Adaptation of sensory systems of gamasid mites (Parasitiformes, Gamasina) to dwelling in different environment

The main complication sensory organs (the palpal organ and the tarsal sensory complex) of several species of gamasid mites were studied in scanning electron microscope. The species examined included permanent ectoparasites (Laelaps agilis, Laelaptidae), parasites of the nasal cavity and respiratory tract of birds (Sternostoma tracheocolum and Ptilonyssus reguli, Rhinonyssidae), dwellers of the sea littoral zone (Parasitus kempersi, Parasitus immanis, Parasitidae), and mites found on soil and on plants (Amblyseius barkeri, Parasitidae). Similar sensillar types, including olfactory SW-WP sensilla, contact chemo-mechanosensory (SW-UP and DW-UP) sensilla, termo-chemo-mechanosensitive (DW-WP) sensilla of two types, and tactile (NP) sensilla were found in all these species, excluding endoparasites, where some sensillar types (in particular, DW-WP sensilla with slit-like pores) are absent. It was shown that the topography of olfactory SW-WP sensilla of the tarsal complex reflects taxonomic position and phylogenetic history of mite genera, whereas the number of certain sensillar types and the degree of their development reflect ecological specialization of species. The palpal organ is characterized by rather uniform structure in mites of different families, dwellers of different environments, except for the endoparasites of the family Rhinonyssidae, where this organ is strongly reduced.     

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

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

革螨跗感器的结构和功能

本文进一步研究了:④厩真厉螨截肢前后的爬行行为,表明第1对足其感觉功能;②用古拉广厉螨分别截各对足驱避反应的对比试验,见到只有当截去第1对足跗节时失去嗅觉功能,而截去第Ⅱ、Ⅲ、Ⅳ对足时,各组都仍有嗅觉功能;⑤对格氏血厉螨、厩真厉螨、毒厉螨和鼠颚毛厉螨进行涂漆前后的驱避试验,显示当跗感器被涂满封闭,则嗅觉功能消失;④以0.5%结晶紫或龙胆紫液染色的截肢标本,观察了厩真厉螨、毒厉螨、格氏血厉螨、古拉广厉螨及尾足螨股一种螨,足Ⅰ跗节末端凹窝中,至少都存在两类感毛,钝钉型感毛和长而尖的刚毛型感毛;⑤厩真厉螨雌螨和幼螨跗感器的钝钉毛分别为8根和5根,另外各有2根短而尖的毛,分别测定了长度,描述了形态特点;⑥透射电镜观察厩真厉螨、毒厉螨等的跗感器钝钉毛,毛外围有表皮壁,壁上有很多微孔,内有中心腔,腔内有树突。属化感器——嗅觉器;⑦电生理技术研究,当用氨和醋酸的气体刺激厩真厉螨、毒厉螨的离体足Ⅰ时,均产生明显的应激电位差,充分证明足Ⅰ辩节有嗅觉功能。     

Palpal sensory organ in the chicken mite Dermanyssus gallinae (Acari: Dermanyssidae)

Palptarsus of the chicken mite bears 5 single-wall upper-pore (SW-UP) chemo-mechanoreceptor sensilla (type A); 4 double-wall upper-pore (DW-UP) chemosensitive sensilla (type B), and 6 no-pore (NP) mechanoreceptor sensilla (type M). The author assumes that sensilla of the type A participate in perception of the aggregation pheromone; of the type B, in perception of trophic stimuli; and of the type M, in determination of mechanical properties of the substrate.     

Comparative study of the sensory system in gamasid mites Rhinonyssus rhinolethrum,Rh. subrhinolethrum and Ptilonyssus motacillae (Mesostigmata: Gamasina: Rhinonyssidae), parasites in the nasal cavity of birds

Sensillae of the tarsal receptor complex, palpal organ, and body chaetom were examined by means of scanning electron microscopy in three endoparasitic gamasid mites: Rhinonyssus rhinolethrum, Rh. subrhinolethrum and Ptilonyssus motacillae, the parasites of Anser albifrons, Anas crecca, and Motacilla alba, respectively. In the tarsal sensory complex, the scale of reduction of the olfactory sensilla reflects the adaptation of gamasid mites to cavernous parasitism. The topography of this sensilla is specific at generic and species taxonomic levels. In the palpal sensory organ, the number and scale of reduction of two main sensilla types depend on peculiarities of places of blood-sucking.     

Fine structure of tarsal sensory organs in the whip spider Admetus pumilio (Amblypygi, Arachnida).

The sensory organs on the tarsi of the antenniform first legs of the whip spider Admetus pumilio C. L. Koch (Amblypygi, Arachnida) were examined with the scanning and transmission electron microscope. At least four different types of hair sensilla were found: (1) thick-walled bristles, which have the characteristics of contact chemoreceptors (several chemoreceptive dendrites in the lumen plus two mechanoreceptors at the base); (2) short club sensilla, innervated by 4-6 neurons which terminate in a pore on the tip; they are possibly humidity receptors; (3) porous sensilla, which are either innervated by 20-25 neurons and have typical pore tubules, or they have 40-45 neurons but no pore tubules; both types are considered to be olfactory; (4) rod sensilla occur in clusters near segmental borders; they are innervated by only one large dendrite which branches inside the lumen. Other tarsal receptors are the claws, which correspond to contact chemoreceptors, and the pit organ which resembles the tarsal organ of spiders. Compared to other arthropod sensilla, the contact chemoreceptors are very similar to those of spiders, while the porous sensilla correspond structurally to olfactory receptors in insects; the club and rod sensilla seem to be typical for amblypygids.     

Ultrastructure of chemosensory tarsal tip-pore sensilla of Argiope spp. Audouin, 1826 (Chelicerata: Araneae: Araneidae)

While chemical communication has been investigated intensively in vertebrates and insects, relatively little is known about the sensory world of spiders despite the fact that chemical cues play a key role in natural and sexual selection in this group. In insects, olfaction is performed with wall–pore and gustation with tip-pore sensilla. Since spiders possess tip-pore sensilla only, it is unclear how they accomplish olfaction. We scrutinized the ultrastructure of the trichoid tip-pore sensilla of the orb weaving spider Argiope bruennichi—a common Palearctic species the males of which are known to be attracted by female sex pheromone. We also investigated the congener Argiope blanda. We examined whether the tip-pore sensilla differ in ultrastructure depending on sex and their position on the tarsi of walking legs of which only the distal parts are in contact with the substrate. We hypothesized as yet undetected differences in ultrastructure that suggest gustatory versus olfactory functions. All tarsal tip-pore sensilla of both species exhibit characters typical of contact-chemoreceptors, such as (a) the presence of a pore at the tip of the sensillum shaft, (b) 2–22 uniciliated chemoreceptive cells with elongated and unbranched dendrites reaching up to the tip-pore, (c) two integrated mechanoreceptive cells with short dendrites and large tubular bodies attached to the sensillum shaft's base, and (d) a socket structure with suspension fibres that render the sensillum shaft flexible. The newly found third mechanoreceptive cell attached to the proximal end of the peridendritic shaft cylinder by a small tubular body was likely overlooked in previous studies. The organization of tarsal tip-pore sensilla did not differ depending on the position on the tarsus nor between the sexes. As no wall-pore sensilla were detected, we discuss the probability that a single type of sensillum performs both gustation and olfaction in spiders.     

Functional morphology of a double-walled multiporous olfactory sensillum: the sensillum coeloconicum of Bombyx mori (Insecta, Lepidoptera)

The fine structure of coeloconic sensilla of Bombyx mori was studied in cryofixed specimens. These sensilla belong to the category of double-walled wall-pore sensilla. The pegs are approximately 10 mum long, located in pits on the dorsal side of the antennal branches, and longitudinally grooved in their distal half (grooved surface approximately 30 mum(2)). The central lumen contains the outer dendritic segments of usually five receptor cells, and is surrounded by up to 15 partially fused cuticular fingers. The peripheral lumina of these cuticular fingers are filled with material resembling wax-canal filaments. Radial spoke channels ( approximately 600 per peg), each 10-20 nm wide, connect the central lumen with the longitudinal groove channels. Groove and spoke channels are assumed to mediate the transport of odorant molecules from the outer epicuticular surface layers to the sensory dendrites. Thus the double-walled wall-pore sensilla represent a bauplan essentially different from single-walled wall-pore sensilla; the reason, however, why the two types are found together throughout the insect orders remains enigmatic. Other peculiar features of the coeloconic sensilla of the silkmoth are invaginations of the outer dendritic segments and direct contacts between the receptor cell somata. The latter may be the structural correlate to electrophysiological observations indicative of peripheral interaction between the receptor neurons. All three auxiliary cells have elaborately folded apical plasma membranes studded with portasomes and associated with an abundance of mitochondria; basally they often contact tracheal branches. As compared to the auxiliary cells of the single-walled olfactory sensilla of the same species, all the mentioned features are much more prominent and hint to a higher ion pumping activity at the border to the sensillum-lymph cavities.     

Phenol and lactone receptors in the distal sensilla of the Haller's organ in Ixodes ricinus ticks and their possible role in host perception

Extract of steer wool odor was found to excite olfactory receptor(s) in a wall-pore olfactory sensillum on the distal knoll of the Haller's organ. Three active volatile compounds were revealed in this odor by gas chromatography. Electrophysiological experiments revealed two types of receptors (sensory neurons) within the sensilla examined. One type of receptor responded only to phenolic derivatives, such as o-chlorophenol, o-bromophenol, o-methylphenol, 2,6-dichlorophenol, 2,6-dibromophenol, 2,4,6-trichlorophenol, but not to o-nithrophenol, p-methylphenol, 2,5-dichlorophenol, 3,5-dichlorophenol, 2,6-dinithrophenol, 2,6-dimethylphenol, and pentachlorophenol. The other type of receptor responded only to gamma-valerolactone. It is assumed that these cells play an important role in perception of a host from long distances (10-15 m), which is typical of Ixodes ricinus ticks.     

Detection of vibrations in sand by tarsal sense organs of the nocturnal scorpion,Paruroctonus mesaensis

Summary The scorpionParuroctonus mesaensis locates prey by orienting to substrate vibrations produced by movements of the prey in sand. At the end of each walking leg of this scorpion there are two sense organs, the basitarsal compound slit sensillum and tarsal sensory hairs (Figs. 1, 3) that are excited by substrate vibrations conducted through sand. The slit sensilla appear to be most sensitive to surface (Rayleigh) waves while the tarsal sensory hairs respond best to compressional waves (Fig. 7). Both mechanoreceptors were activated by nearby disturbances of the substrate (Fig. 6) but only the slit sensilla responded to insects moving more than 15 cm away. Both receptors are highly sensitive to small amplitude (less than 10 Å) mechanical stimuli applied to the tarsus (Fig. 5).Behavioral studies of scorpions with ablated sense organs (Fig. 2) indicate that the basitarsal compound slit sensilla are necessary for determining vibration source direction.Abbreviation BCSS basitarsal compound slit sensillum (a) Supported by PHS Environmental Science and Regents Intern Fellowships (PB), and by intramural research funds from the University of California (RDF)     

Ultrastructure of chemoreceptive tarsal sensilla in an armored harvestman and evidence of olfaction across Laniatores (Arachnida,Opiliones)

Harvestmen (Arachnida, Opiliones) are especially dependent on chemical cues and are often regarded as animals that rely mainly on contact chemoreception. Information on harvestman sensilla is scarce when compared to other arachnid orders, especially concerning internal morphology. Using scanning (SEM) and transmission (TEM) electron microscopy, we investigated tarsal sensilla on the distal tarsomeres (DT) of all leg pairs in Heteromitobates discolor (Laniatores, Gonyleptidae). Furthermore, we explored the typological diversity of sensilla present on the DT I and II in members of the suborder Laniatores, which include two thirds of the formally described opilionid fauna, using species from 17 families representing all main laniatorian lineages. Our data revealed that DT I and II of H. discolor are equipped with wall-pored falciform hairs (two types), wall-pored sensilla chaetica (two types) and tip-pored sensilla chaetica, while DT III and IV are mainly covered with trichomes (non-sensory) and tip-pored sensilla chaetica. The ultrastructural characteristics support an olfactory function for all wall-pored sensilla and a dual gustatory/mechanoreceptive function for tip-pored sensilla chaetica. Based on our comparative SEM survey, we show that wall-pored sensilla occur in all investigated Laniatores, demonstrating their widespread occurrence in the suborder and highlighting the importance of both legs I and II as the sensory appendages of laniatorean harvestmen. Our results provide the first morphological evidence for olfactory receptors in Laniatores and suggest that olfaction is more important for harvestmen than previously thought.     

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

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

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

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

Tarsal taste sensilla of the autumn gum moth, Mnesampela privata: morphology and electrophysiological activity

Mnesampela privata Guenée (Lepidoptera: Geometridae: Ennominae) is a native Australian geometrid that conducts considerable host assessment prior to ovipositing on its host plants, which belong to the genus Eucalyptus . The leaves of some of their hosts are covered with a particularly thick and waxy cuticle and we have shown that epicuticular waxes influence the oviposition preferences of females. This necessitates that M. privata has evolved specific chemosensory organs to assess the identity and perhaps even the quality of its hosts. In this work, we examined the morphology of tarsal taste sensilla and the sensitivity of their sensory neurones to a range of primary metabolites possibly influential on host assessment and oviposition. The ventral surface of the fifth tarsomere of females bear two parallel rows of up to eight sensilla, each loosely aligned with two parallel rows of five spines. Salts, sugars, and amino acids elicited phasi-tonic multicellular neuronal responses of variable magnitude and form. Two pairs of sensilla are closely apposed to the most distal spine in each row; the sensory neurones associated with these sensilla exhibited notably larger responses to alanine and serine compared with those of all other sensilla. The arrangement of the taste sensilla in close proximity to prominent tarsal spines is unique and could represent an adaptation that enables them to penetrate the wax layer and be brought into contact with primary metabolites present closer to the leaf surface.     

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

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

Ultrastructure of the chemosensitive basiconic single-walled wall-pore sensilla on the antennae in adults and embryonic stages of Locusta migratoria L. (Insecta,Orthoptera)

Summary The structure and embryonic development of the two types (A, B) of basiconic sensilla on the antennae of Locusta migratoria were studied in material that had been cryofixed and freeze-substituted, or chemically fixed and dehydrated. Both types are single-walled wall-pore sensilla. Type-A sensilla comprise 20–30 sensory and 7 enveloping cells. One enveloping cell (thecogen cell secretes the dendrite sheath); four are trichogen cells, projections of which form the trichogen process during the 2nd embryonic molt. The trichogen cells form two concentric pairs proximally. Two tormogen cells secrete the cuticular socket of the sensillum. The dendritic outer segments of the sensory cells are branched. Bifurcate type-A sensilla have also been observed. Type-B sensilla comprise three sensory and four enveloping cells (one thecogen, two trichogen and one tormogen). The trichogen process is formed by the two trichogen cells, each of which gives rise to two projections. The trichogen cells are concentrically arranged. The dendritic outer segments of the sensory cells are unbranched. In the fully developed sensillum, all trichogen and tormogen cells border on the outer receptor lymph cavity. It is suggested that the multicellular organization of the type-A sensilla can be regarded as being advanced rather than primitive.Supported by the Dcutschc Forschungsgemeinschaft (SFB 4/G1)     

Development and organization of the Drosophila olfactory system: an analysis using enhancer traps.

Drosophila uses different olfactory organs at different developmental stages. The larval and adult olfactory organs are morphologically dissimilar and have different developmental origins: the antenno-maxillary complex (AMC), which houses the larval olfactory organ, is histolyzed during metamorphosis; the third antennal segment--the principal adult olfactory organ--derives from an imaginal disc. A screen for genes expressed in both larval and adult olfactory organs, but in relatively few other tissues, has been carried out. Seven enhancer trap lines showing reporter gene expression in both the larval AMC and in certain subsets of the adult antenna are described. The antennal staining pattern of one line shows a striking change over the first few days of adult life, with a time course comparable to that of the development of sexual maturity. A pronounced sexual dimorphism in antennal staining pattern is seen in another line. Some staining patterns resemble the patterns of certain classes of antennal sensilla; others show expression restricted to only a small number of cells. Some lines also show expression associated with other chemosensory organs at either the larval or adult stage, including the maxillary palps, labellum, and anterior wing margin. One line, which also shows staining in the male reproductive tract, is male sterile. The significance of these results is considered in terms of (1) the molecular organization of the olfactory system; (2) the recruitment of olfactory genes for use in two developmental contexts; (3) the sharing of genes among different sensory modalities; (4) the role of olfaction in sexual behavior; and (5) posteclosional changes in the olfactory system.     

The sensory equipment of a spider – A morphological survey of different types of sensillum in both sexes of Argiope bruennichi (Araneae,Araneidae)

Spiders show a wide range of sensory capabilities as evidenced by behavioural observations. Accordingly, spiders possess diverse sensory structures like mechano-, hygro-, thermo- or chemoreceptive sensilla. As to chemoreceptive structures, only trichoid tip-pore sensilla were found so far that were tested for gustation. That spiders are also able to receive airborne signals is corroborated by numerous behavioural experiments but the responsible structures have not been determined yet. Here, we provide sensilla distribution maps of pedipalps and walking legs of both sexes of the wasp spider Argiope bruennichi whose biology and mating system is well explored. By means of scanning electron microscopy, we scrutinized whether there is in fact only one type of trichoid pore sensillum and if so, if there are deviations in the outer structure of the tip-pore sensilla depending on their position on the body. We also describe the external structure and distribution of slit sense organs, trichobothria and tarsal organs. Our study shows that all four sensillum types occur on pedipalps and walking legs of both sexes. As to chemosensory organs, only tip-pore sensilla were found, suggesting that this sensillum type is used for both gustation and olfaction. The highest numbers of tip-pore sensilla were observed on metatarsi and tarsi of the first two walking legs. Mechanosensitive slit sense organs occur as single slit sensilla in rows along all podomers or as lyriform organs next to the joints. The mechanosensitive trichobothria occur on the basal part of tibiae and metatarsi. Tarsal organs occur on the dorsal side of all tarsi and the male cymbium. The distribution maps of the sensilla are the starting point for further exploration of internal, morphological differences of the sensilla from different regions on the body. Cryptic anatomical differences might be linked to functional differences that can be explored in combination with electrophysiological analyses. Consequently, the maps will help to elucidate the sensory world of spiders.     

Fine structure of the antennal sensilla of the millipede Orthomorphella pekuensis (Polydesmida: Paradoxosomatidae)

The fine structural characteristics of various sensory receptors on the antenna of a millipede, Orthomorphella pekuensis, were observed with field emission scanning electron microscopy. The antenna of this millipede has eight segments, called articles. On the surface of the antenna, there are a variety of sensory receptors, including olfactory and mechanical receptors. According to their morphological and fine structural characteristics, we could identify four basic types of antennal sensillum: chaetiform sensilla (CS), trichoid sensilla (TS), basiconic sensilla (BS) and apical cone sensilla (AS). The BS are divided further into three subtypes: large basiconic sensilla (BS₁) on the 5th and 6th articles; small basiconic sensilla (BS₂) on the 5th article; and a distinct type of basiconic spiniform sensilla (BS₃) on the 7th article. The most prominent sensilla are four large AS on the distal tip of the 8th segment. Based on our results, we conclude that the main function of the CS and TS are related to mechanical reception, and that the BS and AS are likely to function in olfactory reception of volatile odors of plants, as these sensilla have base and apex pores, respectively.     

Ultrastructural investigation of the nuchal organs ofPygospio elegans (Polychaeta). I. Larval nuchal organs

The structural differentiation of the nuchal organs during the post-embryonic development ofPygospio elegans is described. The sensory organs are composed of two cell types: ciliated cells and bipolar primary sensory cells, constituting the nuchal ganglion, which is associated with both the sensory epithelium and the brain. Since the sensory neurons are largely integrated into posterolateral parts of the cerebral ganglion, the nuchal organs are primary presegmental structures. The microvilli of the ciliated cells form a cover over the cuticle with a presumed protective function. An extracellular space extends between cuticle and sensory epithelium. The distal dendrites of the sensory cells terminate in sensory bulbs, bearing one modified sensory cilium each that projects into the olfactory chamber, embedded within the secretion of the ciliated cells. During development, the nuchal organs increase in size. This is accompanied by a shift in position, an expansion of the sensory area, and secretory activity of the ciliated cells. The nuchal ganglion differentiates into three nuchal centres forming three distinct sensory areas around the ciliated region. Each nuchal complex reveals two short nuchal nerves comprising the sensory axons, which enter the posterior circumesophageal connective. The sensory cells lying in the brain exhibit neurosecretory activity; the sensory cilia enlarge their surface area by dilating and branching. Nuchal organs accomplish the basic structural adaptions of chemoreceptors and show structural analogies to arthropod olfactory sensilla; thus, there is every reason to suppose chemoreceptor function.