Die digitiformen Sensillen auf dem Maxillarpalpus von Coleoptera

Summary The digitiform sensilla on the distal segment of the maxillar palps ofAgabus bipustulatus (L.) andHydrobius fuscipes (L.) were investigated by electron microscopic methods. Each sensillum is innervated by a single bipolar sensory cell. The sensilla ofHydrobius are associated with three enveloping cells, which enclose an inner and outer receptor lymph cavity. A single enveloping cell only is found in the completely differentiated sensilla ofAgabus. These sensilla do not form an outer lymph cavity. The area beneath the hair base is filled by the distal process of the enveloping cell and by extensions of epidermal cells. Only one extra-cellular space exists, which seems to be homologous to an inner receptor lymph cavity.The outer dendritic segment surrounded by a dendritic sheath runs to the tip of the hair shaft. In the hair shaft the outer dendritic segment divides into several branches. The poreless hair shaft does not rise over the surface of the cuticle, but it is positioned in a narrow shallow groove. Special socket structures or a tubular body do not exist. The digiti-form sensilla possess neither the typical feature of mechanosensitive, nor gustatory or olfactory sensilla. The functional significance of the structural divergences in the sensilla of both species and the presumed function of the sensilla are discussed referring to hygro- and thermo-receptors.

Unserem verehrten Lehrer, Herrn Prof. Dr. H.Risler, dem wir für vielfache Förderung danken möchten, zum 65. Geburtstag gewidmet.     

Antennal sensilla ofNeomysis integer (leach)

Summary The most frequent type of the hair sensilla on the antennae ofNeomysis integer is investigated by electron microscopic methods. The cellular properties of the sensilla are compared with those of other arthropods in order to detect possible homologies.The hairs are innervated by 2, 3, 6, 8, 9, or 10 sensory cells. The dendrites show an inner and outer dendritic segment. Five or six enveloping cells belong to a sensillum. In intermoult stage, processes of all the enveloping cells except the innermost one extend into the hair shaft. The sensory hairs possess only a single liquor cavity, which morphologically is homologous to the inner lymph cavity of insect sensilla. Around the liquor cavity, a supporting structure is located which seems to be identical to the scolopale of chordotonal organs. The six-to tenfold-innervated hairs possess two groups of differently structured dendrites which are regularly arranged on opposite sides of the liquor cavity. The outer dendritic segments are enclosed in a dendritic sheath. It is secreted by the innermost enveloping cell (= dendritic sheath cell of insect sensilla). All the outer dendritic segments terminate in the distal region of the hair shaft which shows a pore at its tip. The possible function of the sensilla is discussed. The double and triple-innervated hairs are considered to be mechano-receptors, whereas the sensilla associated with six to ten sensory cells might be mechano-chemoreceptors.     

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

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

蜚蠊尾须单个细毛型感受器对机械刺激的反应

用钨丝微电极插入蜚蠊尾须细毛感受器的园盘作胞外记录.研究了感受器自发脉冲的统计特性.利用电磁驱动装置控制毛杆的偏转,研究了感受器电位和脉冲对正弦波、方波和阶梯波机械刺激的反应特性,并测定了六列细毛的兴奋方向.     

Ultrastructure and receptor cell responses of the antennal grooved peg sensilla of Triatoma infestans (Hemiptera: Reduviidae)

Ultrastructural examination of grooved-peg (GP) sensilla on the antenna of fifth instar Triatoma infestans nymphs by scanning electron microscopy and transmission electron microscopy reveal that they are 8–18 μm long with a diameter of about 2–2.8 μm at the non-articulated base. Some pegs have a terminal pore. These double-walled wall-pore (dw-wp) sensilla have an outer cuticular wall with 13–18 longitudinal grooves at the distal part of the peg. Groove channels are present at the bottom of the grooves from which radial spoke channels lead into the inner sensillum-lymph cavity. A dendrite sheath connects the tip of the thecogen cell to the inner cuticular wall thus forming separated outer and inner sensillum-lymph cavities. Four or five bipolar receptor cells are ensheathed successively within the GP sensilla by the thecogen cell, trichogen and tormogen cells. The inner dendritic segments of each sensory cell give rise at the ciliary constriction to an unbranched outer dendritic segment which can reach the tip of the sensillum.Electrophysiological recordings from the GP sensilla indicate that they house NH₃, short-chain carboxylic acid and short-chain aliphatic amine receptor cells and can be divided into three functional sub-types (GP 1–3). All GP sensilla carry a receptor cell excited by aliphatic amines, such as isobutylamine, a compound associated with vertebrate odour. GP type 1 and 2 sensilla house, in addition, an NH₃-excited cell whereas the type 2 sensilla also contains a short-chain carboxylic acid receptor. No cell particularly sensitive to either NH₃ or carboxylic acids was found in the grooved-peg type 3 sensilla. GP types 1, 2 and 3 represent ca. 36, 10 and 43% of the GP sensilla, respectively, whereas the remaining 11% contain receptor cells that manifest normal spontaneous activity but do not respond to any of the afore mentioned stimuli.     

中华蚊蝎蛉成虫口器感器的超微结构

口器感器在昆虫取食活动中起着重要作用, 但蚊蝎蛉成虫口器上感器的种类和形态迄今未见报道。我们利用扫描电子显微镜, 观察了中华蚊蝎蛉Bittacus sinensis Walker成虫口器上的感器。结果显示: 中华蚊蝎蛉口器上共有8种感器, 分别为锥形、毛形、刺形、指形、掌状、钟形、柱状感器及Böhm氏鬃毛, 主要集中于内唇、 下颚须以及下唇须上。锥形感器和刺形感器数量最多; 毛形感器主要在下颚轴节、 茎节和下唇的亚颏和前颏有分布; 钟形感器和Böhm氏鬃毛只存在于下唇须和下颚须上。下颚须端节和下唇须端节的感器种类相同, 以锥形感器为主。高度骨化的上颚以及下颚内颚叶与外颚叶上未发现感器分布。简要讨论了口器感器在昆虫分类中的意义。     

Fine structure and sensory apparatus of the mouthparts of the pear psyllid,Cacopsylla chinensis (Yang et Li) (Hemiptera: Psyllidae)

The pear psyllid, Cacopsylla chinensis (Yang et Li) (Hemiptera: Psyllidae), is one of the most significant economic pests of pear in China, causing direct damage through feeding by the highly specialized piercing–sucking mouthparts. The ultrastructural morphology and sensory apparatus of the mouthparts of the adult were examined using scanning and transmission electron microscopy. The piercing–sucking mouthparts of C. chinensis are composed of a three-segmented labium with a deep groove in the anterior side, a stylet fascicle consisting of two mandibular and two maxillary stylets, and a pyramid-shaped labrum. Proximal to the labium, the stylet fascicle forms a large loop within a membranous crumena. Mandibles, with more than ten teeth on the external convex region, can be seen on the distal extremity. Smooth maxillary stylets are interlocked to form a larger food canal and a smaller salivary canal. One dendritic canal housing 2 dendrites is also found in each mandible. Two types of sensilla trichodea, four types of sensilla basiconica, single as well as groups of sensilla campaniformia, and oval flattened sensilla occur in different locations on the labium, whereas a kind of sensilla basiconica is at the junction of the labrum and anteclypeus. Sensilla trichodea and sensilla campaniformia, always present with denticles, are present on the middle labial segment. Three types of sensilla basiconica, two types of sensilla trichodea and two oval flattened sensilla are located on the distal labial segment. The mouthpart morphology and abundance of sensilla located on the labium in C. chinensis are illustrated, along with a brief discussion of their taxonomic and putative functional significance.     

Fine structure of the statocyst sensilla of the mysid shrimp Neomysis integer (Leach, 1814) (Crustacea,Mysidacea)

The fine structure of the statocyst sensilla of Neomysis integer was investigated. The statocyst contains about 35 sensilla, which are composed of two bipolar sensory cells, nine enveloping cells, and a seta. The sensory cells consist of an axon, a perikaryon, and a dendrite. The dendrite contains a proximal segment with a ciliary rootlet and at least one basal body, and a distal segment with a ciliary axoneme (9 × 2 + 0) at its base. The distal segment extends along the peripheral wall of the seta and is in close contact with the wall of the hair shaft. The enveloping cells surround the proximal and distal segments of the dendrite. The innermost enveloping cell contains a scolopale rod. It surrounds the receptor lymph cavity and secretes flocculent material into this cavity. From the tip of the cell a dendritic sheath, which encloses the distal segment of the dendrite, emerges. A peculiar feature of the second enveloping cell is the presence of a scolopale-like rod, which is more slender and less pronounced than in the first enveloping cell. The seta consists of three parts: a socket, a tubular midpart, and a gutter-like apical part, the tip of which penetrates into the statolith. The seta shows over its full length a bilaterally symmetrical axis that is coplanar with the plane in which the seta is bent toward the statolith. The structure of the seta and the position of the distal segments provide morphological evidence for directional sensitivity of the sensilla and for the magnitude of shear on the setal wall being an adequate stimulus.     

The cercal sensilla of the blowfly Lucilia cuprina. I. Structure of the sockets and distal dendritic regions

The functions of the gustatory, olfactory, touch and stress receptors on the cerci of Lucilia cuprina Wied. (Diptera: Calliphoridae) are apparent from the morphology of their distal dendritic segments and associated cuticular structures. Each trichoid mechanoreceptor has a dendrite containing a tubular body at the base of the hairshaft. The suspension fibres and socket septum may be involved in transmitting a stimulus to the dendrite terminal and restoring the hair to its resting position. The campaniform sensilla are considered as trichoid mechanoreceptors with reduced hair shafts and socket structures, reflected in fusion of the suspension fibres into the inner cuticle of the dome and loss of the socket septum. Fusion and reduction of the socket structures is also apparent at the bases of the olfactory pegs. They differ from typical antennal olfactory sensilla in having a flexible socket and relatively thick walls; features which may protect them from damage during ovipositor probing of potential oviposition substrates. The two types of cereal gustatory sensilla differ in their complement of chemosensory dendrites, one has three, the other four, the latter type also has a mechanoreceptive dendrite at the base of the hair shaft. Both types have socket structures resembling those of the trichoid mechanoreceptors.     

Fine structure of the galeal styloconic sensilla of larval Lymantria dispar (Lepidoptera: Lymantriidae)

Lepidopteran larvae possess two pairs of styloconic sensilla located on the maxillary galea. These sensilla, namely the lateral and medial styloconic sensilla, are each comprised of a smaller cone, which is inserted into a style. They are thought to play an important role in host-plant selection and are the main organs involved in feeding. Ultrastructural examination of these sensilla of fifth instar Lymantria dispar (L.) larvae reveal that they are each approximately 70 um in length and 30 um in width. Each sensillum consists of a single sensory peg inserted into the socket of a large style. Each peg bears a slightly subapical terminal pore averaging 317 nm in lateral and 179 nm in medial sensilla. Each sensillum houses five bipolar neurons. The proximal dendritic segment of each neuron gives rise to an unbranched distal dendritic segment. Four of these dendrites terminate near the tip of the sensillum below the pore and bear ultrastructural features consistent with contact chemosensilla. The fifth distal dendrite terminates near the base of the peg and bears ultrastructural features consistent with mechanosensilla. Thus, these sensilla each bear a bimodal chemo-mechanosensory function. The distal dendrites lie within the dendritic channel and are enclosed by a dendritic sheath. The intermediate and outer sheath cells enclose a large sensillar sinus, whereas the smaller ciliary sinus is enclosed by the inner cell. The neurons are ensheathed successively by the inner, intermediate, and outer sheath cells.     

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.     

Cellular organization of the dome sensillum: A presumed chemoreceptor in Gammarus setosus (Gammaridea: Amphipoda)

A previously unknown type of sensillum with a thin cuticular dome and two pairs of pores is described in the amphipod Gammarus setosus. There is only one dome sensillum on each interantennal lobe of the head. The receptor is innervated by two sensory dendrites that bifurcate into two pairs of 9 + 0 cilia, concentrically enclosed by four auxiliary cells—two thecogen, one trichogen, and one tormogen and surrounded by a cluster of accessory cells. The ciliary regions are contained in small inner lymph cavities. The outer segments are sheathed by the apical extensions of the thecogen cells, are looped inside the outer lymph cavity, and come in close contact with lipid spheroids inside the dome. The basal bodies consist of microtubule doublets, which extend into the distal segments where they are interspersed with singlets. The nodal inner dendritic segments join the ventral suspension cord of the organ of Bellonci and enter its ganglion. The application of colloidal lanthanum resulted in intraciliary lanthanum deposits. The dome sensilla are presumed to be chemosensory because their cellular plan has similarities to that of some known olfactory and pheromone-sensitive sensilla in decapod crustaceans and insects. © 1994 Wiley-Liss, Inc.     

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

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

Hair regeneration during moulting in the spiderCiniflo similis (Araneae,Dictynidae)

Summary The mechanoreceptive and chemoreceptive hairs on the legs of the cribellate spiderCiniflo similis were examined during the moulting cycle. In mechanoreceptive hairs the new hair shaft is formed around the extended dentrites, which emerge from near the tip of the newly forming hair and continue to the old sensillum within the extended dendritic sheath. Thus there is no ecdysial canal in the base of the hair shaft as found in insect mechanoreceptive hairs. The dendritic connection with the old hair is maintained until shortly before ecdysis by which time new tubular bodies have developed in the same dendrites at the base of the new hair. In chemoreceptive sensilla the new hair shaft is also formed around the elongated outer segment of the dendrites (19 chemosensitive and 2 mechanosensitive). The two mechanosensitive dendrites develop new tubular bodies at the base of the hair. As ecdysis occurs the old dendritic sheath and dendrites are snapped off at the tip of the new hair but the pore remains open. The ultrastructural evidence indicates that the roles of the three main enveloping cells are as follows: The dendritic sheath cell secretes the dendritic sheath, the middle enveloping cell forms the hair shaft while the outer enveloping cell forms the socket. This pattern corresponds closely to that observed in insecta sensilla. The extreme length of the chemoreceptive dendrites during moulting is mentioned in connection with receptor function. The unique multi-layered nature of the middle enveloping cell is seen as a device for the formation of regularly occurring rows of small spines on the shaft of the hair.     

Proboscis sensilla in Vanessa cardui (Nymphalidae, Lepidoptera): functional morphology and significance in flower-probing

 Morphology and distribution of the proboscis sensilla in Vanessa cardui have been investigated in order to contribute to the understanding of flower-probing behaviour in butterflies. The proboscis has a bend region approximately one-third of the length from the base. A short tip region is characterized by rows of intake slits leading into the food canal. Along the dorsal, lateral and ventral sides of the proboscis, sensilla trichodea, sensilla basiconica and sensilla styloconica are distributed in varying patterns depending on their distance from the b ase. The medial food canal bears one longitudinal row of sensilla basiconica only. The bristle-shaped sensilla trichodea are longer in the proximal region of the proboscis and become gradually shorter towards the tip. They are most frequent in number near to the bend region and near the beginning of the tip region. Sensilla basiconica arranged in longitudinal rows increase in number the more distal they are on the proboscis. The tip region is characterized by rows of sensilla styloconica on the dorsal side whereas the sensilla trichodea are mostly restricted to the ventral side. The ultrastructure suggests that the aporous sensilla trichodea function as mechanosensilla while the uniporous sensilla basiconica act as contact chemosensilla. The sensilla styloconica are regarded as bimodal contact chemo/mechanosensilla since their sensory cones are equipped with a single terminal pore and a tubular body at the base. The mouthpart sensilla appear to provide tactile cues on the positioning of the proboscis and on the degree of its insertion into a floral tube. Furthermore, they receive chemical stimuli on the availability of nectar and on the immersion status of the food canal. Accepted: 12 September 1997     

Fine structure and molting of aesthetasc sense organs on the antennules of the isopod,Asellus aquaticus (Crustacea)

In Asellus aquaticus certain distal antennular segments bear single sensilla referred to as aesthetascs. These show a proximal stem and a distal bulbous region. Depending on its position, each aesthetasc is innervated by either 50-60 or 70-80 bipolar sensory cells, the perikarya of which are situated within the pedunculus. Within the antennular segment the dendrites develop unbranched cilia (9 X 2 + 0 structure). The sensory cells are unusual in that mono- as well as biciliary dendrites are present within a single aesthetasc, the ratio of both types being correlated with the number of sensory cells. Cilia and receptor lymph cavity are enveloped by a set of 3-4 inner and 13-14 outer sheath cells, which terminate at the base of the sensillum, so that the delicate and poreless cuticle of the bulbous region encloses only outer segments within the receptor lymph fluid. A new molting type in arthropods is described in which the outer sheath cells alone build the new cuticle, whereas the inner sheath cells most probably have a protective function. A definition of aesthetascs is proposed based on fine-structural criteria. Functionally the sensilla are considered to be chemoreceptors. This assumption is confirmed by experiments with diluted vital dye as well as lanthanum showing that dissolved substances penetrate the poreless cuticle instantaneously.     

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)     

A cryofixation study of presumptive hygroreceptors on the antennule of a terrestrial isopod

The structure of the apical sensilla on the antennule of the terrestrial isopod Porcellio scaber was examined in cryofixed and freeze substituted (CRF) and chemically fixed and dehydrated (CHF) material. CRF specimens generally showed a preservation superior to CHF material. Only in deeper regions did the tissue show damage from freezing. Each of the 13–22 sensilla contains two sensory cells. In contrast to earlier reports, it was observed that the dendritic segments of these cells are arranged in a unique, concentric manner. In CRF specimens the dendrites reach the tip of the sensilla and border upon the innermost layer of the complicated wall of the peg which is not pierced by pores. Silver-protein and lanthanum failed to penetrate the wall of the sensilla and also did not reach the dendrites via an apical pore, which therefore is regarded as a molting pore. The lymph spaces which, in CHF specimens, are observed around the dendrites and beneath the cuticle within the antennal tip are regarded as artefactual. From similarities in the dendritic structures to insect hygroreceptors and their relationship to the adjoining cuticle it can be assumed that the antennular sensilla in Porcellio are sensitive to humidity changes. Mechanoreception and chemoreception, however, cannot entirely be excluded as possible functions.     

Morphology of the mouthparts of the spittlebug Philagra albinotata Uhler (Hemiptera: Cercopoidea: Aphrophoridae)

Mouthparts associated with feeding behavior and feeding habits are important sensory and feeding structures in insects. To obtain a better understanding of feeding in Cercopoidea, the morphology of mouthparts of the spittlebug, Philagra albinotata Uhler was examined using scanning electron microscopy. The mouthparts of P. albinotata are of the typical piercing–sucking type found in Hemiptera, comprising a cone-shaped labrum, a tube-like, three-segmented labium with a deep groove on the anterior side, and a stylet fascicle consisting of two mandibular and two maxillary stylets. The mandibles consist of a dorsal smooth region and a ventral serrate region near the apical half of the external convex region, and bear five nodules or teeth on the dorsal external convex region on the distal extremity; these are regarded as unique features that distinguish spittlebugs from other groups of Hemiptera. The externally smooth maxillary stylets, interlocked to form a larger food canal and a smaller salivary canal, are asymmetrical only in the internal position of longitudinal carinae and grooves. One dendritic canal is found in each maxilla and one in each mandible. Two types of sensilla trichodea, three types of sensilla basiconica and groups of multi-peg structures occur in different locations on the labium, specifically the labial tip with two lateral lobes divided into anterior sensory fields with ten small peg sensilla arranged in a 5 + 4 + 1 pattern and one big peg sensillum, and posterior sensory fields with four sensilla trichodea. Compared with those of previously studied Auchenorrhyncha, the mouthparts of P. albinotata may be distinguished by the shape of the mandibles, the multi-peg structures and a tooth between the salivary canal and the food canal on the extreme end of the stylets. The mouthpart morphology is illustrated using scanning electron micrographs, and the taxonomic and putative functional significance of the different structures is briefly discussed.     

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.