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

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

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.     

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.     

Fine structure of antennal putative thermo-/hygrosensilla of adult Rhodnius prolixus Stål (Hemiptera: Reduviidae)

At least five nonporous sensilla with inflexible sockets (npsensilla) occur on each antenna of both sexes of adult Rhodnius prolixus. Externally the sensillum appears as a short, rounded peg set into a pit surrounded by a depression. A very electron-dense material occurs in the peg lumen and the inner aspect of the pit. Filamentous extensions of this material radiate into the overlying outlets. Each sensillum is innervated by three neurons with unbranched dendrites. Two dendrites extend to the peg tip and distally are covered by a dendritic sheath. The portion of these dendrites within the sheath contains a large number of microtubules. The third dendrite terminates near the base of the dentritic sheath and partially wraps around the other two dendrites. Three sheath cells are associated with each sensillum. Based on similarities in structure with sensilla of known function it is probable that the np-sensilla of R. prolixus are thermo-/hygrosensilla responding to cold, dryness and wetness. The sensilla have a number of structural similarities with insect rectal sheath cells known to absorb atmospheric water by electroosmosis. Possibly this process leads to volumetric alterations of cuticular elements associated with the dendrites and ultimately to mechanotransduction.     

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.     

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

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

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.     

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.     

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.     

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.     

The gustatory sensilla on the endophytic ovipositor of Odonata

The present paper aims at describing the fine structure of coeloconic sensilla located on the cutting valves of the endophytic ovipositor of two Odonata species, the anisopteran Aeshna cyanea (Aeshnidae) and the zygopteran Ischnura elegans (Coenagrionidae), by carrying out parallel investigations under SEM and TEM. In both species these coeloconic sensilla are innervated by four unbranched neurons forming four outer dendritic segments enveloped by the dendrite sheath. One dendrite terminates at the base of the peg forming a well developed tubular body, while the other three enter the peg after interruption of the dendrite sheath. The cuticle of the peg shows an apical pore and a joint membrane. This last feature, together with the tubular body and the suspension fibers, represent the mechanosensory components of the sensillum while the pore and the dendrites entering the peg allow chemoreception. The ultrastructural organization of these coeloconic sensilla is in agreement with the one reported for insect gustatory sensilla. Our investigation describes for the first time typical insect gustatory sensilla in Odonata. Electrophysiological and behavioral studies are needed to verify the role that these structures can perform in sensing the egg-laying substrata.     

Fine structure of tarsal sensilla of Aedes aegypti (L.) (Diptera: Culicidae)

The tarsi of all three pairs of legs of both sexes of Aedes aegypti (L.) bear spine sensilla, five types of hair sensilla, which are designated A, B, C1, C2 and C3, and campaniform sensilla. Type A and B hairs, spines, and cam-paniform sensilla are innervated by one neuron with a tubular body, a characteristic of cuticular mechanoreceptors. In particular the hairs and spines are tactile receptors and the campaniform sensilla are proprioceptors. The C1, C2, and C3 hair sensilla have the morphological features of contact chemoreceptors. Type C1 and C3 hairs are innervated by five and four neurons, respectively, which extend to the tip of the hair. Type C2 is innervated by five neurons, one of which terminates at the base of the hair in a tubular body while the remaining four extend to the tip of the hair. The role of the type C hairs in oviposition behavior, nectar feeding, and recognition of conspecific females is discussed. Presumed efferent neurosecretory fibers occur near the spine and hair sensilla.     

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.     

The cercal receptor system of the praying mantid,Archimantis brunneriana Sauss.

Summary The cerci of the praying mantid, Archimantis brunneriana Sauss., are paired segmented sensory organs located at the tip of the abdomen. Basally the cercal segments are slightly flattened dorso-ventrally and are fused to such a degree that it is difficult to distinguish them. Distally the segments become progressively more flattened laterally and their boundaries become more obvious.Two types of sensilla are present on the cerci, trichoid sensilla and filiform sensilla. Trichoid hairs are longest on the medial side of the cerci and toward the cercal base. On the proximal cercal segments they are grouped toward the middle of each segment while they are more uniformly distributed on the distal segments. Filiform sensilla are found at the distal end of each segment except the last and are most abundant on the middle segments of the cercus. Both the number of cercal segments and the number of sensilla are variable. Trichoid hairs are highly variable in appearance from short and stout to long and thin. They arise from a raised base, have a fluted shaft, and some have a pore at the tip. They are innervated by from one to five dendrites, one of which is always considerably larger than the others. Some of the dendrites continue out into the shaft of the hair.Filiform hairs have fluted shafts and are mounted in a flexible membrane within a cuticular ring in a depression. They are innervated by a single large sensory neuron, the dendrite of which passes across a flattened area on the inner wall of the lumen of the hair. The dendritic sheath forms the lining of the ecdysial canal and is therefore firmly attached to the hair. The dendrite is attached to the sheath by desmosomes distally and is penetrated by projections of the sheath more proximally. A fibrous cap surrounds the dendrite and may hold it in place relative to the hair.The cercal receptor system of Archimantis is compared to those of cockroaches and crickets.     

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.     

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

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

Atlas of olfactory organs of Drosophila melanogaster 2. Internal organization and cellular architecture of olfactory sensilla

Antennae and maxillary palps of Drosophila melanogaster were studied with the electron microscope on serial sections of cryofixed specimens. The number of epidermal cells roughly equals the number of sensilla, except for regions where the latter are scarce or absent. Each epidermal cell forms about two non-innervated spinules, a prominent subcuticular space and a conspicuous basal labyrinth, suggesting a high rate of fluid transport through the sensory epithelium. The internal organization and fine structure of trichoid, intermediate and basiconic sensilla is very similar. Receptor cell somata are invested by thin glial sheaths extending distad to the inner dendritic segments. Further distally, the thecogen cell forms a sleeve around the dendrites, but an extracellular dendrite sheath is absent. At the base of the cuticular apparatus, the inner sensillum-lymph space around the ciliary and outer dendritic segments is confluent with the large outer sensillum-lymph space formed by the trichogen and tormogen cells. All three auxiliary cells exhibit many features of secretory and transport cells but extend only thin basal processes towards the haemolymph sinus. The bauplan and fine structure of coeloconic sensilla differs in the following aspects: (1) the ciliary segment of the dendrites is located deeper below the base of the cuticular apparatus than in the other sensillum types; (2) a prominent dendrite sheath is always present, separating inner and outer sensillum-lymph spaces completely; (3) the apical microlamellae of the auxiliary cells are more elaborate, but free sensillum-lymph spaces are almost absent; (4) there are always four not three auxiliary cells. Morphometric data are presented on the diameter of inner and outer dendritic segments and on the size of receptor cells, as well as of the receptor and auxiliary cell nuclei. The special fine structural features of Drosophila olfactory sensilla are discussed under the aspects of sensillar function and the localization of proteins relevant for stimulus transduction.     

Functional anatomy of sensory structures on the antennae of Psylliodes chrysocephala L. (Coleoptera: Chrysomelidae)

The ultrastructure and distribution of sensilla on the antennae of the cabbage stem flea beetle, Psylliodes chrysocephala, were investigated using scanning and transmission electron microscopy techniques. Eight different sensillar types were distinguished. These were; hair plate sensilla, sensilla chaetica, three types of sensilla trichodea, sensilla basiconica, grooved peg sensilla and styloconic sensilla. The sensilla chaetica are known to be gustatory receptors. Ultrastructure indicates that the hair plate sensilla and sensilla trichodea type one are probably mechanoreceptors, whilst the sensilla styloconica are probably thermo-hygro receptors. These thermo-hygroreceptors are unusual in that they are innervated by two sensory cells (one hygroreceptor and one thermoreceptor) rather than the more usual triad. The remaining four sensillar types all have a porous hair shaft, indicating an olfactory role. One of these (the grooved peg sensillum) may also have a thermoreceptive function. No sexual dimorphism was found in the structure, number or distribution of the antennal sensilla.     

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.