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.     

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.     

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.     

Ultrastructure of the galeal sensory complex in adults of the Colorado potato beetle, Leptinotarsa decemlineata

ABSTRACT. The structure of galeal sensilla of the Colorado potato beetle, Leptinotarsa decemlineata Say (Coleoptera: Chrysomelidae), is described using electron microscopical methods. Previous electro-physiological studies indicate that these sensilla respond to amino acids, sucrose and plant saps. One physiological type is particularly sensitive to L-alanine and gamma amino butyric acid (GABA).
Three morphologically different types of sensilla occur on the galeal tip. The more numerous apical pegs are not distinguishable from one another on the basis of external structure, although they differ physiologically. Five sensory cells are associated with most apical pegs. One apical peg, the α-sensillum, contains only four cells. All apical pegs have one cell with a tubular body. The remaining cells have unbranched dendrites and are associated with a single apical pore.
Apical hairs differ from the apical pegs by having double innervation. Within the hair shaft, a dendritic sheath is lacking and the sensillar sinus extends to the base of the hair. The function of this hair type is not known.
Numerous mechanosensory hairs which surround the other sensilla are singly innervated and contain a tubular body at the level of the outer dendritic segments.     

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.     

Sensory cell degeneration in the ontogeny of the chemosensitive sensilla in the labial palp-pit organ of the butterfly,Pieris rapae L. (Insecta,Lepidoptera)

Summary The ontogeny of the chemoreceptive sensilla in the labial palp-pit organ was studied in Pieris rapae by examining twelve successive stages between pupation and emergence of the imago, which takes a period of 160 h under the experimental conditions. Mitoses occur until 20 h after pupation. They lead to anlagen of sensilla, 91% of which are comprised of three sensory cells. However, two sensory cells degenerate in each sensillum during a period of 28 h. The same process occurs in anlagen with four sensory cells resulting in bicellular sensilla. Axons grow out only after the number of sensory cells has been reduced. Further consecutive steps in sensory cell differentiation are: (a) outgrowth of dendritic outer segment and dendrite sheath; (b) outgrowth of trichogen process and change in structure of elongating dendrite sheath; (c) deposition of cuticle and pore tubules in the pegs; (d) retraction of trichogen process; (e) increase in diameter of dendritic outer segment accompanied by increase of microtubule number and appearance of regularly spaced electron-dense bodies at tubular doublets; (f) branching of dendritic outer segment; and (g) transformation of the dendritic branches into curled lamellae and partial destruction of the dendrite sheath. The unique process of sensory cell degeneration is interpreted as an event that revokes a step towards a possible functional improvement of the labial palp-pit organ during further evolutionSupported by the Deutsche Forschungsgemeinschaft (SFB 4/G1)     

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.     

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.     

The mechanosensory apparatus of the femoral tactile spine of the cockroach,Periplaneta americana

Summary Tactile spines are large cuticular sense organs that appear to provide insects with a sense of touch which is spatially coarse but of great sensitivity. Cockroach legs have a number of these spines on each leg and a particularly prominent spine on the end of each femur, the femoral tactile spine. The ease of recording afferent activity from this spine during mechanical stimulation has made it one of the most thoroughly studied insect mechanoreceptors and yet it has never been examined by electron microscopy.We report here the results of an examination of the femoral tactile spine by both scanning and transmission electron microscopy, as well as by light microscopy. The spine is shown to be innervated by a single sensory bipolar neuron with its soma located in the base of the spine. A canal through the wall of the spine leads to the outside and emerges just above the junction between the base of the spine and its articulating socket membrane. The sensory dendrite of the neuron passes from the soma through this canal and forms a modified ciliary sensory ending with the typical dendritic sheath and dense tubular body that is characteristic of insect mechanosensory cuticular sensilla. The tubular body is embedded in a cuticular terminal plug which closes the exterior end of the canal but appears to be fastened to the spine by a very flexible ring of cuticle. This plug is connected to the socket membrane by a specialized socket attachment which presumably serves to move the plug relative to the wall of the spine during movement of the spine within the socket. The morphology of this sensillum is discussed in terms of the possible ways in which it is stimulated by movements of the spine and also in light of the dynamic behaviour of the receptor which is now very well described.Supported by the Canadian Medical Research Council. The authors gratefully acknowledge the expert technical assistance of Sita Prasad and Rodney Gramlich     

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.     

Innervation of the cercal sensilla on the ovipositor of the Australian sheep blowfly (Lucilia cuprina)

ABSTRACT. The ovipositor of the female sheep blowfly, Lucilia cuprina (Wied.) (Diptera: Calliphoridae), has a complement of cercal sensilla that includes long, medium and short tactile hairs, two campaniform domes, four olfactory pegs, and ten double-channelled gustatory hairs. This sensory array is suited to assess oviposition site resources, prior to and during the laying of an egg batch.
The tactile hairs and campaniform sensilla are each innervated by a single, tubular body containing dendrite. The olfactory pegs are each innervated by a single, moderately branched dendrite, which gains access to the external environment via pores at the bottom of deep grooves in the peg wall. The gustatory hairs fall into two categories. Four hairs have a single, tubular body containing dendrite at their base, and four unbranched dendrites running up to the hair tip which has a terminal pore. Six of the taste hairs have no tubular body containing dendrite at the base, and three unbranched dendrites running up to a terminal pore.     

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.     

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 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.     

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.     

Anatomy of the antennal dorsal organ in female of Neodryinus typhlocybae (Hymenoptera: Dryinidae): A peculiar sensory structure possibly involved in perception of host vibration

Neodryinus typhlocybae (Hymenoptera: Dryinidae) is a natural enemy of the planthopper Metcalfa pruinosa, which was introduced from North America into Europe and has become established in various regions as a pest species. Vibrational signals play a crucial role in the communication of M. pruinosa, which appears to be exploited by N. typhlocybae. Scanning and transmission electron microscopy have shown that the antennae of N. typhlocybae females have peculiar and complex sensory structures: deep longitudinal grooves that house long sensilla trichodea, termed here “Antennal Dorsal Organs.” Such structures were not present on male antennae. These sensilla extend for the length of the grooves, without contact with the groove cuticle. Their hair shaft is empty and aporous, and inserted into a specialized socket, underneath which there is a cuticular ampulla‐like chamber. Each sensillum is associated with two sensory neurons: one terminates at the proximal end of the dendritic sheath; the other continues into the sensillum sinus and is enclosed in the dendritic sheath. This second sensory neuron then enters the ampulla‐like chamber through the circular opening, and then terminates with a conspicuous tubular body at the shaft base. The possible involvement of this peculiar structure in the context of host recognition mechanism is discussed. J. Morphol. 277:128–137, 2016. © 2015 Wiley Periodicals, Inc.     

Properties of the trochanteral hair plate and its function in the control of walking in the cockroach.

1. The physiological properties of the group of long hair sensilla of the trochanteral hair plate in the cockroach metathoracic leg were studied. The sensilla were divided into type I and type II according to their responses to imposed displacements. 2. Type I hair sensilla responded to dynamic displacements whereas type II hair sensilla responded to both dynamic and static displacements. The hair sensilla are normally excited by phasic flexion movements of the femur near the end of leg protraction. 3. Activity in the trochanteral hair plate afferents had a short latency excitatory effect on the motoneurone producing slow extension movements of the femur and an inhibitory effect on the femur flexor motoneurones. 4. Removal of the trochanteral hair plate in one leg caused overstepping of that leg in a walking animal due to exaggerated flexion of the femur. This change in leg movement can be explained by the removal of the inhibitory influence from the hair plate afferents to the femur flexor motoneurones. 5. We conclude that one function of the trochanteral hair plate is to limit femur flexion during a step cycle.     

Structure and differentiation of the sensilla of the ventral sensory field on the maxillary palps ofPeriplaneta americana (Insecta,Blattodea), paying special attention to the ciliogenesis of the sensory cells

Summary The embryonic development of palpal contact chemosensitive sensilla was studied from 42% of development up to the hatching of the larvae. Ciliogenesis of the sensory cells can be observed at the earliest stages investigated. A complex consisting of two basal bodies and a cap-like ciliary vesicle is localized in the dendritic inner segment. It migrates apically and fuses with the cytoplasmic membrane. At the same time, microtubule doublets of the distal basal body elongate, thus generating the dendritic outer segment. Furthermore, the typical accessory structures of a motile cilium are formed. Although the central pair of microtubules is lacking, the dendritic outer segment can be considered as a modified motile cilium. At about 84% of development the hair structure starts to be formed. Whereas the socket is generated by the tormogen cell, the trichogen cell produces the hair shaft and terminal porus. The dendrite sheath, which rises above the newly formed hair, is attached apically to the embryonic cuticle forming an irregular pore. In larvae and imagines, the inner surface of the dendrite sheath is highly differentiated. A range of circular ledges and filamentous structures wrapping around the dendritic outer segments can be distinguished. These may have a stabilizing function. Furthermore, in cryofixed specimens, the dendritic outer segments possess regularly spaced swellings which are about 1 m in length and about 0.5 m in diameter. Their functional significance is still unclear.     

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 the tibiotarsal and pretarsal sensory organs in Monobella grassei banyulensis Deharveng (Collembola : Neanuridae)

The fine structure of the tibiotarsal and pretarsal sensory organs of Monobella grassei banyulensis Deharveng (Collembola : Neanuridae) has been examined by electron microscopy.Three types of sensory organs have been observed. (1) the most numerous setae of the tibiotarsus are classic mechanosensitive setae with one bipolar sensory cell, whose distal outer segment ends in a tubular body. (2) Two small setae are arranged on each side of the basal part of the claw; they show 3 sensory cells, 2 of which are mechanosensitive cells of the scolopidial type; the outer segments of the 2 mechanosensitive cells end at the base of the sensory hair. The dendrite of the 3rd sensory cell extends into the hair shaft. (3) Two similar chordotonal sensilla link the tibiotarsus and the pretarsus; each sensillum is composed of 2 bipolar sensory cells enveloped in sheath cells. The first type of sensory organ shows the characteristics of insect exteroceptive mechanosensitive hairs. The mechanosensitive cells of the 2nd and 3rd tibiotarsus sensory organs are probably proprioceptive and control the movements of the pretarsus in relation to the tibiotarsus. Two features are noteworthy: (1) the association of the scolopidial cells with a chemosensitive one has never been observed in other insect sensory organs, except in the Collembola; and (2) there is an important morphological diversity in the ciliary roots of the various scolopidial cells, which are in other respects very similar.