by featuring relatively small visual and olfactory neuropils that contrast with well-developed higher integrative centers, the hemiellipsoid bodies. We propose that these structures in vent shrimps may fulfill functions in addition to higher order sensory processing and suggest a role in place memory. Our study promotes vent shrimps as fascinating models to gain insights into sensory adaptations to peculiar environmental conditions, and the evolutionary transformation of specific brain areas in Crustacea.

The "amphi"-brains of amphipods: new insights from the neuroanatomy of Parhyale hawaiensis (Dana, 1853)

BACKGROUND

Over the last years, the amphipod crustacean Parhyale hawaiensis has developed into an attractive marine animal model for evolutionary developmental studies that offers several advantages over existing experimental organisms. It is easy to rear in laboratory conditions with embryos available year-round and amenable to numerous kinds of embryological and functional genetic manipulations. However, beyond these developmental and genetic analyses, research on the architecture of its nervous system is fragmentary. In order to provide a first neuroanatomical atlas of the brain, we investigated P. hawaiensis using immunohistochemical labelings combined with laser-scanning microscopy, X-ray microcomputed tomography, histological sectioning and 3D reconstructions.

RESULTS

As in most amphipod crustaceans, the brain is dorsally bent out of the body axis with downward oriented lateral hemispheres of the protocerebrum. It comprises almost all prominent neuropils that are part of the suggested ground pattern of malacostracan crustaceans (except the lobula plate and projection neuron tract neuropil). Beyond a general uniformity of these neuropils, the brain of P. hawaiensis is characterized by an elaborated central complex and a modified lamina (first order visual neuropil), which displays a chambered appearance. In the light of a recent analysis on photoreceptor projections in P. hawaiensis, the observed architecture of the lamina corresponds to specialized photoreceptor terminals. Furthermore, in contrast to previous descriptions of amphipod brains, we suggest the presence of a poorly differentiated hemiellipsoid body and an inner chiasm and critically discuss these aspects.

CONCLUSIONS

Despite a general uniformity of amphipod brains, there is also a certain degree of variability in architecture and size of different neuropils, reflecting various ecologies and life styles of different species. In contrast to other amphipods, the brain of P. hawaiensis does not display any striking modifications or bias towards processing one particular sensory modality. Thus, we conclude that this brain represents a common type of an amphipod brain. Considering various established protocols for analyzing and manipulating P. hawaiensis, this organism is a suitable model to gain deeper understanding of brain anatomy e.g. by using connectome approaches, and this study can serve as first solid basis for following studies.

Crustacean olfactory systems: A comparative review and a crustacean perspective on olfaction in insects

Malacostracan crustaceans display a large diversity of sizes, morphs and life styles. However, only a few representatives of decapod taxa have served as models for analyzing crustacean olfaction, such as crayfish and spiny lobsters. Crustaceans bear multiple parallel chemosensory pathways represented by different populations of unimodal chemosensory and bimodal chemo- and mechanosensory sensilla on the mouthparts, the walking limbs and primarily on their two pairs of antennae. Here, we focus on the olfactory pathway associated with the unimodal chemosensory sensilla on the first antennal pair, the aesthetascs. We explore the diverse arrangement of these sensilla across malacostracan taxa and point out evolutionary transformations which occurred in the central olfactory pathway. We discuss the evolution of chemoreceptor proteins, comparative aspects of active chemoreception and the temporal resolution of crustacean olfactory system. Viewing the evolution of crustacean brains in light of energetic constraints can help us understand their functional morphology and suggests that in various crustacean lineages, the brains were simplified convergently because of metabolic limitations. Comparing the wiring of afferents, interneurons and output neurons within the olfactory glomeruli suggests a deep homology of insect and crustacean olfactory systems. However, both taxa followed distinct lineages during the evolutionary elaboration of their olfactory systems. A comparison with insects suggests their olfactory systems ö especially that of the vinegar fly ö to be superb examples for "economy of design". Such a comparison also inspires new thoughts about olfactory coding and the functioning of malacostracan olfactory systems in general.

Brain architecture of the Pacific White Shrimp Penaeus vannamei Boone, 1931 (Malacostraca, Dendrobranchiata): correspondence of brain structure and sensory input?

Penaeus vannamei (Dendrobranchiata, Decapoda) is best known as the "Pacific White Shrimp" and is currently the most important crustacean in commercial aquaculture worldwide. Although the neuroanatomy of crustaceans has been well examined in representatives of reptant decapods ("ground-dwelling decapods"), there are only a few studies focusing on shrimps and prawns. In order to obtain insights into the architecture of the brain of P. vannamei, we use neuroanatomical methods including X-ray micro-computed tomography, 3D reconstruction and immunohistochemical staining combined with confocal laser-scanning microscopy and serial sectioning. The brain of P. vannamei exhibits all the prominent neuropils and tracts that characterize the ground pattern of decapod crustaceans. However, the size proportion of some neuropils is salient. The large lateral protocerebrum that comprises the visual neuropils as well as the hemiellipsoid body and medulla terminalis is remarkable. This observation corresponds with the large size of the compound eyes of these animals. In contrast, the remaining median part of the brain is relatively small. It is dominated by the paired antenna 2 neuropils, while the deutocerebral chemosensory lobes play a minor role. Our findings suggest that visual input from the compound eyes and mechanosensory input from the second pair of antennae are major sensory modalities, which this brain processes.

Comparative analyses of olfactory systems in terrestrial crabs (Brachyura): evidence for aerial olfaction?

Adaptations to a terrestrial lifestyle occurred convergently multiple times during the evolution of the arthropods. This holds also true for the "true crabs" (Brachyura), a taxon that includes several lineages that invaded land independently. During an evolutionary transition from sea to land, animals have to develop a variety of physiological and anatomical adaptations to a terrestrial life style related to respiration, reproduction, development, circulation, ion and water balance. In addition, sensory systems that function in air instead of in water are essential for an animal's life on land. Besides vision and mechanosensory systems, on land, the chemical senses have to be modified substantially in comparison to their function in water. Among arthropods, insects are the most successful ones to evolve aerial olfaction. Various aspects of terrestrial adaptation have also been analyzed in those crustacean lineages that evolved terrestrial representatives including the taxa Anomala, Brachyura, Amphipoda, and Isopoda. We are interested in how the chemical senses of terrestrial crustaceans are modified to function in air. Therefore, in this study, we analyzed the brains and more specifically the structure of the olfactory system of representatives of brachyuran crabs that display different degrees of terrestriality, from exclusively marine to mainly terrestrial. The methods we used included immunohistochemistry, detection of autofluorescence- and confocal microscopy, as well as three-dimensional reconstruction and morphometry. Our comparative approach shows that both the peripheral and central olfactory pathways are reduced in terrestrial members in comparison to their marine relatives, suggesting a limited function of their olfactory system on land. We conclude that for arthropod lineages that invaded land, evolving aerial olfaction is no trivial task.

Central projections of antennular chemosensory and mechanosensory afferents in the brain of the terrestrial hermit crab (Coenobita clypeatus; Coenobitidae, Anomura)

The Coenobitidae (Decapoda, Anomura, Paguroidea) is a taxon of hermit crabs that includes two genera with a fully terrestrial life style as adults. Previous studies have shown that Coenobitidae have evolved a sense of spatial odor localization that is behaviorally highly relevant. Here, we examined the central olfactory pathway of these animals by analyzing central projections of the antennular nerve of Coenobita clypeatus, combining backfilling of the nerve with dextran-coupled dye, Golgi impregnations and three-dimensional reconstruction of the primary olfactory center, the antennular lobe. The principal pattern of putative olfactory sensory afferents in C. clypeatus is in many aspects similar to what have been established for aquatic decapod crustaceans, such as the spiny lobster Panulirus argus. However, there are also obvious differences that may, or may not represent adaptations related to a terrestrial lifestyle. In C. clypeatus, the antennular lobe dominates the deutocerebrum, having more than one thousand allantoid-shaped subunits. We observed two distinct patterns of sensory neuron innervation: putative olfactory afferents from the aesthetascs either supply the cap/subcap region of the subunits or they extend through its full depth. Our data also demonstrate that any one sensory axon can supply input to several subunits. Putative chemosensory (non-aesthetasc) and mechanosensory axons represent a different pathway and innervate the lateral and median antennular neuropils. Hence, we suggest that the chemosensory input in C. clypeatus might be represented via a dual pathway: aesthetascs target the antennular lobe, and bimodal sensilla target the lateral antennular neuropil and median antennular neuropil. The present data is compared to related findings in other decapod crustaceans.

The eyestalk transcriptome of red swamp crayfish Procambarus clarkii

The red swamp crayfish (Procambarus clarkii, Girard 1852) is among the most economically important freshwater crustacean species, and it is also considered one of the most aggressive invasive species worldwide. Despite its commercial importance and being one of the most studied crayfish species, its genomic and transcriptomic layout has only been partially studied. Illumina RNA-sequencing was applied to characterize the eyestalk transcriptome and identify its most characterizing genes. A collection of 83,170,732 reads from eyestalks was obtained using Illumina paired-end sequencing technology. A de novo assembly was performed with the Trinity assembly software generating 119,255 contigs (average length of 1,007 bp) and identifying the first sequenced transcriptome in this species. The eyestalk is a major site for the production of neurohormones and controls a variety of physiological functions such as osmotic regulation, molting, epidermal color patterns and reproduction. Hence, its transcriptomic characterization is interesting and potentially instrumental to the elucidation of genes which have not been comprehensively described yet. Moreover, the availability of such a large amount of information supported the characterization of molecular families which have never been described before. The P. clarkii eyestalk transcriptome reported here provides a resource for improving the knowledge of the still incompletely defined neuroendocrinology of this species and represents an important source of data for all the interested carcinologists.

Brain anatomy of the marine isopod Saduria entomon Linnaeus, 1758 (Valvifera, Isopoda) with special emphasis on the olfactory pathway

Representatives of at least six crustacean taxa managed to establish a terrestrial life style during their evolutionary history and the Oniscidea (Isopoda) are currently held as the most successfully terrestrialized malacostracan crustaceans. The brain architecture of terrestrial isopods is fairly well understood and studies on this field suggest that the evolutionary transition from sea to land in isopods coincided with a considerable size reduction and functional loss of their first pair of antennae and associated brain areas. This finding suggests that terrestrial isopods may have no or poor abilities to detect volatile substances but that their chemosensory ecology is most likely restricted to contact chemoreception. In this study, we explored how the brain of a marine isopod and particularly its olfactory system compares to that of terrestrial relatives. Using histochemical and immunohistochemical labeling, brightfield and confocal laser-scan microscopy, we show that in the marine isopod Saduria entomon aesthetascs on the first pair of antennae provide input to a well defined deutocerebrum (DC). The deutocerebral chemosensory lobes (DCL) are divided into spherical neuropil compartments, the olfactory glomeruli (og). Secondary processing areas in the lateral protocerebrum (lPC) are supplied by a thin but distinct projection neuron tract (PNT) with a contralateral connection. Hence, contrary to terrestrial Isopoda, S. entomon has at least the neuronal substrate to perceive and process olfactory stimuli suggesting the originally marine isopod lineage had olfactory abilities comparable to that of other malacostracan crustaceans.

Structure of the olfactory receptor organs, their GABAergic neural pathways, and modulation of mating behavior, in the giant freshwater prawn, Macrobrachium rosenbergii

In the giant male prawn, Macrobrachium rosenbergii, the olfactory system is thought to be the main pathway for modulating sexual behavior through pheromone perception. In this report, we first used gross anatomical, histological, and SEM methods to describe the structures of the olfactory receptors (sensilla setae), their neural pathways, and possible role in modulating mating behavior. On the surfaces of antennule and antenna filaments there are four types of sensory receptors, viz single spike-like setae, single flagellum-like setae, multiple flagella-like setae, and aesthetascs (ASs). The ASs, which had previously been proposed to be odor receptor setae, are found only on the short filament of lateral antennule (slAn). Each AS on the slAn connects with olfactory receptor neurons (ORNs), whose axons form an outer central antennule nerve (ocAnNv), which then connects with the olfactory neutrophil (ON) of the brain. Thus, the slAn is the major olfactory organ that conveys sensory inputs from each AS to the ON within the deutocerebrum. GABA immunoreactivity was present in ASs, neurons of ORNs, inner central antennular, lateral tegumentary nerve, ocAnNv and the ON, inferring that GABA is the likely neurotransmitter in modulating olfaction. Disruption of the slAn by ablation or covering with Vaseline, resulted in significant reduction of mating behavior, indicating that this organ is crucial for sex pheromone perception. Identification of the active pheromones and further bioassays are now being performed.