CO2 sensitive receptors on labial palps of Rhodogastria moths (Lepidoptera: Arctiidae): physiology, fine structure and central projection

Ultrastructural and Descriptive Study on the Adult Body Surface of Heortia vitessoides (Lepidoptera: Crambidae)

Heortia vitessoides Moore, 1885 (Lepidoptera: Crambidae) is an economically important lepidopteran pest that caused severe damage to the plantation area of Aquilaria sinensis (Lour.) Gilg, 1825 (Thymelaeaceae), resulting in extensive defoliation of the trees during an epidemic. In this study, we used scanning electron microscopy (SEM) to analyze the external morphology and ultrastructure of sensilla on various body parts of H. vitessoides. Specifically, seven, four, four, and five types of sensilla were found, respectively, on the antennae, proboscis, labial palps, and legs. We described the types, distributions, and sexual dimorphism of these sensilla on antennae, and found that the number and size of sensilla differed significantly between males and females. This study provides crucial information for future investigations into the function of these sensilla in H. vitessoides.

A mouthpart transcriptome for Spodoptera frugiperda adults: identification of candidate chemoreceptors and investigation of expression patterns

Moth mouthparts, consisting of labial palps and proboscis, not only are the feeding device but also are chemosensory organs for the detection of chemical signals from surrounding environment. Up to now, the chemosensory systems in the mouthpart of moths are largely unknown. Here, we performed systematic analyses of the mouthpart transcriptome of adult Spodoptera frugiperda (Lepidoptera: Noctuidae), a notorious pest that spreads worldwide. A total of 48 chemoreceptors, including 29 odorant receptors (ORs), 9 gustatory receptors (GRs), and 10 ionotropic receptors (IRs), were annotated. Further phylogenetic analyses with these genes and homologs from other insect species determined that specific genes, including ORco, carbon dioxide receptors, pheromone receptor, IR co-receptors, and sugar receptors, were transcribed in the mouthpart of S. frugiperda adults. Subsequently, expression profiling in different chemosensory tissues demonstrated that the annotated ORs and IRs were mainly expressed in S. frugiperda antennae, but one IR was also highly expressed in the mouthparts. In comparison, SfruGRs were mainly expressed in the mouthparts, but 3 GRs were also highly expressed in the antennae or the legs. Further comparison of the mouthpart-biased chemoreceptors using RT-qPCR revealed that the expression of these genes varied significantly between labial palps and proboscises. This study provides the first large-scale description of chemoreceptors in the mouthpart of adult S. frugiperda and provides a foundation for further functional studies of chemoreceptors in the mouthpart of S. frugiperda as well as of other moth species.

Gustation in insects: taste qualities and types of evidence used to show taste function of specific body parts

The insect equivalent of taste buds are gustatory sensilla, which have been found on mouthparts, pharynxes, antennae, legs, wings, and ovipositors. Most gustatory sensilla are uniporous, but not all apparently uniporous sensilla are gustatory. Among sensilla containing more than one neuron, a tubular body on one dendrite is also indicative of a taste sensillum, with the tubular body adding tactile function. But not all taste sensilla are also tactile. Additional morphological criteria are often used to recognize if a sensillum is gustatory. Further confirmation of such criteria by electrophysiological or behavioral evidence is needed. The five canonical taste qualities to which insects respond are sweet, bitter, sour, salty, and umami. But not all tastants that insects respond to easily fit in these taste qualities. Categories of insect tastants can be based not only on human taste perception, but also on whether the response is deterrent or appetitive and on chemical structure. Other compounds that at least some insects taste include, but are not limited to: water, fatty acids, metals, carbonation, RNA, ATP, pungent tastes as in horseradish, bacterial lipopolysaccharides, and contact pheromones. We propose that, for insects, taste be defined not only as a response to nonvolatiles but also be restricted to responses that are, or are thought to be, mediated by a sensillum. This restriction is useful because some of the receptor proteins in gustatory sensilla are also found elsewhere.

Identification and expression analysis of chemosensory receptors in the tarsi of fall armyworm, Spodoptera frugiperda (J. E. Smith)

Chemosensation of tarsi provides moths with the ability to detect chemical signals which are important for food recognition. However, molecular mechanisms underlying the chemosensory roles of tarsi are still unknown. The fall armyworm Spodoptera frugiperda is a serious moth pest that can damage many plants worldwide. In the current study, we conducted transcriptome sequencing with total RNA extracted from S. frugiperda tarsi. Through sequence assembly and gene annotation, 23 odorant receptors 10 gustatory receptors and 10 inotropic receptors (IRs) were identified. Further phylogenetic analysis with these genes and homologs from other insect species indicated specific genes, including ORco, carbon dioxide receptors, fructose receptor, IR co-receptors, and sugar receptors were expressed in the tarsi of S. frugiperda. Expression profiling with RT-qPCR in different tissues of adult S. frugiperda showed that most annotated SfruORs and SfruIRs were mainly expressed in the antennae, and most SfruGRs were mainly expressed in the proboscises. However, SfruOR30, SfruGR9, SfruIR60a, SfruIR64a, SfruIR75d, and SfruIR76b were also highly enriched in the tarsi of S. frugiperda. Especially SfruGR9, the putative fructose receptor, was predominantly expressed in the tarsi, and with its levels significantly higher in the female tarsi than in the male ones. Moreover, SfruIR60a was also found to be expressed with higher levels in the tarsi than in other tissues. This study not only improves our insight into the tarsal chemoreception systems of S. frugiperda but also provides useful information for further functional studies of chemosensory receptors in S. frugiperda tarsi.

Gustatory function of sensilla chaetica on the labial palps and antennae of three tortricid moths (Lepidoptera: Tortricidae)

In adult Lepidoptera the labial palps are best known for their role in CO₂ detection, but they can also bear sensilla chaetica which function is unknown. The number and distribution of sensilla chaetica in labial palps was studied using a bright field microscope. To determine if these sensilla have a gustatory function, we performed single sensillum electrophysiology recordings from palp and antennal sensilla of adult moths of Cydia pomonella (L.), Grapholita molesta (Busck) and Lobesia botrana (Denis and Shieffermüller). Each sensillum was stimulated with 3 doses of one of four test stimulus (sucrose, fructose, KCl and NaCl). Overall, responses (spikes/s^-1) increased with dose, and were higher in the palps than in the antennae, and higher to sugars than to salts. With sugars the response increased with concentration in the palp but not in the antenna. With salts there was a drop in response at the intermediate concentration. The number and position of sensilla chaetica on labial palps was variable among individuals. Sensilla were located in the most exposed areas of the palp. Differences in sensilla distribution were detected between species. Such differences among species and between palps and antenna suggest that taste sensilla on the palps have an unforeseen role in adaptation.

The mouthparts and sensilla of the adult tomato leafminer moth, Tuta absoluta (Meyrick, 1917) (Lepidoptera: Gelechiidae)

In this study, investigation of the morphology and distribution of mouthpart sensilla in the adult tomato leafminer moth, Tuta absoluta Meyrick (Lepidoptera: Gelechiidae), using a scanning electron microscope (SEM) is performed. SEM studies examining the mouthparts of Gelechiidae have been noted to be rare; moreover, there have been few investigations on the sensory structures of adult T. absoluta, despite the fact that it is an important pest. In adult T. absoluta, mouthpart structures include the labrum, maxillae comprising two maxillary galeae that form a proboscis, two maxillary palps, and two labial palps. Mandibles were not observed in all studied specimens (30 adults). The proboscis resembles that in other lepidopterous insects and is 1.48 ± 0.08 mm long. Its outer surface is covered with lamellar scales, dense microtrichia, and four sensillum types, that are, aporous sensilla chaetica, uniporous sensilla chaetica, uniporous sensilla styloconica, and aporous sensilla squamiformia. Uniporous sensilla basiconica were also found on the internal face of the proboscis. The labial palps are three-segmented and are known to bear three types of sensilla: squamiformia (present on all three segments), campaniformia, and coeloconica. In this study, we focus on the functional importance of the morphology and distribution of sensilla campaniformia on these labial palps.

Fine Structure and Olfactory Reception of the Labial Palps of Spodoptera frugiperda

The olfactory system of insects is essential in many crucial behaviors, such as host seeking, mate recognition, and locating oviposition sites. Lepidopteran moths possess two main olfactory organs, including antennae and labial palps. Compared to antennae, the labial palps are relatively specific and worthy of further investigation due to the labial-palp pit organ (LPO), which contains a large number of sensilla located on the tip segment. The fall armyworm, Spodoptera frugiperda, is a worldwide lepidopteran pest, which can damage more than 350 plants and cause significant economic losses. In this study, we surveyed the structure of the labial palps and LPO of S. frugiperda using a super-high magnification lens zoom 3D microscope. Then, the distribution and fine structure of sensilla located in the LPO of S. frugiperda were investigated using scanning electron microscopy. Subsequently, the electrophysiological responses of labial palps to CO₂ and 29 plant volatiles were recorded by using electrolabialpalpography. Our results showed the fine structure of labial palps, the LPO, and the sensilla located in the LPO of S. frugiperda. Moreover, we demonstrated that the labial palps are olfactory organs that respond to both CO₂ and other volatile compounds. Our work established a foundation for further study of the roles of labial palps in insect olfactory related behaviors. Further investigations on the function of labial palps and their biological roles together with CO₂ and volatile compound responses in S. frugiperda are necessary, as they may provide better insect behavioral regulators for controlling this pest.

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

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

Neuronal architecture of the second-order CO2 pathway in the brain of a noctuid moth

Many insects possess the ability to detect fine fluctuations in the environmental CO₂ concentration. In herbivorous species, plant-emitted CO₂, in combination with other sensory cues, affect many behaviors including foraging and oviposition. In contrast to the comprehensive knowledge obtained on the insect olfactory pathway in recent years, we still know little about the central CO₂ system. By utilizing intracellular labeling and mass staining, we report the neuroanatomy of projection neurons connected with the CO₂ sensitive antennal-lobe glomerulus, the labial pit organ glomerulus (LPOG), in the noctuid moth, Helicoverpa armigera. We identified 15 individual LPOG projection neurons passing along different tracts. Most of these uniglomerular neurons terminated in the lateral horn, a previously well-described target area of plant-odor projection neurons originating from the numerous ordinary antennal-lobe glomeruli. The other higher-order processing area for odor information, the calyces, on the other hand, was weakly innervated by the LPOG neurons. The overlapping LPOG terminals in the lateral horn, which is considered important for innate behavior in insects, suggests the biological importance of integrating the CO₂ input with plant odor information while the weak innervation of the calyces indicates the insignificance of this ubiquitous cue for learning mechanisms.

Morphology and histology of vom Rath's organ in brush-footed butterflies (Lepidoptera: Nymphalidae)

Vom Rath’s organ, located at the distal end of the third segment of the labial palp, is one of the recognized synapomorphies of Lepidoptera (Insecta). Information about the structural and histological morphology of this organ is sparse. The structure of vom Rath’s organ in four species of Nymphalidae, three frugivorous: Fountainea ryphea (Charaxinae: Anaeini), Morpho helenor achillaena (Satyrinae: Morphini) and Hamadryas epinome (Biblidinae: Ageroniini), and the nectarivorous species Aeria olena (Danainae: Ithomiini) is described by means of scanning electron microscopy and histology. The species showed significant differences in the cavity shape, setal morphology and arrangement, opening shape and location, associated with the organization of cell groups, type of axon, and degree of development. These differences do not seem to be related to feeding habit. No cell groups were found in Actinote thalia (Heliconiinae: Acraeini) and Heliconius erato phyllis (Heliconiinae: Heliconiini), and for the first time the absence of vom Rath’s organ is documented in the clade Ditrysia. A terminology is proposed to improve understanding of the organ morphology, with an extensive analysis of the previous descriptions.

Revisiting the Labial Pit Organ Pathway in the Noctuid Moth, Helicoverpa armigera

Lepidopteran species detect CO2 via a specialized organ located on the peripheral segment of the labial palps, the labial palp pit organ (LPO). Based on tracing of LPO sensory neurons targeting one distinct antennal-lobe glomerulus, Kent and her colleagues described the projections originating from the LPO in the sphinx moth as "an accessory olfactory pathway in Lepidoptera" already in the 1980 ties. In spite of similar reports from studies of other lepidopteran species, however, it has been an unresolved issue whether additional termination areas of the labial nerve, such as the gnathal ganglion (GNG) and the ventral nerve cord, are actually output sites of LPO neurons. Since the previous studies have interpreted slightly differently about the projection pattern occurring from the classical mass staining, we performed selective mass staining from the inside of the pit and from the outer surface of the peripheral palp. The results demonstrated that the LPO sensory neurons project exclusively to the LPO glomerulus (LPOG), whereas the non-LPO sensory neurons target the GNG and the ventral nerve cord. Additional iontophoretic staining of individual LPO sensory neurons, performed from the LPO and the LPOG, showed three morphological neuron types: one bilateral targeting the LPOG in both antennal lobes, one unilateral targeting the ipsilateral LPOG only, and one contralateral targeting the LPOG in the other antennal lobe. Finally, to explore putative differences in the projection pattern of neurons housed by two previously reported sensillum types in the pit, i.e., hair-shaped sensilla located distally and club-shaped sensilla located proximally, we performed mass staining from two different levels of the peripheral palp. We found a projection pattern implying stronger innervation of the ipsi- than the contralateral LPOG in both staining experiments.

Identification of Candidate Chemosensory Receptors in the Antennae of the Variegated Cutworm, Peridroma saucia Hübner, Based on a Transcriptome Analysis

Insect chemoreception, including olfaction and gustation, involves several families of genes, including odorant receptors (ORs), ionotropic receptors (IRs), and gustatory receptors (GRs). The variegated cutworm Peridroma saucia Hübner (Lepidoptera: Noctuidae) is a worldwide agricultural pest that causes serious damage to many crops. To identify such olfactory and gustatory receptors in P. saucia, we performed a systematic analysis of the antennal transcriptome of adult P. saucia through Illumina sequencing. A total of 103 candidate chemosensory receptor genes were identified, including 63 putative ORs, 10 GRs, 24 IRs, and 6 ionotropic glutamate receptors (iGluRs). Phylogenetic relationships of these genes with those from other species were predicted, and specific chemosensory receptor genes were analyzed, including ORco, pheromone receptors (PRs), sugar receptors, CO2 receptors, and IR co-receptors. RT-qPCR analyses of these annotated genes revealed that 6 PRs were predominantly expressed in male antennae; 3 ORs, 1 GR, 2 IRs, and 2 iGluRs had higher expression levels in male than in female antennae; and 14 ORs, 1 GR, and 3 IRs had higher expression levels in female than in male antennae. This research increases the understanding of olfactory and gustatory systems in the antennae of P. saucia and facilitates the discovery of novel strategies for controlling this pest.

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

Background

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

Results

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

Conclusions

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

Electronic supplementary material

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

Glomerular Organization of the Antennal Lobes of the Diamondback Moth, Plutella xylostella L

The antennal lobe of the moth brain is the primary olfactory center processing information concerning pheromones and plant odors. Plutella xylostella is a major worldwide pest of cruciferous vegetables and its behavior is highly dependent on their olfactory system. However, detailed knowledge of the anatomy and function of the P. xylostella olfactory system remains limited. In the present study, we present the 3-Dimentional (3-D) map of the antennal lobe of P. xylostella, based on confocal microscopic analysis of glomerular segmentation and Neurobiotin backfills of Olfactory Receptor Neurons (ORNs). We identified 74–76 ordinary glomeruli and a macroglomerular complex (MGC) situated at the entrance of the antennal nerve in males. The MGC contained three glomeruli. The volumes of glomeruli in males ranged from 305.83 ± 129.53 to 25440.00 ± 1377.67 μm³. In females, 74–77 glomeruli were found, with the largest glomerulus ELG being situated at the entrance of the antennal nerve. The volumes of glomeruli in females ranged from 802.17 ± 95.68 to 8142.17 ± 509.46 μm³. Sexual dimorphism was observed in anomalous supernumerary, anomalous missing, shape, size, and array of several of the identified glomeruli in both sexes. All glomeruli, except one in the antennal lobe (AL), received projections of antennal ORNs. The glomeruli PV1 in both sexes received input from the labial palp nerve and was assumed as the labial pit organ glomerulus (LPOG). These results provide a foundation for better understanding of coding mechanisms of odors in this important pest insect.

Sensilla on six olfactory organs of male Eogystia hippophaecolus (Lepidoptera: Cossidae)

Eogystia hippophaecolus (Hua et al.) is an important boring pest that primarily damages sea buckthorn, causing large ecological and economic damages in China. In this study, we used scanning electron microscopy to investigate the sensilla on six olfactory tissues of male E. hippophaecolus: antennae, labial palps, external genitals, propodeum, mesopodium, and metapedes. On the antennae, two types of sensillum trichodea, two types of sensillum basiconica, a type of sensillum coeloconica, and Böhm's bristles were found. The labial palps had sensilla trichodea and chaetica. On the external genitals, three types of sensilla trichodea were the only sensilla. Böhm bristles were found on the base of the tibia and at the terminus of the tarsus. Most sensilla were distributed on the tarsus of the three pairs of legs, and notably, a majority of which were sensilla trichodea and sensilla chaetica, were on the pretarsus and telotarsus of the three pairs of legs. In this study, the distribution, number, type, and morphology of the sensilla on six olfactory organs of E. hippophaecolus were determined, which established the foundation for a future immunohistochemical search of olfactory proteins. RESEARCH HIGHLIGHTS: Eogystia hippophaecolus is a serious pest of seabuckthorn. The distribution, number, type, and morphology of olfactory sensilla of E. hippophaecolus are determined. The function and distribution of sensilla are compared with those of other insects.

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

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

Effects of elevated CO2 on the fitness and potential population damage of Helicoverpa armigera based on two-sex life table

We evaluated the direct effects of three different atmospheric CO2 concentrations (380 ppm, 550 ppm and 750 ppm) on the population parameters of the cotton bollworm, Helicoverpa armigera fed an artificial diet. The life history and fitness of H. armigera were analyzed using an age-stage, two-sex life table. Our results showed significantly longer larval durations and lower female pupal weight under elevated CO2 than under ambient CO2. Additionally, the fecundity of H. armigera was lower under elevated CO2 than under ambient CO2. H. armigera reared under elevated CO2 conditions showed lower intrinsic and finite rates of increase but higher net consumption and finite consumption rates than H. armigera reared under ambient CO2 conditions. According to population projections, a much smaller total population size and reduced consumption capacities would be expected in an elevated CO2 atmosphere due to higher mortality and lower growth rate compared with ambient CO2 levels. These results indicate that the fitness of and potential damage caused by H. armigera would be affected by increased CO2 relative to ambient CO2 concentrations. Additional studies on the long-term direct and indirect effects of elevated CO2 levels on H. armigera are still needed.

Functional validation of the carbon dioxide receptor in labial palps of Helicoverpa armigera moths

Adult moths possess an organ in their labial palps, the labial-palp pit organ, which is specialized for sensing carbon dioxide (CO2). They use CO2 as a cue to detect healthy plants and find food or lay eggs on them. The molecular bases of the CO2 receptor in Drosophila melanogaster and Aedes aegypti have been reported, but the molecular mechanisms of the CO2 receptor in Lepidoptera remains elusive. In this study, we first re-examined three putative Helicoverpa armigera CO2 gustatory receptor genes (HarmGr1, HarmGr2, and HarmGr3), and then analyzed expression patterns of them. RT-PCR results verified they were predominantly expressed in the labial palps of H. armigera. Thus, we used in situ hybridization to localize the expression of three genes in the labial palps. We found that all three genes were co-expressed in the same cells of the labial palps. Next, we employed the Xenopus laevis oocyte expression system and the two-electrode voltage-clamp recording to study the function of the three genes. Results showed that only oocytes co-expressing HarmGr1 and HarmGr3 or co-expressing HarmGr1, HarmGr2 and HarmGr3 gave robust responses to NaHCO3. Finally, we confirmed that the sensory cells in labial palps of both females and males show dose dependent responses to CO2 stimuli by using single sensillum recording. Our work uncovers that HarmGr1 and HarmGr3 are indispensable and sufficient for CO2 sensing in labial palps of H. armigera.

Carbon dioxide detection in adult Odonata

The present paper shows, by means of single-cell recordings, responses of antennal sensory neurons of the damselfly Ischnura elegans when stimulated by air streams at different CO2 concentrations. Unlike most insects, but similarly to termites, centipedes and ticks, Odonata possess sensory neurons strongly inhibited by CO2, with the magnitude of the off-response depending upon the CO2 concentration. The Odonata antennal sensory neurons responding to CO2 are also sensitive to airborne odors; in particular, the impulse frequency is increased by isoamylamine and decreased by heptanoic and pentanoic acid. Further behavioral investigations are necessary to assign a biological role to carbon dioxide detection in Odonata.

Attraction to Carbon Dioxide from Feeding Resources and Conspecific Neighbours in Larvae of the Rhinoceros Beetle Trypoxylus dichotomus

Saprophagous (feeding on decaying matter) insects often use carbon dioxide (CO₂) as a cue for finding food. Humus-feeding larvae of the giant rhinoceros beetle Trypoxylus dichotomus exhibit a clumped distribution in natural microhabitats, but the mechanisms driving the distribution were unknown. Herein, I examined whether larvae use CO₂ as a cue for fermented humus and aggregate in the vicinity of the food. I found that (i) larvae of T. dichotomus are strongly attracted to CO₂, (ii) larvae orient toward highly fermented humus when given a choice between highly and poorly fermented humus, (iii) the highly fermented humus emits more CO₂ than the poorly fermented humus, and (iv) larvae grow larger when fed highly fermented humus rather than poorly fermented humus. The clumped distribution of larvae is probably formed along the concentration gradient of CO₂ induced by heterogeneity of fermented organic materials in soil. My laboratory experiments also revealed that larvae are chemically attracted to each other. Moreover, CO₂ concentrations in soil were increased by the larval respiration, and small amounts of CO₂ (much less than emitted during respiration by a single larva) were sufficient for larval attraction. These results suggest that not only response to fermented food resources, but also respiratory CO₂ from conspecifics may lead to aggregation. Enhanced densities resulted in reduced weight gain under experimental conditions. However, exploiting a high-value resource at enhanced densities still led to greater body weight compared to individually exploiting a low-value resource. This demonstrates the adaptive value of the response to CO₂ sources in this species.

Carbon dioxide receptor genes in cotton bollworm Helicoverpa armigera

Carbon dioxide (CO2) is important in insect ecology, eliciting a range of behaviours across different species. Interestingly, the numbers of CO2 gustatory receptors (GRs) vary among insect species. In the model organism Drosophila melanogaster, two GRs (DmelGR21a and DmelGR63a) have been shown to detect CO2. In the butterfly, moth, beetle and mosquito species studied so far, three CO2 GR genes have been identified, while in tsetse flies, four CO2 GR genes have been identified. In other species including honeybees, pea aphids, ants, locusts and wasps, no CO2 GR genes have been identified from the genome. These genomic differences may suggest different mechanisms for CO2 detection exist in different insects but, with the exception of Drosophila and mosquitoes, limited attention has been paid to the CO2 GRs in insects. Here, we cloned three putative CO2 GR genes from the cotton bollworm Helicoverpa armigera and performed phylogenetic and expression analysis. All three H. armigera CO2 GRs (HarmGR1, HarmGR2 and HarmGR3) are specifically expressed in labial palps, the CO2-sensing tissue of this moth. HarmGR3 is significantly activated by NaHCO3 when expressed in insect Sf9 cells but HarmGR1 and HarmGR2 are not. This is the first report characterizing the function of lepidopteran CO2 receptors, which contributes to our general understanding of the molecular mechanisms of insect CO2 gustatory receptors.

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.

Sensory organs of the antenna of two Fannia species (Diptera: Fanniidae)

The latrine fly (Fannia scalaris) and lesser house fly (Fannia canicularis) are ubiquitous fanniid species of forensic and medical importance. The external morphology of sensilla on the antennae is studied using a stereoscopic microscope and scanning electron microscope, and the internal structure of the antennae is revealed by paraffin sections under the light microscope and laser scanning confocal microscope. Only grooved bristles are found on the scape and pedicel. Four major types of surface sensilla are found on the antennal funiculus and are classified as: (a) trichoid sensilla, (b) two subtypes of basiconic sensilla, (c) clavate sensilla, and (d) coeloconic sensilla. Density of each sensilla type of the two species is calculated. A remarkable difference is observed that the higher density of clavate sensilla occurs on the posteroventral surface in F. scalaris, whereas they are on the anterodorsal surface in F. canicularis. Several cuticular depressions that were previously known as "olfactory pits" are observed on the funiculus of both species. Combining with previous studies, they are suggested to be classified into two types: pit and sacculus. Pit is single-chambered and filled with one type of sensilla, whereas the sacculus is a multi-chambered pit organ containing several types of sensilla. Surprisingly, the pits observed in our study are mostly situated at the lateral region of the antennal funiculus, whereas in other flies, they are found on the posteroventral or anterodorsal surface of antennal funiculus. In addition, the sacculus found in our study houses only basiconic-like sensilla, which is remarkably different from other findings. Moreover, observation under light microscope indicates that previous data of the complexity and the number of the "olfactory pits," which are an important type of efficient sensory convergence, could be underestimated. Thus, more precise methods such as paraffin section or transmission electron microscope are suggested for further study.

Candidate chemosensory genes in female antennae of the noctuid moth Spodoptera littoralis

Chemical senses are crucial for all organisms to detect various environmental information. Different protein families, expressed in chemosensory organs, are involved in the detection of this information, such as odorant-binding proteins, olfactory and gustatory receptors, and ionotropic receptors. We recently reported an Expressed Sequence Tag (EST) approach on male antennae of the noctuid moth, Spodoptera littoralis, with which we could identify a large array of chemosensory genes in a species for which no genomic data are available.

Here we describe a complementary EST project on female antennae in the same species. 18,342 ESTs were sequenced and their assembly with our previous male ESTs led to a total of 13,685 unigenes, greatly improving our description of the S. littoralis antennal transcriptome. Gene ontology comparison between male and female data suggested a similar complexity of antennae of both sexes. Focusing on chemosensation, we identified 26 odorant-binding proteins, 36 olfactory and 5 gustatory receptors, expressed in the antennae of S. littoralis. One of the newly identified gustatory receptors appeared as female-enriched. Together with its atypical tissue-distribution, this suggests a role in oviposition. The compilation of male and female antennal ESTs represents a valuable resource for exploring the mechanisms of olfaction in S. littoralis.

An Expressed Sequence Tag collection from the male antennae of the Noctuid moth Spodoptera littoralis: a resource for olfactory and pheromone detection research

BACKGROUND

Nocturnal insects such as moths are ideal models to study the molecular bases of olfaction that they use, among examples, for the detection of mating partners and host plants. Knowing how an odour generates a neuronal signal in insect antennae is crucial for understanding the physiological bases of olfaction, and also could lead to the identification of original targets for the development of olfactory-based control strategies against herbivorous moth pests. Here, we describe an Expressed Sequence Tag (EST) project to characterize the antennal transcriptome of the noctuid pest model, Spodoptera littoralis, and to identify candidate genes involved in odour/pheromone detection.

RESULTS

By targeting cDNAs from male antennae, we biased gene discovery towards genes potentially involved in male olfaction, including pheromone reception. A total of 20760 ESTs were obtained from a normalized library and were assembled in 9033 unigenes. 6530 were annotated based on BLAST analyses and gene prediction software identified 6738 ORFs. The unigenes were compared to the Bombyx mori proteome and to ESTs derived from Lepidoptera transcriptome projects. We identified a large number of candidate genes involved in odour and pheromone detection and turnover, including 31 candidate chemosensory receptor genes, but also genes potentially involved in olfactory modulation.

CONCLUSIONS

Our project has generated a large collection of antennal transcripts from a Lepidoptera. The normalization process, allowing enrichment in low abundant genes, proved to be particularly relevant to identify chemosensory receptors in a species for which no genomic data are available. Our results also suggest that olfactory modulation can take place at the level of the antennae itself. These EST resources will be invaluable for exploring the mechanisms of olfaction and pheromone detection in S. littoralis, and for ultimately identifying original targets to fight against moth herbivorous pests.

Feeding mechanisms of adult Lepidoptera: structure, function, and evolution of the mouthparts

The form and function of the mouthparts in adult Lepidoptera and their feeding behavior are reviewed from evolutionary and ecological points of view. The formation of the suctorial proboscis encompasses a fluid-tight food tube, special linking structures, modified sensory equipment, and novel intrinsic musculature. The evolution of these functionally important traits can be reconstructed within the Lepidoptera. The proboscis movements are explained by a hydraulic mechanism for uncoiling, whereas recoiling is governed by the intrinsic proboscis musculature and the cuticular elasticity. Fluid uptake is accomplished by the action of the cranial sucking pump, which enables uptake of a wide range of fluid quantities from different food sources. Nectar-feeding species exhibit stereotypical proboscis movements during flower handling. Behavioral modifications and derived proboscis morphology are often associated with specialized feeding preferences or an obligatory switch to alternative food sources.

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 CO(2) 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.

Olfactory perception: receptors, cells, and circuits

Remarkable advances in our understanding of olfactory perception have been made in recent years, including the discovery of new mechanisms of olfactory signaling and new principles of olfactory processing. Here, we discuss the insight that has been gained into the receptors, cells, and circuits that underlie the sense of smell.

Neural detection of gases--carbon dioxide, oxygen--in vertebrates and invertebrates

Carbon dioxide (CO(2)) and oxygen (O(2)) are important cues that can signal the presence of food, predators, and environmental stress. Here we will review recent studies on the mechanisms of how the olfactory system detects these two molecules. In both vertebrates and invertebrates, the two molecules are detected by subsets of specialized olfactory neurons. In addition, the signal transduction cascades for sensing these two gases appear to be different from those for sensing typical odorants. CO(2) and O(2) signals can evoke stereotypical innate behaviors such as attraction and avoidance in many animal species. Future studies on the neural pathways underlying CO(2) and O(2) sensing may shed light on the circuit mechanisms of these behaviors.

Hybrid neurons in a microRNA mutant are putative evolutionary intermediates in insect CO2 sensory systems

Carbon dioxide (CO2) elicits different olfactory behaviors across species. In Drosophila, neurons that detect CO2 are located in the antenna, form connections in a ventral glomerulus in the antennal lobe, and mediate avoidance. By contrast, in the mosquito these neurons are in the maxillary palps (MPs), connect to medial sites, and promote attraction. We found in Drosophila that loss of a microRNA, miR-279, leads to formation of CO2 neurons in the MPs. miR-279 acts through down-regulation of the transcription factor Nerfin-1. The ectopic neurons are hybrid cells. They express CO2 receptors and form connections characteristic of CO2 neurons, while exhibiting wiring and receptor characteristics of MP olfactory receptor neurons (ORNs). We propose that this hybrid ORN reveals a cellular intermediate in the evolution of species-specific behaviors elicited by CO2.

Roles and effects of environmental carbon dioxide in insect life

Carbon dioxide (CO(2)) is a ubiquitous sensory cue that plays multiple roles in insect behavior. In recent years understanding of the well-known role of CO(2) in foraging by hematophagous insects (e.g., mosquitoes) has grown, and research on the roles of CO(2) cues in the foraging and oviposition behavior of phytophagous insects and in behavior of social insects has stimulated interest in this area of insect sensory biology. This review considers those advances, as well as some of the mechanistic bases of the modulation of behavior by CO(2) and important progress in our understanding of the detection and CNS processing of CO(2) information in insects. Finally, this review briefly addresses how the ongoing increase in atmospheric CO(2) levels may affect insect life.

Electrophysiological responses from receptor neurons in mosquito maxillary palp sensilla

We recently completed an electrophysiological study of the receptor neurons found in the sensilla basiconica on the maxillary palps of mosquitoes. Our results describe a class of receptor neurons whose properties could provide the afferent input required for some aspects of CO2-modulated host-locating behaviour. First, these neurons have apparent thresholds (150-300 ppm) which are at, or below, the concentration of CO2 (300-330 ppm) normally reported for ambient air. Second, their concentration-response functions are steep, such that small (50 ppm) fluctuations in concentration elicit reliable changes in activity. Third, they behave like absolute CO2 detectors in that their ability to respond to step increases in CO2 concentration is little influenced by the background concentration of CO2. And fourth, a linear extrapolation of the observed response function to the levels that might be expected near vertebrate hosts suggests that these neurons have sufficient dynamic range to cover those CO2 concentrations that should be encountered during a large portion of the behaviour likely involved in host location. The mosquito CO2 receptor neuron thus has an appropriately low threshold and a steep concentration-response function, it is not desensitized by ambient levels of stimulation, and it has a dynamic range appropriate for the distribution of CO2 concentrations expected in the environment. In addition, this sensillum contains two other receptor neurons, neither of which respond to CO2. One of these neurons responds to stimulation with very low doses of another behaviourally relevant compound, 1-octen-3-ol.

Olfactory receptors on the maxillary palps of small ermine moth larvae: evolutionary history of benzaldehyde sensitivity

In lepidopterous larvae the maxillary palps contain a large portion of the sensory equipment of the insect. Yet, knowledge about the sensitivity of these cells is limited. In this paper a morphological, behavioral, and electrophysiological investigation of the maxillary palps of Yponomeuta cagnagellus (Lepidoptera: Yponomeutidae) is presented. In addition to thermoreceptors, CO₂ receptors, and gustatory receptors, evidence is reported for the existence of two groups of receptor cells sensitive to plant volatiles. Cells that are mainly sensitive to (E)-2-hexenal and hexanal or to (Z)-3-hexen-1-ol and 1-hexanol were found. Interestingly, a high sensitivity for benzaldehyde was also found. This compound is not known to be present in Euonymus europaeus, the host plant of the monophagous Yponomeuta cagnagellus, but it is a prominent compound in Rosaceae, the presumed hosts of the ancestors of Y. cagnagellus. To elucidate the evolutionary history of this sensitivity, and its possible role in host shifts, feeding responses of three Yponomeuta species to benzaldehyde were investigated. The results confirm the hypothesis that the sensitivity to benzaldehyde evolved during the ancestral shift from Celastraceae to Rosaceae and can be considered an evolutionary relict, retained in the recently backshifted Celastraceae-specialist Y. cagnagellus.

A single population of olfactory sensory neurons mediates an innate avoidance behaviour in Drosophila

All animals exhibit innate behaviours in response to specific sensory stimuli that are likely to result from the activation of developmentally programmed neural circuits. Here we observe that Drosophila exhibit robust avoidance to odours released by stressed flies. Gas chromatography and mass spectrometry identifies one component of this 'Drosophila stress odorant (dSO)' as CO2. CO2 elicits avoidance behaviour, at levels as low as 0.1%. We used two-photon imaging with the Ca2+-sensitive fluorescent protein G-CaMP to map the primary sensory neurons governing avoidance to CO2. CO2 activates only a single glomerulus in the antennal lobe, the V glomerulus; moreover, this glomerulus is not activated by any of 26 other odorants tested. Inhibition of synaptic transmission in sensory neurons that innervate the V glomerulus, using a temperature-sensitive Shibire gene (Shi(ts)), blocks the avoidance response to CO2. Inhibition of synaptic release in the vast majority of other olfactory receptor neurons has no effect on this behaviour. These data demonstrate that the activation of a single population of sensory neurons innervating one glomerulus is responsible for an innate avoidance behaviour in Drosophila.

Sensory processing of ambient CO2 information in the brain of the moth Manduca sexta

Insects use information about CO2 to perform vital tasks such as locating food sources. In certain moths, CO2 is involved in oviposition behavior. The labial palps of adult moths that feed as adults have a pit organ containing sensory receptor cells that project into the antennal lobes, the sites of primary processing of olfactory information in the brain. In the moth Manduca sexta and certain other species of Lepidoptera, these receptor cells in the labial-palp pit organ have been shown to be tuned to CO2, and their axons project to a single, identified glomerulus in the antennal lobe, the labial-palp pit organ glomerulus. At present, however, nothing is known about the function of this glomerulus or how CO2 information is processed centrally. We used intracellular recording and staining to reveal projection (output) neurons in the antennal lobes that respond to CO2 and innervate the labial-palp pit organ glomerulus. Our results demonstrate that this glomerulus is the site of first-order processing of sensory information about ambient CO2. We found three functional types of CO2-responsive neurons (with their cell bodies in the antennal lobe or the protocerebrum) that provide output from the antennal lobe to higher centers in the brain. Some physiological characteristics of those neurons are described.

Development and connectivity of olfactory pathways in the brain of the lobster Homarus americanus

The main output pathways from the olfactory lobes (primary olfactory centers) and accessory lobes (higher-order integrative areas) of decapod crustaceans terminate within both of the main neuropil regions of the lateral protocerebrum: the medulla terminalis and the hemiellipsoid body. The present study examines the morphogenesis of the lateral protocerebral neuropils of the lobster, Homarus americanus, and the development of their neuronal connections with the paired olfactory and accessory lobes. The medulla terminalis was found to emerge during the initial stages of embryogenesis and to be the target neuropil of the output pathway from the olfactory lobe. In contrast, the hemiellipsoid body is first apparent during mid-embryonic development and is innervated by the output pathway from the accessory lobe. The dye injections used to elucidate these pathways also resulted in the labeling of a previously undescribed pathway linking the olfactory lobe and the ventral nerve cord. To increase our understanding of the morphology of the olfactory pathways in H. americanus we also examined the connectivity of the lateral protocerebral neuropils of embryonic lobsters. These studies identified several interneuronal populations that may be involved in the higher-order processing of olfactory inputs. In addition, we examined the neuroanatomy of ascending pathways from the antenna II and lateral antenna I neuropils (neuropils involved in the processing of chemosensory and tactile inputs). These studies showed that the ascending pathways from these neuropils innervate the same regions of the medulla terminalis and that these regions are different from those innervated by the olfactory lobe output pathway.

An alternative approach to the identification of respiratory central chemoreceptors in the brainstem

Central chemoreceptors (CCRs) play a crucial role in autonomic respiration. Although a variety of brainstem neurons are CO(2) sensitive, it remains to know which of them are the CCRs. In this article, we discuss a potential alternative approach that may allow an access to the CCRs. This approach is based on identification of specific molecules that are CO(2) or pH sensitive, exist in brainstem neurons, and regulate cellular excitability. Their molecular identity may provide another measure in addition to the electrophysiologic criteria to indicate the CCRs. The inward rectifier K(+) channels (Kir) seem to be some of the CO(2) sensing molecules, as they regulate membrane potential and cell excitability and are pH sensitive. Among homomeric Kirs, we have found that even the most sensitive Kir1.1 and Kir2.3 have pK approximately 6.8, suggesting that they may not be capable of detecting hypocapnia. We have studied their biophysical properties, and identified a number of amino acid residues and molecular motifs critical for the CO(2) sensing. By comparing all Kirs using the motifs, we found the same amino acid sequence in Kir5.1, and demonstrated the pH sensitivity in heteromeric Kir4.1 and Kir5.1 channels to be pK approximately 7.4. In current clamp, we show evidence that the Kir4.1-Kir5.1 can detect P(CO(2)) changes in either hypercapnic or hypocapnic direction. Our in-situ hybridization studies have indicated that they are coexpressed in brainstem cardio-respiratory nuclei. Thus, it is likely that the heteromeric Kir4.1-Kir5.1 contributes to the CO(2)/pH sensitivity in these neurons. We believe that this line of research intended to identify CO(2) sensing molecules is an important addition to current studies on the CCRs.

Olfaction in the Colorado potato beetle: ultrastructure of antennal sensilla in Leptinotarsa sp

Sensillae on the antennae of the Colorado potato beetle, Leptinotarsa decemlineata are described using scanning (SEM) and transmission (TEM) electron microscopy and compared with SEM observations of antennal sensilla in L. haldemani and L. texana. In all the three species, 13 distinct sensillar types were identified with a higher density of sensilla in the more polyphagous species, L. decemlineata than in the moderately host specific L. haldemani and the highly host specific L. texana. Cuticular specializations and the predominance of olfactory sensilla are discussed in relation to host specificity in the three species.

Olfaction in the Queensland fruit fly, Bactrocera tryoni. II: Response spectra and temporal encoding characteristics of the carbon dioxide receptors

Single-unit electrophysiology was used to record the nerve impulses from the carbon dioxide receptors of female Queensland fruit flies, Bactroera tryoni. The receptors responded to stimulation in a phasic-tonic manner and also had a period of inhibition of the nerve impulses after the end of stimulation. at high stimulus intensities. The cell responding to carbon dioxide was presented with a range of environmental odorants and found to respond to methyl butyrate and 2-butanone. The coding characteristics of the carbon dioxide cell and the ability to detect other odorants are discussed, with particular reference to the known behavior of the fly.

Morphology of antennal sensilla auricillica and their detection of plant volatiles in the Herald moth, Scoliopteryx libatrix L. (Lepidoptera: Noctuidae)

In the Herald moth Scoliopteryx libatrix there are single superficial auricillic sensilla, as well as groups of s. auricillica located in cavities on the antennae. Two sensory neurones, with different dendrite diameters innervate each of these sensilla. The diameter of the smaller dendritic segment is roughly half that of the larger one. The larger dendritic outer segment branches profusely in the lumen of the sensillum, whereas the smaller dendrite has few branches. Electrophysiological recordings from s. auricillica located in the medial part of the cavity revealed a receptor neurone responding to Delta-3-carene. In addition to these neurones, recordings made deeper and more laterally into the cavity showed neurones that responded to (+/-)-linalool, alpha-pinene and green leaf volatiles.

Ultrastructure and physiology of the CO2 sensitive sensillum ampullaceum in the leaf-cutting ant Atta sexdens

The sensilla ampullacea on the apical antennomere of the leaf-cutting ant Atta sexdens were investigated regarding both their responses to CO2 and their ultrastructure. By staining the sensillum during recording, we confirmed that the sensilla ampullacea are responsible for CO2 perception. We showed that the sensory neurons of the sensilla ampullacea are continuously active without adaptation during stimulation with CO2 (test duration: 1 h). This feature should enable ants to assess the absolute CO2 concentration inside their nests. Sensilla ampullacea have been found grouped mainly on the dorso-lateral side of the distal antennal segment. Scanning and transmission electron microscopic investigations revealed that the external pore opens into a chamber which connects to the ampulla via a cuticular duct. We propose protection against evaporation as a possible function of the duct. The ampulla houses a peg which is almost as long as the ampulla and shows cuticular ridges on the external wall. The ridges are separated by furrows with cuticular pores. The peg is innervated by only one sensory neuron with a large soma. Its outer dendritic segment is enveloped by a dendritic sheath up to the middle of the peg. From the middle to the tip numerous dendritic branches (up to 100) completely fill the distal half of the peg. This is the first report of a receptor cell with highly branched dendrites and which probably is tuned to CO2 exclusively.

The maxillary palp sensory pathway of Orthoptera

Primary sensory projections and arborisations of higher-order neurons associated with the maxillary palps were examined in Tettigoniidae, Gryllidae, Tetrigidae and Acrididae representing the two sub-orders of Orthoptera, Ensifera and Caelifera. Anterograde filling and Golgi impregnation of maxillary receptor neurons revealed two patterns of innervation, the ensiferous and the caeliferous type. In both ensiferans and caeliferans, receptor neurons arborised within the tritocerebrum, the antennal motor- and mechano-sensory centre and the lobus glomerulatus. In ensiferans, additional areas of innervation were found in the lobus glomerulatus and in a previously undescribed neuropil, here referred to as the accessory lobus glomerulatus. In relation to the anatomical data a putative functional segregation of the neuropil into gustatory-, olfactory- and mechano-sensory centres is implied.

Carbon-dioxide sensing structures in terrestrial arthropods

Sensory structures that detect atmospheric carbon dioxide have been identified and described to the subcellular level in adults of Lepidoptera, Diptera, Hymenoptera, Isoptera, Chilopoda, and Ixodidae, as well as in lepidopteran larvae. The structures are usually composed of clusters of wall-pore type sensilla that may form distinct sensory organs, often recessed in pits or capsules. In insects, they are located on either the palps or the antennae, in chilopods on the head capsule, and in ixodids on the forelegs. In the two cases where the central projections have been examined (Lepidoptera and mosquitoes), the clustering is preserved to the level of second order neurons, which are located in the deutocerebrum. Individual sensilla usually contain a single receptor neuron that is sensitive to CO(2); it may be accompanied by other neurons that respond to other olfactory qualities. The distal dendritic processes of CO(2)-sensitive neurons invariably show an increased surface area, dividing into many cylindrical branches or into lamellar structures. Lamellar membranes are often closely linked to arrays of microtubules. Fine pore canal tubules are usually associated with the cuticular pores.

Molecular cloning, by a novel approach, of a cDNA encoding a putative olfactory protein in the labial palps of the moth Cactoblastis cactorum

We have used the CapFinder technology, without the library construction step, to amplify and clone full-length cDNAs expressed in the labial palps (CO2-sensing organs) of the moth Cactoblastis cactorum. The validity of our approach is exemplified by the sequence analysis of a 597-bp cDNA clone, designated CLP-1, that contains a 390-bp open reading frame (ORF) flanked by motifs characteristic to a full-length cDNA. The ORF in CLP-1 encodes a predicted polypeptide that is 47% identical to a novel protein, OS-D, found exclusively in the olfactory antennal segment of Drosophila melanogaster. Both CLP-1 and OS-D have primary structures that do not bear sequence similarity to any previously characterised proteins including odorant-binding proteins (OBPs) in vertebrates and pheromone-binding proteins (PBPs) in moths. Although they share features common to OBPs and PBPs, such as the presence of signal peptides and cysteine motifs, they clearly belong to a distinct class of olfactory proteins that appear to be unique to insects. The relative abundance of the CLP-1 message in the labial palps of females leads to the suggestion that this protein is involved in the CO2-sensing cascade. Our results suggest that the experimental procedure can be used as an alternative, rapid method to identify genes expressed in a particular organ, or tissue, especially in situations when the amount of available tissue is a limiting factor.