Morphological and ultrastructural characterization of olfactory sensilla in Drosophila suzukii: Scanning and transmission electron microscopy

Drosophila suzukii is a serious horticultural and quarantine pest, damaging various berry crops. Although the active use of olfactory communication in D. suzukii is well-known, their olfactory sensory system has not been comprehensively reported. Therefore, the present study was carried out to understand the morphology, distribution and ultrastructure of olfactory sensilla present in the antennae and maxillary palps of D. suzukii, through scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The olfactory sensilla on the antennae of D. suzukii in both sexes could be classified into three major morphological types, basiconic, trichoid and coeloconic sensilla, according to their shapes. The antennal basiconic sensilla were further divided into three subtypes and the antennal trichoid sensilla into two subtypes, respectively, according to the size of individual sensillum. In contrast to the antennal olfactory sensilla showing diverse morphology, basiconic sensilla was the only type of olfactory sensilla in the maxillary palps of D. suzukii. The basiconic sensilla in the maxillary palps could be further classified into three subtypes, based on their size. Our SEM and TEM observations indicated that multiple nanoscale pores are present on the surface of all types of olfactory sensilla in the antennae and maxillary palps, except coeloconic sensilla. The difference in the morphological types and the distribution of olfactory sensilla suggests that their olfactory functions are different between antennae and maxillary palps in D. suzukii. The results of this study provide useful information for further studies to determine the function of olfactory sensilla in D. suzukii and to understand their chemical communication system.     

Morphological and functional heterogeneity in olfactory perception between antennae and maxillary palps in the pumpkin fruit fly,Bactrocera depressa

The morphology and ultrastructure of the olfactory sensilla on the antennae and maxillary palps were investigated through scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and their responses to five volatile compounds were measured using electroantenogram (EAG) and electropalpogram (EPG) techniques in the pumpkin fruit fly, Bactrocera depressa (Shiraki; Diptera: Tephritidae). Male and female B. depressa displayed distinct morphological types of olfactory sensilla in the antennae and maxillary palps, with predominant populations of trichoid, basiconic, and coeloconic sensilla. Basiconic sensilla, the most abundant type of olfactory sensilla in the antennae, could be further classified into two different types. In contrast, the maxillary palps exhibited predominant populations of a single type of curved basiconic sensilla. High‐resolution SEM observation revealed the presence of multiple nanoscale wall‐pores on the cuticular surface of trichoid and basiconic sensilla, indicating that their primary function is olfactory. In contrast, coeloconic sensilla displayed several longitudinal grooves around the sensillum peg. The TEM observation of individual antennal olfactory sensilla indicates that the basiconic sensilla are thin‐walled, while the trichoid sensilla are thick‐walled. The profile of EAG responses of male B. depressa was different from their EPG response profile, indicating that the olfactory function of maxillary palps is different from that of antennae in this species. The structural and functional variation in the olfactory sensilla between antennae and maxillary palps suggests that each plays an independent role in the perception of olfactory signals in B. depressa.     

The organization of the chemosensory system in Drosophila melanogaster: a rewiew

This review surveys the organization of the olfactory and gustatory systems in the imago and in the larva of Drosophila melanogaster, both at the sensory and the central level. Olfactory epithelia of the adult are located primarily on the third antennal segment (funiculus) and on the maxillary palps. About 200 basiconic (BS), 150 trichoid (TS) and 60 coeloconic sensilla (CS) cover the surface of the funiculus, and an additional 60 BS are located on the maxillary palps. Males possess about 30% more TS but 20% fewer BS than females. All these sensilla are multineuronal; they may be purely olfactory or multimodal with an olfactory component. Antennal and maxillary afferents converge onto approximately 35 glomeruli within the antennal lobe. These projections obey precise rules: individual fibers are glomerulus-specific, and different types of sensilla are associated with particular subsets of glomeruli. Possible functions of antennal glomeruli are discussed. In contrast to olfactory sensilla, gustatory sensilla of the imago are located at many sites, including the labellum, the pharynx, the legs, the wing margin and the female genitalia. Each of these sensory sites has its own central target. Taste sensilla are usually composed of one mechano-and three chemosensory neurons. Individual chemosensory neurons within a sensillum respond to distinct subsets of molecules and project into different central target regions. The chemosensory system of the larva is much simpler and consists essentially of three major sensillar complexes on the cephalic lobe, the dorsal, terminal and ventral organs, and a series of pharyngeal sensilla.     

The organization of the chemosensory system in Drosophila melanogaster: a rewiew

This review surveys the organization of the olfactory and gustatory systems in the imago and in the larva of Drosophila melanogaster, both at the sensory and the central level. Olfactory epithelia of the adult are located primarily on the third antennal segment (funiculus) and on the maxillary palps. About 200 basiconic (BS), 150 trichoid (TS) and 60 coeloconic sensilla (CS) cover the surface of the funiculus, and an additional 60 BS are located on the maxillary palps. Males possess about 30% more TS but 20% fewer BS than females. All these sensilla are multineuronal; they may be purely olfactory or multimodal with an olfactory component. Antennal and maxillary afferents converge onto approximately 35 glomeruli within the antennal lobe. These projections obey precise rules: individual fibers are glomerulus-specific, and different types of sensilla are associated with particular subsets of glomeruli. Possible functions of antennal glomeruli are discussed. In contrast to olfactory sensilla, gustatory sensilla of the imago are located at many sites, including the labellum, the pharynx, the legs, the wing margin and the female genitalia. Each of these sensory sites has its own central target. Taste sensilla are usually composed of one mechano-and three chemosensory neurons. Individual chemosensory neurons within a sensillum respond to distinct subsets of molecules and project into different central target regions. The chemosensory system of the larva is much simpler and consists essentially of three major sensillar complexes on the cephalic lobe, the dorsal, terminal and ventral organs, and a series of pharyngeal sensilla.     

Distribution of chemo- and mechanoreceptors on the antennae and maxillae of Busseola fusca larvae

The stem borer Busseola fusca (Fuller) (Lepidoptera: Noctuidae) is a major pest of maize, Zea mays L., and sorghum, Sorghum bicolor (L.) Moench (both Poaceae), in sub-Saharan Africa. Like in many other lepidopteran insects, the success of B. fusca in recognizing and colonizing a limited variety of plants is based on the interaction between its sensory systems and the physicochemical characteristics of its immediate environment. The sensilla on the maxillary galeae of B. fusca larvae are typical of Lepidoptera and comprise two uniporous styloconic sensilla, which are contact chemoreceptors, three basiconic sensilla, and two aporous sensilla chaetica. The maxillary palp is two-segmented and has eight small basiconic sensilla at the tip, which were also found to be gustatory. The antennae of B. fusca larvae are short and simple. The sensilla of the antenna are composed of two aporous sensilla chaetica, three multiporous cone-shaped basiconic sensilla, three small basiconic sensilla, and one aporous styloconic sensillum. The basiconic sensillum located on the third antennal segment displayed a contact chemoreception response. The other basiconic sensilla did not show any action potential activity in tip-recording tests. The significant and positive dose–response curve obtained for the antennal basiconic sensillum with sucrose indicated for the first time the presence of gustatory chemoreceptors on the antennae of a lepidopteran larva.     

Olfactory sensilla on the antenna and maxillary palp of the sheep head fly,Hydrotaea irritans (Fallen) (Diptera : Muscidae)

Antennae and maxillary palps of both sexes of the Sheep Head fly Hydrotaea irritans (Diptera : Muscidae) were investigated using scanning electron microscopy to describe the types, morphology, and distribution of olfactory sensory structures. Only socketed bristles and microtrichia were found on the scape of the antennae. These structures were also observed on the pedicel together with a group of 7–8 as yet undescribed sensilla, whose function is unknown. Olfactory sensilla were not found on these 2 segments or on the arista. Large numbers of olfactory sensilla and microtrichia were present on the funiculus. The former included sensilla trichodea (thick-walled, multiporous sensilla), sensilla styloconica and 6 types of sensilla basiconica (thin-walled, multiporous sensilla), 4 of which occurred individually and 2 of which were found in groups. An olfactory pit containing groups of thin-walled multiporous sensilla was located on the dorsomedian side of the funiculus. All sensilla basiconica were classified on morphological characteristics. The maxillary palps were covered with microtrichia and socketed bristles, but only 1 type of olfactory sensillum was found. This was a type of sensillum basiconicum that differed from any of those found on the antennae. No differences were found in sensilla diversity and distribution between males and females.     

Functional anatomy of sensory structures on the antennae of Psylliodes chrysocephala L. (Coleoptera: Chrysomelidae)

The ultrastructure and distribution of sensilla on the antennae of the cabbage stem flea beetle, Psylliodes chrysocephala, were investigated using scanning and transmission electron microscopy techniques. Eight different sensillar types were distinguished. These were; hair plate sensilla, sensilla chaetica, three types of sensilla trichodea, sensilla basiconica, grooved peg sensilla and styloconic sensilla. The sensilla chaetica are known to be gustatory receptors. Ultrastructure indicates that the hair plate sensilla and sensilla trichodea type one are probably mechanoreceptors, whilst the sensilla styloconica are probably thermo-hygro receptors. These thermo-hygroreceptors are unusual in that they are innervated by two sensory cells (one hygroreceptor and one thermoreceptor) rather than the more usual triad. The remaining four sensillar types all have a porous hair shaft, indicating an olfactory role. One of these (the grooved peg sensillum) may also have a thermoreceptive function. No sexual dimorphism was found in the structure, number or distribution of the antennal sensilla.     

Comparative morphology of sensilla on antenna,maxillary palp and labial palp of larvae of Eucryptorrhynchus scrobiculatus (Olivier) and E. brandti (Harold) (Coleoptera: Curculionidae)

Eucryptorrhynchus scrobiculatus (Olivier) and E. brandti (Harold) are two wood boring pests of Ailanthus altissima (Mill.) Swingle (tree of heaven) and the variety Ailanthus altissima var. Qiantouchun. These beetles attack healthy trees and bore into roots and trunks during the larval stage. We studied the typology, distribution and morphostructure of the sensilla on the antennae, maxillary palps and labial palps of E. scrobiculatus and E. brandti larvae using scanning and transmission electron microscopy. The results showed the following: (i) the antennae of the two weevil larvae had two types of sensilla, sensilla basiconica (S.b.1 and S.b.2) and sensilla twig basiconica (S.tb.1‐S.tb.3), with S.tb.4 observed only on the antennae of E. brandti larvae; (ii) the maxillary palps had three types of sensilla, S.b.2, S.tb. (S.tb.2, S.tb.3 and S.tb.5) and digitiform sensilla; (iii) the labial palps had two types of sensilla, S.b.2 and S.tb. (S.tb.2, S.tb.3 and S.tb.5); (iv) the quantity and distribution of sensilla on the antennae, maxillary palps and labial palps remained constant between E. scrobiculatus and E. brandti larvae; and (v) sensilla basiconica had distinct sidewall pores, an apical pore was observed on sensilla twig basiconica, and digitiform sensilla were oval in shape, with a distinct apical pore. Based on the microstructure of the cuticle wall and dendrite, we hypothesized that these sensilla functioned as olfactory, gustatory and hygro‐/thermo‐receptors, respectively. We discuss the relationships among types of sensilla and the types of damage caused by the larvae inside the host tree to understand olfactory and gustatory receptor mechanisms. The results of this study will provide a firm basis for future electrophysiological studies.     

Central projections of olfactory receptor neurons from single antennal and palpal sensilla in mosquitoes

In insects, olfactory receptor neurons (ORNs) are located in cuticular sensilla, that are present on the antennae and on the maxillary palps. Their axons project into spherical neuropil, the glomeruli, which are characteristic structures in the primary olfactory center throughout the animal kingdom. ORNs in insects often respond specifically to single odor compounds. The projection patterns of these neurons within the primary olfactory center, the antennal lobe, are, however, largely unknown.We developed a method to stain central projections of intact receptor neurons known to respond to host odor compounds in the malaria mosquito, Anopheles gambiae. Terminal arborizations from ORNs from antennal sensilla had only a few branches apparently restricted to a single glomerulus. Axonal arborizations of the different neurons originating from the same sensillum did not overlap.ORNs originating from maxillary palp sensilla all projected into a dorso-medial area in both the ipsi- and contralateral antennal lobe, which received in no case axon terminals from antennal receptor neurons. Staining of maxillary palp receptor neurons in a second mosquito species (Aedes aegypti) revealed unilateral arborizations in an area at a similar position as in An. gambiae.     

Sensilla on the palps and legs of the adult soft tick argas persicus oken (IXODOIDEA: ARGASIDAE) and their projections to the central nervous system

The sensory structures present on the palps and legs of adult Argas persicus Oken (Ixodoidea: Argasidae) were studied by light, scanning and transmission electron microscopy. The number, distribution, surface morphology and the fine structure of the prominent sensilla present on these appendages were determined. The palps have 2 morphologically prominent types of sensilla: one with a grooved surface of the hair and the other having a non-grooved hair. The TEM distinguishes at least 4 prominent subtypes in grooved sensilla with single or double lumina and dendrites occupying the periphery of the central lumen or distributed all over the central lumen. Amongst the sensilla with non-grooved hair-shaft, a rare type of Olfactory Mechanoreceptive (OM) sensillum was found on the palps and the first legs of A. persicus. At the base of the hair-shaft, the OM sensillum has 2 mechanosensory dendrites. The hair-shaft of the sensillum has a porous cuticle, characteristic of an olfactory sensillum. The lumen of the hair-shaft is invested with branching dendrites from 3–8 neurons, which are surrounded by 4 sheath cells. The sensilla on the legs, including those present in the Hallers organ, are of at least 3 prominent categories. (i) Single wall with un-innervated hair-shaft. (ii) Single wall, multiporous sensillum with dendrites present in the hair shaft. (iii) Double walls with spoke channels and dendrites present in the central lumen. Sensory projections from the crown of sensilla located on the distal end of the palp extend to the palpal and suboesophageal (SOG) ganglia. Projections in the SOG extend further to the contralateral side. Sensilla in the Hallers organ project to the first pedal ganglion and to the anterodorsal region of supraoesophageal ganglion. As expected, the primary sensory projections from the sensilla of the other 3 legs extend to the respective pedal ganglia.     

Antennal sensilla of male and female of the nut weevil,Curculio nucum Linnaeus, 1758 (Coleoptera: Curculionidae)

The nut weevil, Curculio nucum (Linnaeus, 1758), is the main pest in hazelnut orchards (Corylus avellana L.). Semiochemicals are interesting bio control tools that could be used to manipulate the pest behaviour and to control pest populations. The study of the sensorial equipment of the insect antennae provides information on the importance of olfaction in the adult life for host plants and mate findings as well as on the putative other senses. Before electrophysiological investigation, the knowledge of antennae equipment is also necessary. The aim of this study is to determine the types, number and location of sensilla on the antennae of male and female adult C. nucum in order to determine their implication in seeking a sexual partner and a host plant. The 12-segmented antenna comprises a scape, a 7-segmented funicle and a 4-segmented club. Out of the nine sensillum types listed, three are present on the scape and the funicle and seven types on the club which gathers 71–73% of the total of sensilla. Tactile aporous sensilla chaetica C1, gustatory uniporous sensilla chaetica C2, olfactory multiporous sensilla basiconica B1 and B2 are found on both the dorsal and ventral surfaces of the club in both sexes. Thermo-hygroreceptive dome-shaped sensilla D, olfactory multiporous sensilla basiconica B3 and olfactory multiporous fluted sensilla basiconica F are found exclusively on the ventral surface of the club, suggesting that these sensilla are utilized in host plant acceptance during antennal tapping. The sexual dimorphism concerns only the numbers of sensilla chaetica C1 and sensilla basiconica B2.     

Influence of precocene II on the sensory system of antennae and mouthparts in larvae of the fruit tree tortricid <Emphasis Type="Italic">Archips podana</Emphasis> Scop. (Lepidoptera: Tortricidae)

The influence of precocene II, an antijuvenile agent, on morphological characters of the chemoreceptor apparatus of antennae and mouthparts was studied in fifth instar A. podana larvae. Treatment with different doses of precocene was performed at the egg stage. It proved to cause changes in the form and number of basiconic sensilla on the maxillary palps and galea and in the size of basiconic sensilla on the second and third antennal segments. The results are discussed with respect to the influence of precocenes on the insect sensory system and the role of the juvenile hormone in regulation of its development.     

Re-Classification of Drosophila melanogaster Trichoid and Intermediate Sensilla Using Fluorescence-Guided Single Sensillum Recording

Drosophila olfactory receptor neurons are found within specialized sensory hairs on antenna and maxillary palps. The linking of odorant-induced responses to olfactory neuron activities is often accomplished via Single Sensillum Recordings (SSR), in which an electrode inserted into a single sensory hair records the neuronal activities of all the neurons housed in that sensillum. The identification of the recorded sensillum requires matching the neuronal responses with known odor-response profiles. To record from specific sensilla, or to systematically screen all sensillar types, requires repetitive and semi-random SSR experiments. Here, we validate an approach in which the GAL4/UAS binary expression system is used for targeting specific sensilla for recordings. We take advantage of available OrX-Gal4 lines, in combination with recently generated strong membrane targeted GFP reporters, to guide electrophysiological recordings to GFP-labeled sensilla. We validate a full set of reagents that can be used to rapidly screen the odor-response profiles of all basiconic, intermediate, and trichoid sensilla. Fluorescence-guided SSR further revealed that two antennal trichoid sensilla types should be re-classified as intermediate sensilla. This approach provides a simple and practical addition to a proven method for investigating olfactory neurons, and can be extended by the addition of UAS-geneX effectors for gain-of-function or loss-of-function studies.     

The pedipalp of Pseudocellus pearsei (Ricinulei, Arachnida) - ultrastructure of a multifunctional organ

Ricinulei possess movable, slender pedipalps with small chelae. When ricinuleids walk, they occasionally touch the soil surface with the tips of their pedipalps. This behavior is similar to the exploration movements they perform with their elongated second legs. We studied the distal areas of the pedipalps of the cavernicolous Mexican species Pseudocellus pearsei with scanning and transmission electron microscopy. Five different surface structures are characteristic for the pedipalps: (1) slender sigmoidal setae with smooth shafts resembling gustatory terminal pore single-walled (tp-sw) sensilla; (2) conspicuous long, mechanoreceptive slit sensilla; (3) a single, short, clubbed seta inside a deep pit representing a no pore single walled (np-sw) sensillum; (4) a single pore organ containing one olfactory wall pore single-walled (wp-sw) sensillum; and (5) gustatory terminal pore sensilla in the fingers of the pedipalp chela. Additionally, the pedipalps bear sensilla which also occur on the other appendages. With this sensory equipment, the pedipalps are highly effective multimodal short range sensory organs which complement the long range sensory function of the second legs. In order to present the complete sensory equipment of all appendages of the investigated Pseudocellus a comparative overview is provided.     

The quill mite <Emphasis Type="Italic">Syringophilopsis fringilla</Emphasis> (Fritsch) (Acari: Trombidiformes: Syringophilidae): the structure of sensory organs providing feeding of the parasite in the feather quill

The structure of the sensory organs situated on palps and chelicerae of the quill mite Syringophilopsis fringilla (Fritsch, 1958) was examined with the use of scanning and transmitting electron microscopy. The tarsal segment of the palps bears 8 sensilla of three types: two contact chemo-mechanoreceptor sensilla, a single chemoreceptor (olfactory) sensillum, and five tactile mechanoreceptor sensilla. All other sensilla situated on basal palpal segments and on cheliceral stylets are represented exclusively by tactile mechanoreceptors. A proprioceptor sensillum was revealed in the movable digit of chelicerae; the modified cilia of dendrites of 5 sensory neurons of this sensillum run inside the inner non-sclerotized core of the stylet and end at different levels in its apical part, attaching to electron-dense rods connected with a sclerotized sheath of the stylet. The authors assume that the proprioceptor sensillum of the stylet detects the strength of the pressure of the stylet of the movable digit on the quill wall during its piercing, and palpal sensilla determine the optimal place for this process.     

Morphology and untrastructure of the antennal sense organs in tenebrionid larvae <Emphasis Type="Italic">Tenebrio molitor</Emphasis> L. and <Emphasis Type="Italic">Zophobas rugipes</Emphasis> Kirsch (Coleoptera: Tenebrionidae)

The distribution, external morphology, and ultrastructure of various types of sensilla in the antennae of tenebrionid larvae Tenebrio molitor and Zophobas rugipes are studied by means of scanning and transmission electron microscopy. On the antennae of T. molitor there are sensilla of four basic morphological types: basiconic, styloconic, trichoid, and papillate sensilla. On the antennae of Z. rugipes, in addition to the aforementioned ones, there are placoid sensilla. Ultrastructure points to olfactory function of basiconic and placoid sensilla. Other sensillum types are contact chemoreceptors.     

Sex-specific antennal sensory system in the ant <Emphasis Type="Italic">Camponotus japonicus</Emphasis>: structure and distribution of sensilla on the flagellum

The antennae are a critically important component of the ant’s highly elaborated chemical communication systems. However, our understanding of the organization of the sensory systems on the antennae of ants, from peripheral receptors to central and output systems, is poorly understood. Consequently, we have used scanning electron and confocal laser microscopy to create virtually complete maps of the structure, numbers of sensory neurons, and distribution patterns of all types of external sensilla on the antennal flagellum of all types of colony members of the carpenter ant Camponotus japonicus. Based on the outer cuticular structures, the sensilla have been classified into seven types: coelocapitular, coeloconic, ampullaceal, basiconic, trichoid-I, trichoid-II, and chaetic sensilla. Retrograde staining of antennal nerves has enabled us to count the number of sensory neurons housed in the different types of sensilla: three in a coelocapitular sensillum, three in a coeloconic sensillum, one in an ampullaceal sensillum, over 130 in a basiconic sensillum, 50–60 in a trichoid-I sensillum, and 8–9 in a trichoid-II sensillum. The basiconic sensilla, which are cuticular hydrocarbon-receptive in the ant, are present in workers and unmated queens but absent in males. Coelocapitular sensilla (putatively hygro- and thermoreceptive) have been newly identified in this study. Coelocapitular, coeloconic, and ampullaceal sensilla form clusters and show biased distributions on flagellar segments of antennae in all colony members. The total numbers of sensilla per flagellum are about 9000 in unmated queens, 7500 in workers, and 6000 in males. This is the first report presenting comprehensive sensillar maps of antennae in ants.     

Immunolocalization of general odorant-binding protein in antennal sensilla of moth caterpillars

Antennae of Bombyx mori and Helicoverpa armigera larvae were immunolabelled with antisera raised against the pheromone-binding protein or the general odorant-binding protein 2 of Antheraea polyphemus to assign the expression of these proteins to individual sensilla and to compare the localization pattern with that in sensilla of adult moths. Specific labelling of antennal sensilla was only obtained with the antiserum against general odorant-binding protein 2. Among the few sensilla present on the antenna the three large sensilla basiconica, which are suspected to be olfactory in function, were labelled. These sensilla are compound sensilla consisting of several sensillum units which form a common sensory hair. The hair is single-walled and pierced by many pores. Labelling of sensillum compartments was the same as in sensilla of adults. Prominent labelling of the sensillum lymph is accompanied by labelling of secretory organelles in the two outermost auxiliary cells and of endocytotic pathways in all sensillum cells. The results suggest that general odorant-binding protein is expressed in single-walled multiporous sensilla of presumed olfactory function on the antenna of moth larvae. The overall identity of the localization pattern for general odorant-binding protein between larval and adult sensilla implies a similar role of these proteins in olfactory stimulus transduction.