Sensilla of the antennae and the labial and maxillary palps of Blattella germanica (L.) (Dictyoptera: Blattellidae): Their classification and distribution

The various sensilla on the antennae and on the labial and maxillary palps of Blattella germanica (L.) were studied. Thick-walled chemoreceptors with fluted shafts and articulated bases are located on the antennae and on the labial and maxillary palps. Thin-walled chemoreceptors, without fluted shafts or articulated bases, are restricted to the flagellar segments of the antennae and to the distal segments of the palps. Antennae of adult males have more thin-walled chemoreceptors than do those of females. Hair-plate sensilla are found at the scape-head and scape-pedicel joints, and at the joints of segments on the palps. Campaniform sensilla are concentrated as a ring around the distal margin of the pedicel, and are also scattered singly on the scape, pedicel, and flagellar segments of the antennae, and on the first segment of the maxillary palps. Occasionally, a few sensilla coeloconica and cold receptor sensilla are found on the antennal flagellum.     

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.     

Taste receptors in Colorado beetle larvae

Electrophysiological studies show that at least six sensilla on the various mouthparts of larval Colorado potato beetles (Leptinotarsa decemlineata) contain chemosensory cells. These sensilla are found on the galea, maxillary palps, and labial palps. Sodium chloride, sucrose, some amino acids, and chlorogenic acid are among the effective stimuli. The sensitivity spectrum of these cells correlates with known larval phagostimulants.     

Ultrastructure and distribution of sensilla on the maxillary and labial palps of Chlorophorus caragana (Coleoptera: Cerambycidae)

Chlorophorus caragana is a species of long‐horned beetle that damages Caragana davazamcii Sancz. (Fabales: Papilionaceae) bushes in desert areas in China. The beetles cause substantial damage to local forestry plantations and the environment. Sensilla on the maxillary and labial palps of coleopterans a allow the insects to recognize their host plants. We used scanning and transmission electron microscopy to study the ultrastructure, distribution, and abundance of various sensilla on the maxillary and labial palps of C. caragana. We found four types of sensilla including ten subtypes: one of Böhm's bristles, three of sensilla chaetica, one of digitiform sensilla, and five of sensilla twig basiconica. The types and distribution of the sensilla on the maxillary and labial palps were highly similar between males and females. Finally, this article discusses the functions of the sensilla of related species in recognizing hosts and the significance of gustation studies in the context of the control of C. caragana.     

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.     

Ultrastructure of sensilla on the maxillary and labial palps of three species (Coleoptera: Staphylinidae: Staphylininae)

Most species of Staphylinidae are predators in an agroecosystem. They acquire prey information from their environment through receptors. In this study, the sensilla on maxillary and labial palps of Philonthus kailiensis, Philonthus lewisius and Quedius robustus were examined with scanning electron microscopy to identify and analyse the external morphology and distribution of the sensilla to enhance our knowledge of the sensilla of Staphylinidae and provide a rationale of taxonomical studies on the two genus. Results showed that the sensilla are classified into six types: Böhm bristles, sensilla chaetica, sensilla furcate, sensilla coeloconica, sensilla placodea and sensilla basiconica. No sexual dimorphism exists among the three species. The relationships and functions of sensilla on maxillary and labial palps were also speculated. There may be a certain correlation between the sensilla on maxillary and labial palps of the staphylinid and its habitat.     

Morphology of antennal,maxillary palp and labial palp sensilla in different larval instars of the Asian long‐horned beetle,Anoplophora glabripennis (Motschulsky) (Coleoptera: Cerambycidae)

The Asian long‐horned beetle (ALB) is one of the most important international trunk‐boring insect pests of forest trees, which has caused enormous losses in the forests of China, the USA and some countries in Europe. To investigate the morphology of the antennal, maxillary palp and labial palp sensilla in the five larval instars of the ALB, scanning electron microscopy was used to determine the typology, morphology, number and distribution of the sensilla. The antennae had two types of sensilla: the sensilla twig basiconica (s.t.b.) and sensilla basiconica. Three different types of sensilla were observed on the maxillary palps, which were the sensilla digitiformia, the sensilla chaetica and the s.t.b. The labial palps had two types of sensilla: the sensilla chaetica and the s.t.b. The s.t.b. had seven total subtypes, and the sensilla basiconica had two subtypes. The typology, quantity and distribution of sensilla on the antennae, maxillary palps and labial palps remained constant in the five larval instars, but the dimensions of all sensilla types increased throughout the development. We discussed the mechanoreception and the chemoreception of ALB larvae inside the host tree to provide a theoretical understanding and information for further research on the behaviour and the electrophysiology of this devastating forest pest.     

Les pieces buccales et leurs recepteurs sensoriels chez l'imago de Micropterix calthella L. (Lepidoptera : Micropterigidae)

The aim of our study is to describe the mouthparts of Micropterix calthella (Lepidoptera) and their sensilla by the scanning electron microscope (SEM). The imaginal mouthparts are primitive. They are well adapted for collecting and grinding pollen of Ranunculus flowers. The maxillary palps show specialized structures for collecting pollen: mushroom-shaped hairs and blade-like structures. The brushes on the epipharynx, the teeth on the mandibles and the triturating basket on the hypopharynx grind the pollen to powder before absorption.Sensilla of different types have been observed on the mouthparts. The labrum, galeae, laciniae, and labial palps possess sensilla chaetica. Epipharynx, galeae, laciniae, and hypopharynx show sensilla basiconica. Sensilla trichoidea have been observed on the laciniae and sensilla campaniformia on the maxillary palps and the hypopharynx. At the top of each maxillary palp is a perforated area of unknown function. Two cavities are present at the distal extremity of each labial palp: one with sensilla coeloconica and the other with a perforated heart-shaped organ. The mushroom-shaped hairs, the blade-like structures, and the perforated heart-shaped organs have not yet been described in other insects.     

Sexual dimorphism of adult labial palps of the peach fruit moth Carposina sasakii Matsumura (Lepidoptera: Carposinidae) with notes on their sensilla

The labial palps and their sensilla of the peach fruit moth Carposina sasakii Matsumura, a serious pest of pome fruits in eastern Asia, were investigated using light microscopy and scanning and transmission electron microscopy. The labial palps are three‐segmented and exhibit distinct sexual dimorphism in length: much longer in the female than in the male. Four types of sensilla were found on the labial palps: campaniform, squamiform, chaetic and flattened sensilla. The campaniform sensilla are present on the first segment alone. The squamiform sensilla are located on all the three segments. The chaetic sensilla are mainly present on the third segment. The flattened sensilla are grooved with wall pores and situated in a labial palp‐pit organ, which is located at the apex of the distal segment of the labial palp. The sexual dimorphism of the labial palp and the labial palp‐pit organ was briefly discussed.     

小菜蛾成虫下唇须感器的超微结构及其神经元中枢投射

【目的】明确小菜蛾Plutella xylostella成虫下唇须感器的形态结构及感器神经元的投射。【方法】利用光学显微镜观察和扫描电子显微镜观察下唇须结构和感器类型,利用神经回填技术和激光共聚焦显微镜观察下唇须感器神经元在脑部的投射。【结果】小菜蛾成虫下唇须共3节,其上存在Böhm氏鬃毛、钟形感器、鳞形感器、锥形感器、微毛形感器5种不同类型的感器和一个陷窝器结构。Böhm氏鬃毛短小尖细,钟形感器形如顶部凹陷的圆帽,两种感器均分布于下唇须第1节,且大小上均无雌雄二型差异;鳞形感器形同柳叶,锥形感器粗而直,均散生于下唇须的第2和3节,两种感器在大小上均存在雌雄二型差异,其中雌性的鳞形感器显著大于雄性的,根据其雌雄二型差异现象推测雌蛾的鳞形感器可能与感受寄主植物挥发物有关;下唇须第3节中上部具有一个圆形陷窝器结构,雄虫的陷窝器内径为5.68±0.33μm,雌虫的为6.03±0.23μm,雌雄间无显著性差异;凹坑内长有表面光滑的微毛形感器。小菜蛾下唇须感器神经元主要投射于脑部咽下神经节、每个触角叶的下唇须陷窝器神经纤维球和腹神经索3条通路。【结论】阐明了小菜蛾下唇须感器的类型、分布和形态特征及其感器神经元在脑部的投射形态,为深入了解小菜蛾下唇须感器的生理和功能奠定了基础。     

Ultrastructure of sensilla of the female mountain pine beetle,Dendroctonus ponderosae hopkins (Coleoptera : scolytidae)

The objective of this study is to provide a morphological foundation for electrophysiological studies on the sensilla of D. ponderosae. The sensilla on the antennae, labial and maxillary palps, galeae and fore-tarsi are described. The antennal club has several types of sensilla: (1) multiporous non-socketed pegs of varying lengths innervated by 2 neurons, (2) uniporous socketed pegs innervated by 5 neurons, (3) pegs innervated by only 1 neuron at the base, (4) tapered uniporous cones innervated by 4 neurons, and (5) fluted multiporous, non-socketed cones innervated by 4 neurons. All hairs on the fore-tarsi are innervated by 1 neuron at the base. The labial and maxillary palp-tips have: (1) digitiform organs, (2) campaniform organs, (3) uniporous, socketed pegs innervated by 5 neurons, and (4) non-porous pegs innervated by 3 neurons. Two small, uniporous, socketed pegs innervated by 4 neurons, are set into the maxillary palp-tip sidewall, and a single, sharp-tipped peg, innervated by 1 neuron, is found on the tip. These 2 types are not seen on the labial palps. The ultrastructure of each type of sensillum found on both the maxillary and labial palps is similar; however, the number of uniporous and non-porous pegs on the labial palps is about half those of the maxilla. The maxillary galeae each have 3 types of sensilla: (1) numerous pegs of various sizes innervated by 1 neuron, (2) a single, possibly uniporous socketed peg innervated by 5 neurons, and (3) another single non-porous non-socketed peg innervated by 2 neurons.     

Expression of odorant-binding and chemosensory proteins and spatial map of chemosensilla on labial palps of Locusta migratoria (Orthoptera: Acrididae)

The sensilla on labial palps in Locusta migratoria were observed and mapped using light microscopy, scanning and transmission electron microscopy. A dome region on the tip of the fourth segment (distal segment) of labial palps is mainly covered with sensilla chaetica (about 98%), and few sensilla basiconica (2%). The total number of both types of sensilla is significantly higher in females than in males. Sensilla chaetica can be further subdivided into three groups containing 6, 7 or 10 neurons. Immunocytochemical localization of odorant-binding protein (OBP) and chemosensory proteins (CSPs) was performed on ultrathin sections of sensilla on labial palps. The antiserum against odorant-binding protein from Locusta migratoria (LmigOBP) only labelled sensilla basiconica, with gold granules only found in the sensillum lymph. Chemosensory protein instead was specifically present in the outer sensillum lymph of all three subgroups of sensilla chaetica with antiserum against CSP-I from Schistocerca gregaria (SgreCSP-I). In contrast these three subgroups were never labelled with antiserum against CSP-II from Locusta migratoria (LmigCSP-II). In addition, a few sensilla chaetica could not be stained with any of the antisera used.     

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.     

Sensory organs of the antennae and mouthparts of beetle larvae (Coleoptera)

The sensilla located on the antennae and maxillary and labial palps of the larvae of 64 beetle species from 22 families were studied using electron microscopy. The larvae of beetles living in different habitats and having different trophic specializations possess a uniform structure of the sensory organs. They are composed of two groups of sensilla on the apical and subapical segments of the antennae, one apical group of sensilla on both maxillary and labial palps, and one or several digitiform sensilla on the lateral surface of the maxillary and, occasionally, labial palp. The external morphology of the sensory organs is adaptive and represents modifications of the initial type. Band-shaped sensilla or placoid sensilla, clearly different from the initial sensory organs, appear in some taxa as rare exceptions, while other groups display either partial reduction of the receptor organs (Gyrinidae) or reduction of the cuticular parts of the sensilla (Cantharidae).     

The integument and sensilla of the pregenital abdomen of the Desert locust (Schistocerca gregaria) (Orthoptera: Acrididae)

The integument and sensilla of the abdominal segments II–VII, together with the tracheal and nerve supply, are described in so far as they can be seen using whole mounts. Such a method often enables the neurons of the sensilla to be seen clearly and to be associated definitely with a certain cuticular structure. The sensilla are compared with those already known from the palps, and it is concluded that probably all those on the abdomen also occur on the palps.     

七星瓢虫唇须感器分布和统计特征的雌雄差异

应用扫描电镜观察七星瓢虫Coccinella septempunctata的唇须和附节,以便确认在感应农药残留时起主要作用的化学感受器。研究证明分布在下颚须上的感器类型包括两种锥形感器（兼有机械感受器和化学感受器的功能）和一种钟形感器。同时本文首次报道了对这些感器精确的形态测量,明确了其性二型性。对下颚须的测量表明其左右末节的长度差异与七星瓢虫的性别之间有明显的相关性。文中还记述了下唇须上一种新的钟形感器,其在每个下唇须上的数量大约为12～17个。经测量这些机械感受器的平均直径为2.4 μm。早期研究中所记述的锥形感器被证实是下唇须上仅有的化学感应器,并且首次证明这些锥形感器的数量与性别相关,雄性平均为18个,雌性平均为16个。附节上也存在性别差异： 第2跗节跗垫的宽度在雄性中是从后向前逐渐增大,而在雌性中刚好相反。本文还就这些差异的意义以及新发现感器的功能进行了讨论。     

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.     

Morphology,distribution, ultrastructure,and possible function of the external sensilla on antennae and mouthparts of Echinothrips americanus Morgan (Thysanoptera: Thripidae)

To better understand the semiochemical detection and reception mechanisms that involved in intra- and interspecific communication, host selection and mating behavior of Echinothrips americanus (Thysanoptera: Thripidae), scanning electron microscopy was used to examine the external morphology, distribution and ultrastructure of the sensilla on antennae and mouthparts. According to serial SEM images, seven types of antennal sensilla in total were observed in both sexes. Sensilla on mouthparts were observed concentrating on the maxillary palps, labial palps, and paraglossa, which is consistent with the previous conclusions. Possible function of the external sensilla is also discussed.     

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.     

Sensing the Underground - Ultrastructure and Function of Sensory Organs in Root-Feeding Melolontha melolontha (Coleoptera: Scarabaeinae) Larvae

Introduction

Below ground orientation in insects relies mainly on olfaction and taste. The economic impact of plant root feeding scarab beetle larvae gave rise to numerous phylogenetic and ecological studies. Detailed knowledge of the sensory capacities of these larvae is nevertheless lacking. Here, we present an atlas of the sensory organs on larval head appendages of Melolontha melolontha. Our ultrastructural and electrophysiological investigations allow annotation of functions to various sensory structures.

Results

Three out of 17 ascertained sensillum types have olfactory, and 7 gustatory function. These sensillum types are unevenly distributed between antennae and palps. The most prominent chemosensory organs are antennal pore plates that in total are innervated by approximately one thousand olfactory sensory neurons grouped into functional units of three-to-four. In contrast, only two olfactory sensory neurons innervate one sensillum basiconicum on each of the palps. Gustatory sensilla chaetica dominate the apices of all head appendages, while only the palps bear thermo-/hygroreceptors. Electrophysiological responses to CO₂, an attractant for many root feeders, are exclusively observed in the antennae. Out of 54 relevant volatile compounds, various alcohols, acids, amines, esters, aldehydes, ketones and monoterpenes elicit responses in antennae and palps. All head appendages are characterized by distinct olfactory response profiles that are even enantiomer specific for some compounds.

Conclusions

Chemosensory capacities in M. melolontha larvae are as highly developed as in many adult insects. We interpret the functional sensory units underneath the antennal pore plates as cryptic sensilla placodea and suggest that these perceive a broad range of secondary plant metabolites together with CO₂. Responses to olfactory stimulation of the labial and maxillary palps indicate that typical contact chemo-sensilla have a dual gustatory and olfactory function.