Morphogenesis during molting of the setae in the statocyst sensilla of the mysid shrimp Neomysis integer (Leach, 1814) (crustacea,mysidacea)

The morphogenesis of the setae in the statocyst sensilla of Neomysis integer was studied. Immediately before ecdysis, a new seta lies inverted between the enveloping cells. All of the nine enveloping cells, except the first one, secrete a well defined part of the new seta. The second, third, fourth, and fifth have a trichogen function; the sixth has a trichogen-tormogen function; and the seventh, eighth, and ninth enveloping cells have a tormogen function. It could not be established whether the dendritic sheath is replaced at molt. In the second and third enveloping cells, there is a differential secretion of cuticular material forming the wall of the distal part of the seta. As a consequence, this wall is not homogeneous. The possible role of this heterogeneity in the formation of the gutter-like apical part of the seta is discussed. A mechanism is proposed by which the pore develops at the transition between the midpart and the apical gutter-like part. Before ecdysis, the distal segments of the sensory cells are still connected with the wall of the old seta in the same way as during intermolt. No degeneration is apparent in the distal segments during preparation for the molt. These morphological findings suggest that sensitivity of the sensilla must be maintained until the moment of the ecdysis.     

Structure of the auditory system of the weta Hemideina crassidens (Blanchard, 1851) (Orthoptera,Ensifera, Gryllacridoidea,Stenopelmatidae)

Summary This study of the ultrastructure of the auditory sensilla of the New Zealand weta, Hemideina crassidens, is the first such study on a member of the orthopteran Superfamily Gryllacridoidea. Ultrastructure of the auditory sensilla is similar in all of the tibial mechanosensory organs, here called subgenual organ, intermediate organ and crista acoustica by analogy with comparable structures in Tettigoniidae.Distal to each sensory soma is a dendrite containing multiple ciliary rootlets that fuse into a single ciliary root. This splits into nine root processes that pass around the outside of the proximal basal body and then rejoin at the level of the distal basal body, distal to which the dendrite has a modified ciliary structure with a circlet of nine peripheral paired tubes and rods as it passes through the proximal extracellular space. It is then enclosed by a zone of scolopale cell cytoplasm before expanding into a dilatation within the distal extracellular space. In some sensilla this space is partially occluded by electron dense material which is part of the scolopale cell. Distal to the dilatation the cilium shrinks and ends surrounded by the scolopale cap.Accessory cells consist of glia enwrapping the sensory neuron in the region of its soma, the scolopale cell surrounding the ciliary portion of the dendrite, and the attachment cell surrounding the scolopale cell and scolopale cap and connected to them by desmosomes. The attachment cells are filled with microtubules in differing densities and orientations. Lamellae are present in the acellular matrix surrounding the attachment cells. Banded fibres, presumably of collagen, are also present in the matrix.     

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.     

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.     

Femoral chordotonal organ in the legs of an insect,Chrysoperla carnea(Neuroptera)

The femoral chordotonal organ (FCO) inChrysoperla carneais situated in the distal part of the femur and consists of two scoloparia, which are fused at their distal end. The distal scoloparium contains 17-20 scolopidia, and the proximal one six scolopidia. Each scolopidium consists of two sensory cells and three types of enveloping cells (glial, scolopale and attachment cell). The sensory cells of different scolopidia do not lie at the same level in the FCO. Therefore the attachment cells of different scolopidia have different lengths. In the FCO, three types of ciliary roots are found in different sensory cells. The dendrite of the sensory cell terminates in a distal process, which has the structure of a modified cilium (9x2+0). The very distal part of the cilium is surrounded by an extracellular electron dense material, the cap, and ends in a terminal dilation. The scolopale cell contains the electron dense scolopale rods, consisting of plentiful microtubules. In their middle third the scolopale rods are fused and form the scolopale. In the FCO septate junctions, desmosomes and hemidesmosomes are found.     

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

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

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.     

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.     

Morphology of the prothoracic discs and associated sensilla of Acanthocnemus nigricans (Coleoptera, Acanthocnemidae)

The Australian ‘little ash beetle’ Acanthocnemus nigricans (Coleoptera, Cleroidea, Acanthocnemidae) is attracted by forest fires. A. nigricans has one pair of unique prothoracic sensory organs and it has been speculated that these organs may play a role in fire detection. Each organ consists of a cuticular disc, which is fixed over an air-filled cavity. On the outer surface of the disc, about 90 tiny cuticular sensilla are situated. The poreless outer peg of a sensillum is 3–5 μm long and is surrounded by a cuticular wall. One ciliary sensory cell innervates the peg. As a special feature, the outer dendritic segment is very short already terminating below the cuticle. A massive electron-dense cylindrical rod, which most probably represents the hypertrophied dendritic sheath, extends through the cuticular canal connecting the tip of the outer dendritic segment to the peg. The dendritic inner segment and the soma are fused indistinguishably. Thin, leaflike extensions of glial cells deeply extend into that conjoint and considerably enlarged compartment which also contains large numbers of mitochondria. In summary, the sensilla of the sensory disc of A. nigricans represent a new type of insect sensillum of hitherto unknown function. The possible role of the prothoracic sensory organ in fire detection is discussed.     

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

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

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

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

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.     

An Ultrastructural Account of Otoplanid Turbellaria Neuroanatomy II. The statocyst design: evolutionary and functional implications

The statocyst architecture in the three otoplanid species Notocaryoturbella bigermaria Lanfranchi, 1969, Otoplana truncaspina Lanfranchi, 1969 and Parotoplanella heterorhabditica Lanfranchi, 1969 is compared. Common features are: (a) a fibrillar collagen-like, 0.2 μm thick, investing capsule continuous with the brain capsule; (b) an inner wall made up of six or more flattened and overlapping parietal cells; (c) a statolith forming cell hanging from the dorsal side down in the lumen, with a large statolith containing vacuole; (d) a bilateral pair of spindle shaped accessory cell groups, adjoining the statolith cell and sending projections to the wall—nerve projections run through the capsule; (e) one accessory cell enveloping the other cells of the group has a filament containing cytoplasm, the filaments coverging into a hemidesmosome making contact with a projection coming from a parietal cell; (f) muscles from the longitudinal body musculature inserting onto the capsule externally. The lack of ciliary structures differentiates the turbellarian statocyst from the majority of invertebrate statocysts. The developmental origin, the phylogenetical meaning and the functional and adaptive value of the statocyst in Turbellaria are here commented on.     

Reduction of sensory cells in antennal sensilla correlated with changes in feeding behavior in the beetle Loricera pilicornis (Insecta,Coleoptera, Carabidae)

Summary The structure of the setae on the proximal antennal segments of the beetle Loricera pilicornis is described using electron microscopical methods. These setae are part of a prey-capturing apparatus and are inserted within flexible sockets. They have no central lumen.Four or five sensory cells are connected to each seta. One cell is characterized as a mechanoreceptor due to the presence of a tubular body and the location of its dendritic outer segment. The other sensory cells are of two types. One type shows the usual features of sensillar receptors except that the dendritic outer segments end beneath the seta within the cuticular sheath. In the other type all parts of the cell, including the perikaryon, appear undersized, and no axon was found. In a single case a sixth cell was found which lacks any process, although, due to its location, it belongs to the sensory cell group.The enveloping cells also deviate from the usual pattern. Trichogen and tormogen cells have no membrane folds nor microvilli. From the membrane of the thecogen cell, where it borders on the inner receptor lymph cavity, invaginations have developed which form voluminous membrane whorls. Portasomes are found on these membranes.On the basis of the structural features we hypothesize that the setae represent sensilla undergoing stepwise reduction, losing primordial gustatory units whilst the prey-capturing mechanism is optimized.Dedicated to Professor Dr. Dietrich Schneider on occasion of his 65th birthday     

The fine structure and possible function of the sensory setae of the penis of Balanus balanoides (L.)

The structure and innervation of the sensory setae which are present in large numbers on the penis of Balanus balanoides (L.) have been established by scanning and transmission electron microscopy. Each seta contains a small number of sensory dendrites surrounded by an extracellular supporting tube which is presumed to be secreted by the enclosing sheath cell. The dendrites, which distally extend beyond both the sheath cell and supporting tube, terminate at the tip of the seta within a pore-like invagination of the cuticle and thus are in direct contact with the environment. Proximally bundles of dendrites pass into the penis tissue where they are surrounded by several sheath cells. The supporting tube terminates at a point within the body of the penis where a series of intracellular rods arise. The ciliary character of the dendrites is evident in this region, the microtubules being organized into the (9 × 2) +2 pattern. It is deduced that the sensilla are chemosensory; their structure is compared with that of other crustacean sensilla which are presumed to be chemosensory.     

Ultrastructure of the sensory palps ofTetranchyroderma papii (Gastrotricha,Macrodasyida)

Summary The sensory palps of the macrodasyoid gastrotrichTetranchyroderma papii contain processes from two types of cell: 22–23 bipolar primary sensory cells and two to three support cells. In the proximal region of the palp each sensory cell contains a short ciliary segment with a basal body and from this ciliary segment a longer distal segment lacking axonemal microtubules extends through the major part of the length of the palp. Each support cell process bears microvilli and contains a conspicuous bundle of microtubules running the entire length of the process. The cell bodies of both cell types are situated in the epidermis of the head region. The palps are interpreted as having a chemosensory function. They are considered to be homologous to the posterior cephalic sensory organ ofTurbanella cornuta, but not the head tentacles ofChordodasys antennatus or nematode amphids.     

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 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.     

Digitiform Sensilla on the Maxillar Palp of Coleoptera

The fine structure of the digitiform sensilla on the distal segment of the maxillar palps of Tenebrio and Dermestes is described. Each sensillum is associated with a single sensory cell and three enveloping cells, which enclose two receptor lymph cavities. The inner receptor lymph cavity of both species shows a different structural feature. Branches of the outer dendritic segments, which contain numerous microtubules, run to the tip of the hairshaft. A dendritic sheath extends to the apex of the peg. The hairshaft possesses a second canal, which is free of dendrites. The poreless hairshaft is inserted in a cuticular canal; the longer distal part of the shaft is positioned in a narrow superficial groove. The digitiform sensilla do not show the typical structures of mechanosensitive sensilla. The absence of pores in the setal wall does not point to a function as olfactory or gustatory hairs. The presumed function of the sensilla is discussed in relation to thermo-, hygro- and CO₂-receptors.