The cercal sensilla of the blowfly Lucilia cuprina. II. Structure of the enveloping cells and the basal regions of the sensory dendrites

The gustatory, olfactory, touch and stress receptors on the cerci of Lucilia cuprina Wied. (Diptera: Calliphoridae) have either two or three enveloping cells. The gustatory and olfactory sensilla have three enveloping cells: a tormogen, trichogen and thecogen cell. The tormogen and trichogen cells contribute to a sub-cuticular sensillar lumen which divides into two lobes basally. The thecogen cell forms a lumen around the dendrites. Distally the dendrites lie in the contents of the thecogen lumen within the dendritic sheath. Proximally the dendrites embed in the thecogen cell which has an expanded, microlamellate lumen basally. The sensillar lumen of the mechanosensory (trichoid mechanoreceptors and campaniform) sensilla is formed by a single enveloping cell: the presumptive tormogen cell. In trichoid mechanoreceptors the thecogen lumen is restricted to the region of the transitional region of the dendrite whereas the thecogen lumen of campaniform sensilla extends proximally although it is not as well-developed as that of the chemoreceptive sensilla. The dendrites of all sensillum types on the cerci have a granular body in the transitional region: a situation which has not been previously reported in chemoreceptive sensilla although common in the mechanoreceptors of Calliphoridae and Sarcophagidae.     

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.     

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

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

Volume and surface of receptor and auxiliary cells in hygro-/thermoreceptive sensilla of moths (Bombyx mori,Antheraea pernyi,and A. polyphemus)

Summary The thermo/hygroreceptive sensilla styloconica of the silkmoths Bombyx mori, Antheraea pernyi, and A. polyphemus were reconstructed from serial sections of cryofixed and chemically fixed specimens. The volume and surface area of the different sensillar cells were calculated from the area and circumference of consecutive section profiles. In addition, data are provided on the length and diameter of the outer and inner dendritic segments of the receptor cells. The morphometric data obtained from the three species are highly consistent and significantly different from those of olfactory sensilla trichodea of the same species. In each sensillum two type-1 receptor cells (hygroreceptors) are associated with one type-2 cell with a lamellated outer dendritic segment, a comparatively thick inner dendritic segment, and a particularly large soma (thermoreceptor). In contrast to olfactory sensilla, the thecogen cell is the largest auxiliary cell forming an extensive apical labyrinth bordering the inner sensillum-lymph space, whereas an inconspicuous trichogen cell and a medium-sized tormogen cell border a comparatively small outer sensillum-lymph cavity. Moreover, both sensillum-lymph spaces are separated from each other not only by the dendrite sheath, but also by the trichogen cell. The results are discussed with regard to recent electrophysiological observations and current hypotheses on the function of sensilla.     

Ultrastructure and ontogeny of the hair sensilla on the funicle of Calliphora erythrocephala (Insecta,Diptera)

Summary Four envelope cells are responsible for the formation of the basiconical sensilla of Calliphora. They are the thecogen, trichogen, and tormogen cells, and envelope cell 4. In early stages of development the still subepithelial sensory cilia are completely enclosed by the innermost thecogen cell. The first formation movements are initiated by a growth thrust of the hair-forming cell into the exuvial space. The sensory cilia only begin to grow into the hair anlage when the hair-forming cell has almost reached its final length. As soon as growth is completed the trichogen cell, tormogen cell, and envelope cell 4 start to excrete cuticular material. The trichogen cell forms the perforated part of the hair shaft and the stimulus-conducting system consisting of the pore tubules. The tormogen cell is responsible for the excretion of the basal non-perforated hair shaft and sheath cell 4 forms the proximal part of the socket region. The thecogen cell only begin to produce dendritic sheath material when the sensory hair is almost complete.Approximately 7–8 days after pupation the tormogen cell degenerates, having, by this time, produced about two-thirds of the sensilla cuticle. The surrounding envelope cells incorporate cell fragments of the tormogen cell. The trichogen cell continues the secretion where the tormogen cell left off. When the secretion of cuticle is finished the sheath cells begin to withdraw towards the proximal direction and to form microvilli on the apical membrane. The resulting outer receptor lymph space is bordered by envelope cell 4 and the trichogen and thecogen cells. The tormogen cell is absent in the sensilla of the imago.Abbreviations DS dendritic sheath - E4 envelope cell 4 - Ex exuvial space - G glial cell - iD inner dendritic segment - iRL inner receptor lymph space - oRL outer receptor lymph space - oD outer dendritic segment - P pore - PT pore tubules - S sensory cell - T thecogen cell - TO tormogen cell - TR trichogen cell Part 1 of a dissertation accepted by the Faculty of Bio- and Geosciences, University of Karlsruhe     

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

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

Structure of isolated sensilla and sensory neurons in vitro: Observations on developing silkmoth antennae

Summary By combined enzymatic and mechanical treatment, it was possible to dissociate the sensory epithelium of developing antennae of male Antheraea polyphemus and A. pernyi silkmoths from the stage of separation of the antennal branches up to the early stages of cuticle deposition. Large numbers of entire developing trichoid sensilla were isolated. These are characterized by a large trichogen cell with a long apical, hair-forming process and a large nucleus. A cluster of 2–3 sensory neurons, enclosed by the thecogen cell, is situated in the basal region. The dendrites run past the nucleus of the trichogen cell into the apical process from which they protrude laterally. The nuclei of the tormogen and a 4^th enveloping cell can be distinguished near the base of the prospective hair. After further dissociation, only the neuron clusters remain, still enclosed by their thecogen cell and often attached to the antennal branch nerve via their axons. It is finally possible to disrupt the thecogen cells and the axons, leaving the sensory neurons with inner dendritic segments and axon stumps. The majority of these neurons can be expected to be olfactory.     

Morphology and ontogeny of single-walled multiporous sensilla of hemimetabolous insects

Comparative morphological investigations were made to determine the common organization plan of single-walled multiporous sensilla. The development of multiporous chemoreceptive sensilla of Gryllus, Oncopeltus and Lepisma follows the same path. Each chemoreceptive sensillum is associated with four types of enveloping cell. During ontogeny, enveloping cell 1 secretes the dendritic sheath. Enveloping cell 4 builds the connection of the hair base with the antennal cuticle. In Gryllus and Oncopehus, enveloping cells 2 and 3 build the hair shaft, the wall pores and pore tubules in nearly equal parts. Enveloping cells 2 and 3 lie side by side in the hair process, in which enveloping cell 2 produces the inner part, enveloping cell 3 the outer part of the hair shaft. In Lepisma the predominant part of the hair shaft with the wall pores is formed by the doubled enveloping cells 3. Interpreting our findings and the literature data, a new proposal is given for the homology of the enveloping cells. In singlewalled chemoreceptors, enveloping cell 1 is considered as thecogen and enveloping cell 4 as tormogen cell. Enveloping cell 2 is interpreted as inner trichogcn cell and enveloping cell 3 as outer trichogen cell.     

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.     

Morphogenesis of the antenna of the male silkmoth. Antheraea polyphemus, III. Development of olfactory sensilla and the properties of hair-forming cells

During adult development of the male silkmoth Antheraea polyphemus, the anlagen of olfactory sensilla arise within the first 2 days post-apolysis in the antennal epidermis (stage 1-3). Approximately on the second day, the primary dendrites as well as the axons grow out from the sensory neurons (stage 4). The trichogen cells start to grow apical processes approximately on the third day, and these hair-forming 'sprouts' reach their definite length around the ninth day (stages 5-6). Then the secretion of cuticle begins, the cuticulin layer having formed on day 10 (stage 7a). The primary dendrites are shed, the inner dendritic segments as well as the thecogen cells retract from the prospective hair bases, and the inner tormogen cells degenerate around days 10/11 (stage 7b). The hair shafts of the basiconic sensilla are completed around days 12/13 (stage 7c), and those of the trichoid sensilla around days 14/15 (stage 7d). The trichogen sprouts retract from the hairs after having finished cuticle formation, and the outer dendritic segments grow out into the hairs: in the basiconic sensilla directly through, and in the trichoid sensilla alongside, the sprouts. The trichogen sprouts contain numerous parallel-running microtubules. Besides their cytoskeletal function, these are most probably involved in the transport of membrane vesicles. During the phase of cuticle deposition, large numbers of vesicles are transported anterogradely from the cell bodies into the sprouts, where they fuse with the apical cell membrane and release their electron-dense contents (most probably cuticle precursors) to the outside. As the cuticle grows in thickness, the surface area of the sprouts is reduced by endocytosis of coated vesicles. When finally the sprouts retract from the completed hairs, the number of endocytotic vesicles is further increased and numerous membrane cisterns seem to be transported retrogradely along the microtubules to the cell bodies. Here the membrane material will most probably be used again in the formation of the sensillum lymph cavities. Thus, the trichogen cells are characterized by an intensive membrane recycling. The sensillum lymph cavities develop between days 16-20 (stage 8), mainly via apical invaginations of the trichogen cells. The imago emerges on day 21.     

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.     

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.     

Ultrastructure des sensilles trochanteriens de Campodea sensillifera conde et mathieu (Diplura : Campodeidea)

The fine structure of the basiconica sensilla situated on the posterior part of trochanters in Campodea sensillifera (Diplura : Campodeidea) reveals that they are probably olfactory and mechano-sensitive setae. Each sensillum is composed of one sensory axis made of 3 dendrites ensheathed by 3 cells (thecogen, trichogen and tormogen); one outer segment ends by a tubular corper without connection with the cuticular layer. The setae are generally racket-shaped. The epicuticular layer of the expanded part is perforated by a lattice of numerous slits, which communicate with underlying canals. The ciliary structures and apex of the tormogen cell are eliminated just before ecdysis. The ciliary microtubules are present in the cavity of the new sensillum, but after ecdysis the microtubules persist only at the lower part of the peduncle. An ecdysial canal appears at the tip of the sensillum.     

Sensillum development in the absence of cell division: the sensillum phenotype of the Drosophila mutant string

We have investigated sensillum development in Drosophila embryos homozygous for mutations in the locus string (stg). In these embryos, cell division is blocked following blastoderm formation. This permits a study of the differentiative fate of undivided precursor cells, in particular those giving rise to the larval sensory organs (sensilla). Of the different cell fates normally represented in the sensilla (i.e., sensory neuron, thecogen cell, trichogen cell, tormogen cell, glia cell), only the phenotype of sensory neurons is expressed morphologically in stg embryos, suggesting that the neuronal fate predominates over the fates of the nonneuronal accessory cells. Consistent with this finding, the P element-lacZ insertion A1-2nd-29, which is a marker for trichogen and tormogen cells in the wild-type embryo, is not expressed in the body wall of the stg embryo. Some sensillum precursor cells appear to express a mixed fate in stg mutants: They express antigens (recognized by the monoclonal antibodies 22C10 and 21A6) which in the wild-type appear in separate cells (sensory neurons and thecogen cell, respectively). The differentiation of undivided cells in stg embryos is not restricted to the peripheral nervous system; in all types of tissues analyzed in this study (e.g., epidermis, intestine, muscle, CNS), precursor cells express characteristics normally exhibited by their progeny.     

The coelocapitular sensillum,an antennal hygro- and thermoreceptive sensillum of the honey bee,Apis mellifera L.

Summary The sensillum coelocapitulum, a hygro- and thermoreceptive sensillum of the honey bee, Apis mellifera, was investigated by electron microscopy. The cuticular apparatus of the sensillum is a mushroomshaped protrusion, devoid of pores, set in a narrow cylindrical pit positioned centrally within a cuticular, shallow depression. There may be three or four receptor cells. Three receptor cells have unbranched sensory cilia, containing densely packed microtubules, which extend distally into the cuticular apparatus and completely fill its cavity. These connecting cilia are of the usual 9+0 type. The fourth receptor, if present, has a thin sensory cilium which terminates beneath the cuticular apparatus. Its connecting cilium has armed outer doublets. The outer cavity is formed by two enveloping cells and is completely sealed off. Lipid deposits are present within the cavity and the tormogen cell. The thecogen cell has scolopale rod-like structures around the inner cavity. Features common to the insect hygro- and thermoreceptive sensilla are discussed in comparison with those of other insects.     

Immunocytochemical localization of pheromone-binding protein in moth antennae

Summary Odorant-binding proteins are supposed to play an important role in stimulus transport and/or inactivation in olfactory sense organs. In an attempt to precisely localize pheromone-binding protein in the antenna of moths, post-embedding immunocytochemistry was performed using an antiserum against purified pheromone-binding protein of Antheraea polyphemus. In immunoblots of antennal homogenates, the antiserum reacted exclusively with pheromone-binding protein of A. polyphemus, and cross-reacted with homologous proteins of Bombyx mori and Autographa gamma. On sections of antennae of male A. polyphemus and B. mori, exclusively the pheromone-sensitive sensilla trichodea are labelled; in A. gamma, label is restricted to a subpopulation of morphologically similar sensilla trichodea, which indicates that not all pheromone-sensitive sensilla contain the same type of pheromone-binding protein and accounts for a higher specificity of pheromone-binding protein than hitherto assumed. Within the sensilla trichodea, the extracellular sensillum lymph of the hair lumen and of the sensillum-lymph cavities is heavily labelled. Intracellular label is mainly found in the trichogen and tormogen cells: in endoplasmic reticulum, Golgi apparatus, and a variety of dense granules. Endocytotic pits and vesicles, multivesicular bodies and lysosome-like structures are also labelled and can be observed not only in these cells, but also in the thcogen cell and in the receptor cells. Cell membranes are not labelled except the border between thecogen cell and receptor cell and the autojunction of the thecogen cell. The intracellular distribution of label indicates that pheromone-binding protein is synthesized in the tormogen and trichogen cell along typical pathways of protein secretion, whereas its turnover and decomposition does not appear to be restricted to these cells but may also occur in the thecogen and receptor cells. The immunocytochemical findings are discussed with respect to current concepts of the function of pheromone-binding protein.     

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)     

Development of adult sensilla on the wing and notum of Drosophila melanogaster

We have investigated the temporal pattern of appearance, cell lineage, and cytodifferentiation of selected sensory organs (sensilla) of adult Drosophila. This analysis was facilitated by the discovery that the monoclonal antibody 22C10 labels not only the neuron of the developing sensillum organ, but the accessory cells as well. The precursors of the macrochaetes and the recurved (chemosensory) bristles of the wing margin divide around and shortly after puparium formation, while those of the microchaetes and the stout and slender (mechanosensory) bristles of the wing margin divide between 9 h and 18 h after puparium formation (apf). The onset of sensillum differentiation follows the terminal precursor division within a few hours. Four of the cells in an individual microchaete organ are clonally related: A single first-order precursor cell divides to produce two second-order precursors; one of these divides into the neuron and thecogen cell, the other into the trichogen cell and tormogen cell. Along the anterior wing margin, two rounds of division generate the cells of the mechanosensory sensilla; here, no strict clonal relationship seems to exist between the cells of an individual sensillum. At the time of sensillum precursor division, many other, non-sensillum-producing cells within the notum and wing proliferate as well. This mitotic activity follows a spatially non-random pattern.     

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.     

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.