Development of antennal sensilla during moulting inNeomysis integer (Leach) (Crustacea,Mysidacea)

Summary The sensilla are associated with 6 enveloping cells. The innermost enveloping cell (e 1) secretes the dendritic sheath (=thecogen cell). All other enveloping cells are involved in the formation of the outer cuticular apparatus in secreting the cuticle of a definite region of the new hair shaft.The development of the new sensilla begins when an exuvial space expands between old cuticle and epithelium. The newly forming hair shafts lie folded back in an invagination of the epidermal tissue. Only a distal shaft part projects into the free exuvial space. The cuticle of the distal and middle shaft region is secreted by the three middle enveloping cells (e 2–e 4) (=trichogen cells), which are arranged around the dendritic sheath.The wall of the cylinder, in which the distal shaft is situated, is formed by the cuticle of the future proximal shaft region. It is secreted by the outer enveloping cells (e 5 and e 6). Furthermore, both enveloping cells form the hair socket (=trichogen-tormogen cells).The outer dendritic segments encased within a dendritic sheath run up through the newly formed hair shaft and continue to the old cuticular apparatus. The connection between sensory cells and old hair shaft is maintained until ecdysis. On ecdysis the old cuticle is shed and the newly formed shaft of the sensillum is everted like the invaginated finger of a glove. The dendritic sheath and the outer dendritic segments break off at the tip of the new hair shaft. Morphologically this moulting process ensures that the sensitivity of the receptors is maintained until ecdysis.The internal organization of the sensory cells shows no striking changes during the moulting cycle. An increased number of vesicles is accumulated distally within the inner dendritic segments and distributed throughout the outer segments of the dendrites. The cytoplasmic feature of the enveloping cells indicates that synthesis and release of substances for the cuticular apparatus of the new sensillum take place.     

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.     

The structure of the postantennal organ in Onychiurus sp. (Insecta: Collembola) and its connection to the central nervous system

Summary The postantennal organ in Onychiurus (group armatus) is a sensory organ comprising one sensory cell, several enveloping cells and cuticular structures.The perikaryon of the sensory cell is located in the central nervous system and distally gives off a dendrite in which one inner and two outer segments are distinguishable. Two ciliary structures connect the outer dendritic segments with the inner segment. The outer segments divide repeatedly, basal to the cuticular structures, into small branches which end distally beneath the cuticular wall. The wall of the cuticular structures is very thin and is pierced by numerous funnel-shaped pores. The pores are filled with electron-dense material which forms a continuous sheath underneath the cuticle. This material encases the small dendritic branches and the processes of the enveloping cells which occupy the lumen of the cuticular structures. There are three types of enveloping cells: one inner, several outer and one basal. Their processes differ in the manner in which they envelop the various regions of the dendrite.At the beginning of moulting outer dendritic branches are not found within the cuticular structures of the organ. They may be assumed to retract inwardly. However, in the later stages, when the cuticle is fully formed, the outer dendritic segments appear to divide. It is assumed that the small dendritic branches reach their targets before ecdysis. The electrondense material which clogs the intermoult cuticular pores is absent until the final stages of the moulting cycle.Supported by a grant from the Deutscher Akademischer Austauschdienst.     

The eversible tentacle organs of Polyommatus caterpillars (Lepidoptera,Lycaenidae): Morphology,fine structure,sensory supply and functional aspects

In their late (3rd and 4th) larval stages, caterpillars of the myrmecophilous lycaenid (Lepidoptera) species Polyommatus coridon and Polyommatus icarus, possess on their 8th abdominal segment two eversible so called tentacle organs (TOs). Previous histological and behavioural results have proposed that the TOs may release a volatile substance that elicits “excited runs” in attendant ants. In our study we investigated for the first time the temporal in- and eversion pattern of TOs. Using nerve tracing, Micro-CT, light- and electron microscopy techniques we studied (i) the histology of the 8th abdominal segment, (ii) the fine structure of the cuticular and cellular apparatus of the TOs, (iii) the attachment sites of the retractor muscle of each TO and (iv) the fine structure of the long slender tentacle hairs which are exposed to the outside, when the TOs are everted and fold back into the TO-sac during inversion. Our data show that the tentacle hairs are typical insect mechanoreceptors, each innervated by a small bipolar sensory cell with a tubular body in the tip of the outer dendritic segment. The latter is enclosed by a cuticular sheath previously called the “internal cuticular duct” and misinterpreted in earlier studies as the space, where the tentacle hairs actively secrete fluids. However, we found no glandular structures nearby or in the wall of the TO-sac. Also we did not reveal any conspicuous signs of secretory activity in one of the enveloping cells belonging to a tentacle hair. Although highly unusual features for an insect mechanoreceptor are: (a) the hair-shaft lumen of tentacle hairs contains flocculent material as well small vesicles and (b) the thin cuticular wall of the hair-shaft and its spines possess few tiny pores. Our data do not support the assumption of previous studies that volatile substances are released via the tentacle organs during their interactions with ants which in turn are supposed to cause excited runs in ants.     

Development of the hair mechanosensilla on the pupal labial palp of the butterfly,Pieris rapae L. (Lepidoptera : Pieridae)

Structure and ontogeny of the hair mechanosensilla on the distal segment of the pupal labial palp of Pieris rapae (Lepidoptera : Pieridae) were investigated in 7 successive stages between 28 hr after pupation and emergence of the imago. There are 7–8 mechanosensilla in the distal region of each palp in both sexes. These sensilla house a single sensory cell characterized by a tubular body, and 3 enveloping cells.At 28 hr after pupation, the anlagen of the hair mechanosensila are visible. Consecutive steps in the formation of the sensilla are: (1) elongation of the outer dendritic segment and of the dendritic sheath; (2) outgrowth of the trichogen cell and cuticle deposition; (3) increase in the diameter of the dendritic outer segment and in the number of microtubules within it; (4) reduction of the distal part of the dendritic outer segment and formation of the tubular body; (5) folding of the membrane of the dendritic outer segment and appearance of the receptor lymph cavity.The tubular body is formed during a period of about 80 hr. Its earliest appearance comprises groups of 3–4 microtubules, which are connected by electron-dense material. The final dense tubular body develops via microtubules linked together by electron-dense material.     

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.     

Apical antennal sensilla in nymphs of Libellula depressa (Odonata: Libellulidae)

Abstract. In an ultrastructural study of the apical antenna of the last nymphal stages of Libellula depressa (Odonata: Libellulidae), we found long sensilla trichodea, 2 sensory pegs, and a coeloconic sensillum on the last article of the flagellum (the distal part of the antenna). The long sensilla trichodea are mechanoreceptors, almost identical to the long filiform hairs of some terrestrial insects and the first sensilla of this kind to be described in aquatic insects. Particular attention was given to the complex coeloconic sensillum, a compound sensillum innervated by 2 groups of 3 neurons wrapped in a dendritic sheath. A cuticular sleeve envelops the distal portion of the outer dendritic segment. The cuticle of the coeloconic sensillum shows wide channels and is contiguous to the underlying granular and fibrillar layer. Similar structures on the antennae of the adults of other dragonflies were identified as chemoreceptors in previous studies. We hypothesize that this larval coeloconic sensillum might likewise have a chemosensory function, responding to molecules that diffuse through the cuticle and the underlying granular and fibrillar layer, as no clear pore or pore-tubule system is visible. Alternative functions are also explored on the basis of morphological details.     

Embryonic development and molting of the antennal coeloconic no pore- and double-walled wall pore sensilla in Locusta migratoria (Insecta,Orthopteroidea)

Summary The embryonic development of antennal coeloconic sensilla was studied at four stages between 132 and 252 h after oviposition in Locusta migratoria. Initially the anlagen of the sensilla consist of 2–4 sensory cells and 3 enveloping cells. Two additional cells contribute later to the formation of socket and pit. The dendritic outer segments of the sensory cells elongate before the trichogen process grows out (ecdysis type I) with exception of one sensory cell in anlagen of poreless (np) sensilla. Other differences between np and double-walled wall pore (dw wp) sensilla are not visible until at least about 220 h after oviposition. Molting, which was studied in four stages, follows ecdysis type I in both sensillum types. The fourth enveloping cell maintains its tight connection to the socket of the sensillum even after apolysis. Its apical portion is torn off and shed together with the old cuticle. The electron-dense material between the dendritic sheath and the cuticular wall of the peg in np sensilla, which is regarded important for stimulus transmission, is not deposited during retraction of the trichogen cell. The concentric walls and spoke channels characteristic of dw wp sensilla result from deposition of cuticular material around wedge-shaped projections of the trichogen cell. The typical trilaminar 15 nm cuticulin layer is produced only on the ridges of these sensilla. The first cuticular lining of the spoke channels is only 7 nm thick and of a different structure. A flocculent material surrounds the outgrowing trichogen process. It is continuous with the filling of the spoke channels and can thus be considered as component of the stimulus-transmitting material in the functioning intermolt dw wp sensilla.     

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.     

The infrared receptor of Melanophila acuminata De Geer (Coleoptera: Buprestidae): ultrastructural study of a unique insect thermoreceptor and its possible descent from a hair mechanoreceptor

The paired infrared organs of Melanophila acuminata consist of 50-100 sensilla situated at the bottom of a pit next to the coxae of the mesothoracic legs, where no exocuticle is developed. Each sensillum is accompanied by a wax gland and has a cuticular lens-like spherule (diameter 12-15 mum) bulging out with its upper hemisphere above the surface, covered only by a thin cuticle of about 1 mum. Distal processes of two enveloping cells surround the entire spherule in the form of a flattened protoplasmatic layer with the exception of a small apical stalk connecting the spherule to the outer cuticle. The spherule is innervated by a single sensory neuron of the ciliary type which is anchored ventrally with the distal tip of its cylindrical and unbranched DOS in the spherule. The insertion of the dendrite, which contains a well-developed tubular body, is always eccentric like in a hair mechanoreceptor (sensillum trichodeum) and there is no evidence of any optical function of the spherule. Three enveloping cells exist, but only one - probably the trichogen cell - forms a relatively small outer receptor lymph cavity. In the posterior wall of the pit - where exocuticle is developed - so-called suppressed systems can be found which remain completely below the cuticle with their otherwise well-developed spherules. Additionally, there is a tendency towards basally flattening and longitudinally stretching of spherules which are situated more peripherally. They strongly resemble the basal regions of hair mechanoreceptors (sensilla trichodea) in their immediate neighbourhood which are also accompanied by wax glands. Because of the existence of these transitional stages and the great ultrastructural resemblance between infrared receptors and hair mechanoreceptors concerning the bauplan of the sensory neurons and their mode of innervating the cuticular apparatus, we conclude that the infrared sensilla are probably derived from hair mechanoreceptors. Based on these results and transmission measurements of infrared radiation through the cuticular components of the organ, a model of the possible function of the infrared receptor is presented.     

Fine structure and role in behavior of sensilla on the terminalia of Aedes aegypti (L.) (Diptera: Culicidae)

The terminalia of male and female Aedes aegypti (L.) bear numerous hairs of various shapes and lengths, all of which are mechanoreceptors. Each hair is innervated by one bipolar neuron which contains ciliary rootlets, two basal bodies, and a region assuming the structure of a non-motile cilium. At the distal tip of the dendrite is a tubular body, a characteristic of cuticular mechanoreceptors. Covering the outer dendritic segment is a cuticular sheath which ends proximally in a net-like felt-work and distally attaches to the hair base. Each hair sensillum has two sheath cells. Presumed efferent fibers are associated with the sheath cells. On the insula of the female terminalia are a few campaniform sensilla, the domes of which are raised into small pegs. The sensilla on the terminalia function in copulation and oviposition and probably in warning. A sequence of neurological events is traced for copulation and oviposition. Other cuticular structures, viz., scales, microtrichia, acanthae, and aedeagal spines, which occur on the terminalia are not innervated.     

Are the most numerous sensilla of terrestrial isopods hygroreceptors? Ultrastructure of the dorsal tricorn sensilla of Porcellio scaber

The ultrastructure of the tricorn sensilla of the woodlouse Porcellio scaber was investigated in cryofixed and freeze-substituted, or chemically fixed specimens. The tricorn sensilla have a foramenized triangular-shaped outer hair and bear a poreless rod-like inner hair. The conical base of the inner hair is connected to the base of the outer hair by a complex cuticular structure. Each sensillum contains three sensory cells. The tip of one of the three dendrites contains a tubular body and is clamped between two bulges of the dendritic sheath. The two other dendrites protrude to the tip of the inner hair, flush against the cuticular wall. The microtubules in the ciliary segments are arranged in nine double tubuli that have neither osmiophilic cores nor arms. The ciliary rootlets are small. The inner segment of the largest dendrite wraps around the two smaller dendrites and one of seven enveloping cells in a mesaxon-like manner. Although this ultrastructure deviates considerably from most crustacean mechanosensitive sensilla, it nevertheless suggests a mechanosensitive function, at least for one of the sensory cells. In many aspects, the tricorn sensilla resemble the thermohygrosensilla of insects. However, our results suggest that the structural criteria for thermo-hygro-sensitivity used in insects cannot simply be applied to crustaceans.     

The intermediate moult (First Ecdysis) of Schistocerca gregaria (Forskål) (Insecta,Orthoptera)

Ecdysis of the embryonic cuticle has been studied in detail. The initiation and mechanical processes involved in this ecdysis, and the associated differential size increases of cuticular parts have been given most attention. The roles of air swallowing and of thoracic contraction are most imoprtant for rupturing the old cuticle, while abdominal contractions assist the whole process. Some specialized cuticular structures are described.     

Development of the filiform hairs on the cerci of Gryllus bimaculatus Deg. (Saltatoria,Gryllidae)

Summary The filiform hairs, mechanoreceptors of Gryllus, pass through six developmental stages during the last larval stage. The cytoplasm of their sense cells suggests intensive synthesis of protein for cellular metabolism and intercytoplasmic exchange of material via glial evaginations. Ultrahistochemical tests demonstrated acid phosphatase in the lysosomes as well as in components of the Golgi apparatus. There was no significant change in the appearance of the sense cell cytoplasm, indicating a maintained functional state also during molting. The new cuticular apparatus is formed after apolysis by the three enveloping cells. Formation of the replacement hairs is initiated by a cytoplasmic outgrowth of the trichogen cell. During morphogenesis of the new hair, the microtubules serve as a cytoskeleton and probably control the flow of vesicles, which contain phenol oxidase, also demonstrated in the Golgi apparatus, and are incorporated into the new cuticle. Bundles of microfibrils are involved in the surface sculpturing of the replacement hair. The trichogen cell also forms a number of structural elements, e.g. the cup and strut marked geometric peculiarities of which indicate that they are important in the spatial orientation of the dendrite and thus also in transduction. Reduction of the apical cell membrane of the tormogen cell after apolysis permits unrestricted growth of the new hair into the exuvial space. The tormogen cell participates in the formation of the joint membrane, parts of the socket and the articulation of the hair.Supported by the Deutsche Forschungsgemeinschaft. The author thanks Mrs. G. Thomas for drawing the diagrams, and Miss I. Grossman and Mrs. M. Ullmann for technical assistance     

Morphology and ultrastructure of the sensory spine,a presumed mechanoreceptor of Sphaeroma hookeri (Crustacea,Isopoda), and remarks on similar spines in other peracarids

The isopod Sphaeroma hookeri and many other isopods and peracarids have a sensory spine with laterally inserting sensory hair, positioned in the apical region of the propodal palm of pereopod 1. This spine is innervated by five to eight sensory cells (each giving rise to one cilium) the dendrites of which can be divided into an inner and outer dendritic segment. The cilia are surrounded by an extracellular, electron-dense dendritic sheath. Thirteen enveloping cells are present. The outer dendritic segment (structure beyond the basal bodies) contains two receptor lymph cavities; the inner one lying within the dendritic sheath is homologous with the inner receptor lymph cavity of insects. Scolopales, or tubular bodies, are lacking; their function is probably accomplished by the dendritic sheath. Apically the sensory hair does not have a pore, and the spine is heavily sclerotized. The inner dendritic segment begins with a basal body from which rootlets of different length and thickness extend into the dendrite. In the latter is an accumulation of vesicles. The dendrites keep close contact with other dendrites and the enveloping cells by desmosomal membrane structures. The possible importance of the sensory spine for phylogenetic studies is discussed.     

Fine structure of the pheromone-sensitive sensilla on the antenna of the hawkmoth, Manduca sexta

The flagellar antenna of the male hawkmoth Manduca sexta carries about 42,000 pheromone-sensitive sensilla trichodea, which are arranged in 'baskets' on the single segments. Each sensillum consists of a cuticular hair up to 500 mum long and is innervated by two bipolar sensory neurons. Each neuron sends an unbranched dendrite into the hair shaft. The dendrite is subdivided by a short ciliary region into an inner and an outer segment. The inner segment is especially rich in smooth vesicles, which accumulate beneath the ciliary region where they seem to fuse with the dendritic membrane. The outer dendritic segment often shows conspicuous 'beads' along its length. Three auxiliary, or enveloping, cells belong to each adult sensillum. These are the thecogen, the trichogen, and the 'outer' cell. Most probably, the latter is not homologous with the 'traditional' tormogen cell from a genealogical point of view.     

Ultrastructure of the phallic glands of the firebrat,Thermobia domestica (packard) (Thysanura : Lepismatidae)

The exocrine glands located in the penis of Thermobia domestica (Thysanura : Lepismatidae) are composed of about 100 distinct units, each containing several cell types: one large secretory cell with an apical reservoir; 2 groups of envelope cells, an inner group of 2 superimposed cells, and an outer group of 4 cells arranged in a ring, and also 2 basal cells, called ciliary cells owing to their elongated processes, which look like the dendrite of a sensory cell. Each functional unit includes cuticular differentiations: a tubular bristle, fixed on a small tubercle; and a long “internal” ductule communicating basally with the reservoir of the glandular cell and opening distally at the tip of the bristle. A study of the modifications affecting the phallic glands during moulting, shows that the inner envelope cells deposit the cuticle that forms the ductule, the outer envelope cells elaborate the cuticule of the tubercle, while a temporary distal projection of only one of these cells ensures the formation of the bristle. In addition, a lengthening of the outer dendritic segment of the 2 ciliary cells takes place before ductule formation, but this segment partially degenerates after ecdysis. These findings are compared with data already obtained on the morphogenesis of other insect integumental glands. In T. domestica, the secretion of the phallic glands is presumed to be used, during the mating sequences, for spinning fine threads before spermatophore deposition.     

The hair-peg organs of the shore crab,Carcinus maenas (Crustacea,Decapoda): Ultrastructure and functional properties of sensilla sensitive to changes in seawater concentration

Summary The hair-peg organs of the shore crab, Carcinus maenas, are modified hair-sensilla. A small hair shaft (peg) is surrounded by a tuft of solid cuticular bristles (hairs). Each hair-peg organ is innervated by 6 sensory neurons, 2 of which have scolopidial (type-I) dendrites. The outer segments of all dendrites pass through a cuticular canal extending to the articulated hair base in which the 2 type-I dendrites terminate. The other 4 (type-II) dendrites reach the clavate tip of the hair shaft and have access to a terminal pore and a large sickle-shaped aperture. Three inner and 8–12 outer enveloping cells belong to a hair-peg organ. The innermost enveloping cell contains a scolopale, which has desmosomal connections to the ciliary rootlets of the type-I dendrites. An inner and an outer sensillum lymph space are present. The ultrastructural features of the dendrites and the cuticular apparatus indicate that the hair-peg organs are bimodal sensilla, comprising 2 mechano- and 4 chemosensitive sensory neurons. Extracellular recordings from the leg nerve indicate that the chemosensitive neurons of the hair-peg organs respond to changes in seawater concentration in the physiological range of Carcinus maenas.Supported by the Deutsche Forschungsgemeinschaft (SFB 45/A1; W. Gnatzy)     

The morphology of spider sensilla. I. Mechanoreceptors

The common tactile hair sensilla of spider tarsi were studied in web spiders (Araneus) and ground spiders (Lycosa, Dugesiella) using scanning and transmission electron microscopy. All of these sensilla are innervated by three bipolar neurons whose dendrites end proximally at the sensillum base. Each dendritic terminal exhibits a tubular body, a dense array of microtubules typical for mechanoreceptive sensilla. A dendritic sheath encloses the outer dendritic segments and connects the dendritic terminals to cuticular components of the hair sensillum in three different ways: (1) A distal extension of the dendritic sheath connects to the midline of the hair base; (2) A forked arrangement of cuticular (?) strands attaches on both lateral sides of the hair base, and (3) The socket cuticle directly contacts a part of the dendritic sheath. The latter connection provides a fixed position for the three dendritic terminals and any movement of the hair shaft could be transmitted via connections (I) and (2). The triple innervation strongly suggests a directional sensitivity of these sensilla.Structural comparison between arachnid and insect mechanoreceptive sensilla indicates that tactile hair sensilla in Arachnida are multi-innervated whereas the corresponding reccptors in Insecta are singly innervated.     

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

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