Fine structure of antennal mechanosensilla of adult Rhodnius prolixus stål (Hemiptera: Reduviidae)

Each antenna of both sexes of adult Rhodnius prolixus has approximately 570 mechanosensitive neurons that innervate five morphologic types of cuticular mechanosensilla: campaniform sensilla, tapered hairs, trichobothria, and type I and type II bristle sensilla. Each campaniform sensillum and tapered hair is presumably innervated by one mechanosensitive bipolar neuron and probably functions in proprioception. The campaniform sensilla being located at the base of the scape could monitor the position of the antenna. Tapered hairs are found at the distal margin of flagellar segment I and projecting laterally from the bases of the pedicel and scape. They probably provide information about the relative positions of the antennal segments. Seven trichobothrium are located on the pedicel and three on flagellar segment I. Each trichobothrium has a long filamentous hair inserted into the base of a socket that extends inwardly as a cuticular tube and is innervated by one bipolar neuron with a tublar body, a parallel arrangement of microtubules associated with electron-dense material. The trichobothria may respond to small variations in air currents. Type I bristles occur at the base of the antenna and are the most numerous type of mechanosensillum; an average of 452 occur on each antenna of females and 440 on males. The bristle is curved toward the antennal shaft and is serrated distally. Type II bristles are located distally and are the second most numerous type of mechanosensillum; an average of 88 were counted on each antenna of females and 94 on males. The type II bristle is straight with small, longitudinal, external grooves and projects laterally from the antennal shaft. Each type I and II bristle sensillum is innervated by a bipolar neuron whose dendrite is divided into an inner and outer segment. The outer segment is encased by a dendritic sheath which may be highly convoluted and distally contains a tubular body. Two sheath cells are associated with each sensillum. Both types of bristle sensilla have a tactile function. The tubular bodies of both types of bristle sensilla have a complex structure indicating that they are very sensitive. Variations in the amount and arrangement of the electron-dense material at the tip of the tubular bodies may reflect differences in viscoelastic properties that underlie functional characteristics.     

Antennal sensilla and their possible functions in the host-plant selection behaviour of Phenacoccus manihoti (Matile-Ferrero) (Homoptera : Pseudococcidae)

Nine different types of sensilla have been identified on the antenna of the cassava mealybug Phenacoccus manihoti (Homoptera : Pseudococcidae) with scanning and transmission electron microscopes. Trichoid sensilla, distributed on all segments of the antenna and innervated by a single mechanoreceptive dendrite, have the characteristics of exteroceptors. A campaniform sensillum located on the pedicel and one basiconic sensillum on the flagellum have the characteristics of proprioceptors. Coeloconic sensilla, located ventrally on the pedicel and flagellum, related to poreless sensilla with inflexible sockets, have the characteristics of thermo/hygroreceptors. Uniporous sensilla with a mechanoreceptive dendrite (smooth pegs P1 and P2, grooved pegs P3) and multiporous chemosensilla (grooved pegs P4 and P5), present on the tip of the flagellum, have, respectively, the characteristics of gustatory and olfactory receptors. The results of this study seem to suggest that the cassava mealybug has sensory equipment on its antennae that can detect, by olfaction and contact, chemicals released by the plant.     

Fine structure of scolopidia in Johnston's organ of female Aedes aegypti compared with that of the male.

The Johnston's organ of the female mosquito, Aedes aegypti, has only three types of scolopidia: types A, B, and C. It lacks the type D scolopidium of the male's organ. The basic structure and the location of each type in the female are similar to the counterparts in the male's organ. A single scolopidium is composed of a scolopale cell, an envelope cell, a long cap, and a third sheath, in addition to the two electron-dense scolopales produced inside the cytoplasm of two satellite cells. Each scolopidium has either two (type A) or three (type B) sensory cells. A type C scolopidium, mononematic in contrast to the amphinematic types A and B scolopidia, has two sensory cells, a microtubular cap cell, two microtubular accessory cells, and a scolopale cell with an intracellular scolopale. Even though the female Johnston's organ has all the components of the male's organ except for the single type D scolopidium, the female's organ shows relatively poorer organization and development. The female has a smaller and thinner basal plate, shorter and thicker prongs, fewer type A sensory cells, and a shorter flagellar flange, in addition to the overall smaller size of the pedicel.The probable function of each scolopidial type is discussed, especially in connexion with the probable identification of a single auditory sensillum in the male.     

Ultrastructure of the peg and hair sensilla on the antenna of larval Aedes aegypti (L.)

The antenna of fourth instar larvae of Aedes aegypti has one peg organ of a basiconic type innervated by four neurons. The dendrites are ensheathed to near their terminations at the peg tip by an electron-dense dendritic sheath and by a cuticular sheath. They have easy communication by diffusion with the external environment only at the tip through a peripheral ensheathing membrane and six slit-channels. One of the dendrites resembles a tubular body proximally and may be mechanoreceptive. The peg generally appears to be a contact chemoreceptor. There are three antennal hairs of a typical sensillum trichodeum type innervated at the base by one neuron each. An intricate terminal mechanism at the insertion of the dendrite in the hair is described. These are believed to be tactile hairs. There are also three antennal hairs each innervated by two neurons. The dendrite from one terminates at the base similar to that of a tactile hair, and is believed to function in a similar mechanoreceptive manner. The dendrite from the second neuron extends naked along the length of the hair lumen. It is believed to be primarily chemoreceptive, in a slow-acting general sensory function. In all the sensilla there appear to be secretions produced in the junction body regions of the dendrites, and there is evidence for accumulation of secretory materials in the dendritic tips in some of the sensilla.     

Johnston's organ in Neodiprion sertifer (Insecta: Hymenoptera)

The fine structure of Johnston's organ in the pine sawfly, Neodiprion sertifer, was studied by electron microscopy to determine if there exists a dimorphism in this organ corresponding to the sexual dimorphism in antennal shape and surface area. The organ is made up of scolopidia that are ultrastructurally similar to those of other insects. The scolopidia, identical in both sexes, comprise three sensory cells bearing two types of sensory processes: Two are shorter and smaller in diameter than the third, which extends into the cuticle of the membrane connecting pedicel and flagellum and terminates at an epicuticular invagination. The dendrites and sensory processes are surrounded by two types of enveloping (glial) cells-a scolopale cell and an attachment cell. Other enveloping cells occur at different levels of the scolopidium. Sexual dimorphism is evident only in the numbers of scolopidial groups: Males have more groups with fewer scolopidia, but both sexes possess about the same total number of scolopidia.     

Sensilla on the maxillary palps of Helicoverpa armigera caterpillars: in search of the CO(2)-receptor

The ultrastructure of sensilla on the maxillary palps of helicoverpa armigera caterpillars has been investigated in order ot find candidates for CO(2)-receptors. The following sensilla are found on the palps: a) 8 chemosensory pegs at the tip; b), a large distal pore plate; c), a smaller proximal pore plate; d), a digitiform organ; e), a campaniform sensillum; and f), 3 scolopidia. Each chemosensory peg at the tip is innervated by 4-5 sensory neurons. Five of these pegs are most probably contact chemoreceptors, because each has a dendrite with a tubular body. The distal pore plate has a porous cuticle and is innervated by 3 sensory neurons, each of which sends a highly branched dendrite into a large cuticular cavity. The proximal pore plate is made up from two fused organs, has also a porous cuticle, and is innervated by two sensory neurons which send their dendrites into a narrow cuticular channel. The digitiform organ is innervated by one sensory cell which sends a highly lamellated dendrite into a narrow channel within a chip-shaped protrusion of the porous cuticle. For several reasons, the digitiform organ is the most probable candidate for the CO(2)-receptor. Another possible candidate is the distal pore plate.     

六斑月瓢虫触角及感觉器扫描电镜观察

[目的] 明确六斑月瓢虫雌雄成虫触角感觉器种类、分布及形态特征。[方法] 利用扫描电子显微镜对六斑月瓢虫雌、雄成虫触角形态及触角感受器超微结构进行观察。[结果] 六斑月瓢虫成虫触角由柄节、梗节和鞭节组成,柄节长度与宽度显著大于梗节长度与宽度;鞭节分为9个亚节,末端3节横向膨大呈锤状。雌雄成虫触角上共有8种感觉器:刺形感觉器(SC)、毛形感觉器(ST)、锥形感觉器(SB)、腔形感觉器(CaS)、钟形感觉器(CS)、哑铃形感觉器(DS)、香肠形感觉器(SS)及B?hm氏鬃毛感觉器(BB)。以毛形感觉器和刺形感觉器分布最广,遍布触角;B?hm氏鬃毛仅存在于触角柄节与梗节;触角鞭节第9亚节顶端密布7种触角感觉器。六斑月瓢虫雌雄成虫触角长度、触角感觉器类型及分布无显著差异。[结论] 六斑月瓢虫成虫触角上共有8种感觉器,其触角可能具有感知机械刺激、识别化学信息素及感受温湿度变化的作用。本研究为进一步了解六斑月瓢虫触角与其行为间的关系提供基础资料。     

Antennal sense organs of the cockroach, Leucophaea maderae

Adult and nymphal antennae of the cockroach, Leucophaea maderae, contain nine or more different morphological types of sense organs. There is no outwardly apparent sexual dimorphism in adult antennae. Nymphs are dificient in gross numbers of sensilla. Sense organs are classified morphologically by their similarity to known types of sensila and are assigned functions on this basis and preliminary electrophysiological data: Sensilla chaetica (A), thick-walled mechanoreceptive hairs in groups on the antennal base; S. chaetica (B), thick-walled setae which are tactile and probably chemoreceptive, occurring in the antennal base and flagellum; S. trichodea (A), thin-walled chemoreceptive hairs of the flagellum; S. trichodea (B), minute hairs on the scape and pedicel; S. basiconica, thin-walled chemoreceptive pegs, and S. coeloconica (?pit-pegs”?) of the flagellum; S. campaniformia and scolopidia, mechanoreceptors in the base and flagellum; plus Johnston's organ and/or connective chrodotonal organs in the pedicel. Calculations based on absolute counts of sensilla and their known innervation yield an estimate of about 3.3 × 10⁴ sensilla and 10⁵ cells per antenna.     

Fine structure of the terminal organ of the house fly larva,Musca domestica L.

Summary The terminal organs of the cephalic lobes of the house fly larva, Musca domestica L., were studied by scanning and transmission electron microscopy. Six different types of sensilla were found: (1) papilla sensillum, (2) pit sensillum, (3) spot sensillum, (4) modified papilla sensillum, (5) knob sensillum, and (6) scolopidium. The papilla, pit, spot, and modified papilla sensilla have the essential structure of contact chemoreceptors, i.e., the unbranched dendritic tips are exposed externally through a single opening. However, a tubular body, which is a characteristic structure of tactile setae, is also present in some of the dendritic tips. We assume these sensilla serve a dual function—contact chemo- and mechanoreception. The role of the knob sensilla is obscure. The scolopidia present in the dorsal and the terminal organ are probably stress detectors. Two basal bodies occur in the dendritic ciliary region of all sensilla. Both of the basal bodies (except in the scolopidia) give rise to the distal ciliary microtubules as well as the proximal rootlets.This research was supported in part by the Office of Naval Research, PHS Research Grant EC-246 and NIH Training Grant ES-00069. Paper No. 3608 of the North Carolina State University Agricultural Experiment Station journal series. The advise of R. A. Steinbrecht is gratefully acknowledged.     

各型桃蚜触角感器的比较研究

该文对桃蚜Myzus persicae(Sulzer)各型触角感器进行了比较研究。扫描电镜观察结果表明桃蚜具有钟形感器、毛形感器、原生感觉圈和次生感觉圈4种类型的触角感器。桃蚜各型触角感器的最大差异主要表现在次生感觉圈上,雄蚜、雌性母和有翅孤雌蚜具有次生感觉圈,雌性蚜、干母和无翅孤雌蚜无;但雄蚜具有的次生感觉圈数目多于雌性母和有翅孤雌蚜,且不仅仅分布在触角第3节,第4、5节也有分布。本文探讨了桃蚜触角感器在化学生态学上的功能作用,各型触角感器的差异与桃蚜寄主选择、迁移及交配行为的关系,并进一步分析了桃蚜触角感器的性二型现象。     

Structure of the tarso-pretarsal chordotonal organ in the imago of Tineola bisselliella Humm. (Lepidoptera : Tineidae)

The tarso-pretarsal chordotonal organ in the imago of Tineola bisselliella (Lepidoptera .: Tineidae) includes 2 groups of scolopidia. (1) A basal group composed of 2 scolopidia, each with one neuron and one scolopidium with 3 neurons, whose dendrites present a typical structure of cilium; the dendrite becomes dense apically; it is a common characteristic for all the mechanoreceptive dendrites. (2) An apical scolopidium with 3 neurons, whose dendrites have the same size and are covered with a conical cap.The combination of single and triple dendrites is unusual in the limb chordotonal organs. The 2 groups of scolopidia are not in contact. The importance of the support structures and fixation structures is discussed. In the ciliary root region, peculiar desmosomes occur between the dendrite and the scolopale cell. For the apical scolopidium, one attachment cell is seen distally and it appears in close association with the articular membrane.     

Antennal sensilla and their nerve innervation in first‐instar nymphs of Porphyrophora sophorae Arch. (Hemiptera: Coccomorpha: Margarodidae)

ABSTRACT Porphyrophora (Hemiptera: Coccomorpha: Margarodidae) is a genus of soil‐inhabiting scale insects. The antennal sensilla and their innervation in the first‐instar nymphs of Porphyrophora sophorae were studied using light microscopy and scanning and transmission electron microscopy to understand the function of these sensilla and determine the sensillar innervation feature on these small antennae. The results show that the six‐segmented antennae of these nymphs have 20–23 sensilla which can be morphologically classified into seven types, for example, one Böhm's bristle (Bb), one campaniform sensillum (Ca), one Johnston's organ (Jo), 13–16 aporous sensilla trichodea (St), two coeloconic sensilla (Co), one straight multiporous peg (Mp1), and one curvy multiporous peg (Mp2). According to their function, these sensilla can be categorized into three categories: mechanoreceptors, that is, Bb, Ca, Jo, and St; thermo/hygroreceptors, that is, Co only; and chemoreceptors, that is, Mp1 and Mp2. The dendrites that innervate the Mp1, Mp2, and Co sensilla combine to form a large nerve tract (NT1) in the antennal lumen. Because NT1 extends through and out of the antenna, the somata of these neurons are present in the lymph cavity of the insect's head. The dendrites that innervate the mechanoreceptors form another nerve tract (NT2). The somata of these neurons are located inside the scape and pedicel. J. Morphol. 277:1631–1647, 2016. © 2016 Wiley Periodicals, Inc.     

The sensory structures of the antennal flagellum in Hyalesthes obsoletus (Hemiptera: Fulgoromorpha: Cixiidae): A functional reduction?

Despite their relevance as harmful pests on plants of economic importance, Hemiptera Fulgoromorpha have been poorly studied as regards their antennal sensory structures. In particular, the flagellum has been neglected and, therefore, to date there are no data on its structural organization and sensory equipment. In order to fill this gap, we carried out a study on the sensillum types and distribution on the flagellum of the planthopper Hyalesthes obsoletus Signoret, an efficient vector of the stolbur phytoplasma, the cause of various crop diseases. In this cixiid species the antenna is composed of three segments, the scape, an enlarged pedicel and a long flagellum. This latter is made of a single segment and presents a basal, bulb-like enlargement from which two processes arise, a short spur and a long arista. Combining scanning electron microscopy, transmission electron microscopy and focused ion beam investigations, we discovered the presence of a total number of 6 sensilla, belonging to 4 different types: a single scolopidium extending from the bulb to the arista, three sensilla styloconica within the cuticular spur and two different sensilla coeloconica inside the bulb. As far as structural data can suggest, these sensilla might be involved in the perception of mechanical stimuli (possibly air-borne vibrations), temperature and humidity variations and CO₂ concentration. The strong reduction in sensillum number in this species is discussed as possible functional specialization of the flagellum itself. The ultrastructure of the sensilla in the flagellum of a species of Fulgoromorpha is here presented for the first time.     

The development of the sensory neuron pattern in the antennal disc of wild-type and mutant (lz3, ssa) Drosophila melanogaster

The development of the sensory neuron pattern in the antennal disc of Drosophila melanogaster was studied with a neuron-specific monoclonal antibody (22C10). In the wild type, the earliest neurons become visible 3 h after pupariation, much later than in other imaginal discs. They lie in the center of the disc and correspond to the neurons of the adult aristal sensillum. Their axons join the larval antennal nerve and seem to establish the first connection towards the brain. Later on, three clusters of neurons appear in the periphery of the disc. Two of them most likely give rise to the Johnston's organ in the second antennal segment. Neurons of the olfactory third antennal segment are formed only after eversion of the antennal disc (clusters t1-t3). The adult pattern of antennal neurons is established at about 27% of metamorphosis. In the mutant lozenge3 (lz3), which lacks basiconic antennal sensilla, cluster t3 fails to develop. This indicates that, in the wild type, a homogeneous group of basiconic sensilla is formed by cluster t3. The possible role of the lozenge gene in sensillar determination is discussed. The homeotic mutant spineless-aristapedia (ssa) transforms the arista into a leg-like tarsus. Unlike leg discs, neurons are missing in the larval antennal disc of ssa. However, the first neurons differentiate earlier than in normal antennal discs. Despite these changes, the pattern of afferents in the ectopic tarsus appears leg specific, whereas in the non-transformed antennal segments a normal antennal pattern is formed. This suggests that neither larval leg neurons nor early aristal neurons are essential for the outgrowth of subsequent afferents.     

Fine structure and primary sensory projections of sensilla located in the sacculus of the antenna of Drosophila melanogaster

Sensilla lining the inner walls of the sacculus on the third antennal segment of Drosophila melanogaster were studied by light and transmission electron microscopy. The sacculus consists of three chambers: I, II and III. Inside each chamber morphologically distinct groups of sensilla having inflexible sockets were observed. Chamber I contains no-pore sensilla basiconica (np-SB). The lumen of all np-SB are innervated by two neurons, both resembling hygroreceptors. However, a few np-SB contain one additional neuron, presumed to be thermoreceptive. Chamber II houses no-pore sensilla coeloconica (np-SC). All np-SC are innervated by three neurons. The outer dendritic segments of two of these neurons fit tightly to the wall of the lumen and resemble hygroreceptor neurons. A third, more electron-dense sensory neuron, terminates at the base of the sensillum and resembles a thermoreceptor cell. Chamber III of the sacculus is divided into ventral and dorsal compartments, each housing morphologically distinct grooved sensilla (GS). The ventral compartment contains thick GS₁, and the dorsal compartment has slender sensilla GS₂. Ultrastructurally, both GS₁ and GS₂ are doublewalled sensilla with a longitudinal slit-channel system and are innervated by two neurons. The dendritic outer segment of one ofthe two neurons innervates the lumen of the GS and branches. On morphological criteria, we infer this neuron to be olfactory. The other sensory neuron is probably thermoreceptive. Thus, the sacculus in Drosophila has sensilla that are predominantly involved in hygroreception, thermoreception, and olfaction. We have traced the sensory projections of the neurons innervating the sacculus sensilla of chamber III using cobaltous lysine or ethanolic cobalt (II) chloride. The fibres project to the antennal lobes, and at least four glomeruli (VM₃, DA₃ and DL_2-3) are projection areas of sensory neurons from these sensilla. glomerulus DL₂ is a common target for the afferent fibres of the surface sensilla coeloconica and GS, whereas the VM₃, DA₃ and DL₃ glomeruli receive sensory fibres only from the GS.     

红火蚁触角及其上感受器的扫描电镜观察

应用扫描电镜对采自我国广东吴川和广西南宁的红火蚁Solenopsis invicta Buren工蚁、有翅雌蚁和雄蚁触角的形态和感受器类型进行了研究。结果表明,工蚁、雌蚁和雄蚁的触角存在着较大的差异。工蚁和雌蚁的触角为膝状,末端2或3节膨大为棒状,雌蚁的触角11节,比工蚁(10节)的多1节。雄蚁的触角为线状,12节,末端没有棒状结构。工蚁和雌蚁的触角上共有7种感受器,分别为毛形、曲毛形、锥形、腔锥形、坛形、刺形感受器和Böhm氏鬃毛,前6种感受器大多集中在触角末端的棒节,棒节以外的各鞭节上主要是毛形感受器,Böhm氏鬃毛则分布于柄节和梗节的基部,鞭节上无此感受器。雄蚁触角上也有7种感受器,但其分布与工蚁的有较大差异,Böhm氏鬃毛的分布与工蚁和雌蚁的相同,锥形感受器分布在末端2节,坛形感受器分布在末端3节,其余的4种感受器较为均匀地分布在各鞭节,而不是聚集在端节。另外,新发现雄蚁中存在一种与已知的毛型感受器不同的超短毛状感受器,定名为超短毛感受器。最后,对红火蚁的触角及感受器在蚁群社会性行为中的作用进行了讨论。     

Ultrastructure of a possible new type of crustacean cuticular strain receptor in Carcinus maenas (crustacea,decapoda)

A hitherto unknown sensillum type, the “intracuticular sensillum” was identified on the dactyls of the walking legs of the shore crab, Carcinus maenas. Each sensillum is innervated by two sensory cells with dendrites of “scolopidial” (type I) organization. The ciliary segment of the dendrite is 5–6 μm long and contains A-tubules with an electron-dense core and dynein arm-like protuberances; the terminal segment is characterized by densely packed microtubules. The outer dendritic segments pass through the endo- and exocuticle enclosed in a dendritic sheath and a cuticulax tube (canal), which is suspended inside a slit-shaped cavity by cuticular lamellae. The dendrites and the cavity terminate in a cupola-shaped invagination of the epicuticle. External cuticular structures are lacking. Three inner and four to six outer enveloping cells are associated with each intracuticular sensillum. The innermost enveloping cell contains a large scolopale that is connected to the ciliary rootlets inside the inner dendritic segments by desmosomes. Scolopale rods are present in enveloping cell 2. Since type I dendrites and a scolopale are regarded as modality-specific structures of mechanoreceptors, and since no supracuticular endorgan is present, the intracuticular sensilla likely are sensitive to cuticular strains. The intracuticular sensilla should be regarded as analogous to insect campaniform sensilla and arachnid slit sense organs.     

Acetylcholinesterase activity in antennal receptor neurons of the sphinx moth Manduca sexta

Summary We have used a cytochemical technique to investigate the distribution of acetylcholinesterase (AChE) activity in the antenna of the sphinx moth Manduca sexta. High levels of echothiophate-insensitive (presumably intracellular) AChE activity were found in six different types of antennal receptors localized in specific regions of the three antennal segments of the adult moth. Mechanosensory organs in the scape and pedicel, the Böhm bristles and Johnston's organ, are innervated by AChE-positive neurons. In each annulus of the antennal flagellum, AChE-positive neurons are associated with six sensilla chaetica and a peg organ, probably a sensillum styloconicum. At least 112 receptor neurons (8–10 per annulus) innervating the intersegmental membranes between the 14 distalmost annuli also exhibit high levels of echothiophate-resistant AChE. In addition, each annulus has more than 30 AChE-positive somata in the epidermis of the scale-covered (back) side of the flagellum, and 4 AChE-positive somata reside within the first annulus of the flagellum. Since none of the olfactory receptor neurons show a high level of echothiophateresistant AChE activity, and all known mechanoreceptors are AChE-positive, apparently intracellular AChE activity in the antenna correlates well with mechanosensory functions and is consistent with the idea that these cells employ acetylcholine as a neurotransmitter.     

The Johnston's organ of three homopteran species: A comparative ultrastructural study

A transmission electron-microscopy study has been carried out on the pedicel of three homopteran species, with particular focus on the leafhopper Scaphoideus titanus Ball. The two other species, the planthoppers Hyalesthes obsoletus Signoret and Metcalfa pruinosa Say, were investigated in order to compare the ultrastructure of the Johnston's organ (JO) among representatives of the Auchenorrhyncha group. The results showed the presence of a well developed JO located within the pedicel. Depending on the species the JO is made of 25 up to 72 scolopidia arranged in a coronal array. Each scolopidium is connective, heterodynal, amphinematic and hosts three structurally dissimilar sensory neurons. Two of them have a type 1 ciliary segment while the third bears a type 2 cilium. The type 2 dendrite tip is associated with a tubular cap and is longer than the others, ending into the cuticle at the base of the flagellum. Other scolopidia with one or two neurons were found in S. titanus, forming an accessory organ. The presence of such a well developed mechanosensory apparatus is discussed in relation with the lifestyle of the three species.     

Fine structure and primary sensory projections of sensilla located in the labial-palp pit organ of Helicoverpa armigera (Insecta)

The fine structure and primary sensory projections of sensilla located in the labial-palp pit organ of the cotton bollworm Helicoverpa armigera (Insecta, Lepidoptera) are investigated by scanning electron and transmission electron microscopy combined with confocal laser scanning microscopy. The pit organ located on the third segment of the labial palp is about 300 μm deep with a 60-μm-wide opening, each structure containing about 1200 sensilla. Two sensillum types have been found, namely hair-shaped and club-shaped sensilla, located on the upper and lower half of the pit, respectively. Most sensilla possess a single dendrite. The dendrite housed by the club-shaped sensilla is often split into several branches or becomes lamellated in the outer segment. As reported previously, the sensory axons of the sensilla in the labial pit organ form a bundle entering the ipsilateral side of the subesophageal ganglion via the labial palp nerve and project to three distinct areas: the labial pit organ glomerulus in each antennal lobe, the subesophageal ganglion and the ventral nerve cord. In the antennal lobe, the labial pit organ glomerulus is innervated by sensory axons from the labial pit organ only; no antennal afferents target this unit. One neuron has been found extending fine processes into the subesophageal ganglion and innervating the labial palp via one branch passing at the base of the labial palp nerve. The soma of this assumed motor neuron is located in the ipsilateral cell body layer of the subesophageal ganglion. Our results provide valuable knowledge concerning the neural circuit encoding information about carbon dioxide and should stimulate further investigations directed at controlling pest species such as H. armigera.