Mechanoreceptors in calanoid copepods: designed for high sensitivity

The mechanoreceptors of the first antennae of Pleuromamma xiphias, a mesopelagic calanoid copepod, are critical for the detection of potential threats. These receptors exceed the physiological performance of other crustacean mechanoreceptors in sensitivity to water velocities as well as in frequency response. A study of these receptors was initiated to elucidate structure–function relationships. Morphologically, the receptors resemble the arthropod scolopidial organs by the presence of a scolopale tube. However, the rigidity of the copepod receptors greatly exceeds those described for crustaceans and other arthropods. The scolopale tube completely encloses the distal dendrites and it is firmly anchored to the cuticle. Microtubules are organized in register and arise from microtubule subfibers associated with crescent-shaped rods which extend from the basal body region to the setal socket. The distal dendrites are filled with a large number of cross-linked microtubules. Termination of the distal dendrites inside the lumen of the setae is gradual with a firm anchoring to the cuticle. A likely mechanism for mechanotransduction would involve a linkage between individual microtubules and mechano-gated channels in the dendritic membrane. The rigidity probably contributes to the high frequency sensitivity, and termination of the dendrite inside the seta increases the overall sensitivity of these receptors.     

Ultrastructure and functional organization of mouthpart sensory setae of the spiny lobster Panulirus argus: new features of putative mechanoreceptors

In comparison with other decapods, the Caribbean spiny lobster Panulirus argus has little diversity in the external morphology of the setae on the mouth apparatus. In mouthpart areas that frequently touch food items only two types of setae can be distinguished: simple setae and cuspidate setae. Simple setae are by far more numerous. The ultrastructural data presented here show that both types of seta are bimodal, in that they both contain mechano- and chemosensory cells as indicated by morphological features. The morphological features divide the sensory cells into three types: type 1, which has a mechanosensory appearance; type 2, which has a chemosensory appearance; and type 3, which is believed to be a mechanoreceptor due to desmosomal connections to a scolopale. All three cell types were found in all examined setae. In an earlier study the simple setae were found to contain two types of mechanosensors: bend-sensitive cells and displacement-sensitive cells. The morphological arrangement of the outer dendritic segment described in the present study cannot explain this division. Instead, it is suggested that the difference in sensitivity is caused by a differential arrangement of their stretch-sensitive ion channels. This hypothesis also provides an explanation for the earlier observation that only bend cells respond to changes in osmolarity.     

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.     

Role of maxilla 2 and its setae during feeding in the shrimp Palaemon adspersus (Crustacea: Decapoda)

The movements of the basis of maxilla 2 in Palaemon adspersus were examined using macro-video recordings, and the morphology of its setae was examined using both scanning and transmission electron microscopy. The basis of maxilla 2 performs stereotypical movements in the latero-medial plane and gently touches the food with a frequency of 3-5 Hz. The medial rim of the basis of maxilla 2 carries three types of seta. Type 1 is serrate, type 2 and 3 are serrulate, and type 2 has a prominent terminal pore. Type 2 is innervated by 18-25 sensory cells whose cilia protrude through the terminal pore and are in direct contact with the external environment. The structure of type 2 setae indicates that they are mainly gustatory, although still bimodal due to their innervation by presumed chemosensory and mechanosensory neurons. Distally, the three types of setae have a complex arrangement of the cuticle involving water-filled canals, which may serve to improve flexibility. Type 1 and 3 setae have fewer sensory cells (4-9) but probably also have a bimodal sensory function. The function of type 1 setae is probably to protect type 2 setae, while type 3 setae might serve to groom the ventral side of the basis of maxilla 1.     

A new glandular sensory organ inCatanema sp. (Nematoda,Stilbonematinae)

Summary A new multicellular glandular sensory organ is described forCatanema sp. (Nematoda, Stilbonematinae). The organs terminate in setae and are distributed in six longitudinal rows along the body. Two types of glandular cells (type A and type B), one monociliary sensory cell and one undifferentiated epidermal cell are combined in the basiepidermal organ. A comparison of epidermal glands as well as sensory organs in Nematoda is made. A causal relationship between the development of such complex, large and numerous glandular sensory organs and the occurrence of species-specific, symbiotic epibacteria inCatanema sp. seems probable, although there is no simple correlation between the distribution of these organs and epibacteria. A mucous cover over the bacterial layer, released by the glandular sensory organs, may create a microenvironment for the interaction between epibionts and host.Abbreviations (used in figures) a amphid - A1–A4 type 1–4 granules of type A gland cell - an annuli - b bacteria - B1–B3 type 1–3 granules of type B gland cell - bl basal lamina - bp basal part of seta - bz basal zone of cuticle - c cuticle - ca canal - cg caudal gland - ci cilium - cz cortical zone of cuticle - d dictyosomes - e epidermis - e co extracellular coat - em extracellular matrix - ep epicuticle - f filaments - gcA type 1 gland cell - gcB type 2 gland cell - i lp inner labial papillae - m mitochondrion - me membranes of type 2 gland cell - mo mouth opening - mz median zone of cuticle - n nucleus - nu nucleolus - p process - pv primary vesicle of type A gland cell - r ribosomes - s seta - sc sensory cell - sp secretory product - tj tight junction - tp terminal part of seta - uc undifferentiated epidermal cell - va vacuoles or vesicles of epidermal cells - ve vesicles of sensory cell     

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.     

腐食酪螨成螨形态观察

利用光学显微镜观察腐食酪螨成螨外部形态特征。通过观察发现,腐食酪螨雄性成螨体长约300～440μm,表皮光滑,附肢4对呈淡棕色,各足均着生有刚毛和感器;胛内毛（sci）较胛外毛（sce）长;基节上毛（ps）膨大且有刺状侧突;背毛（d）4对,d1最短,d4最长;腹面可见圆形的肛吸盘和前肛毛（pra）各1对。雌螨体型较雄螨大,长约486～492μm,生殖区位于腹面第Ⅲ、Ⅳ基节之间,生殖毛3对;肛门达躯体末端,周围着生有肛毛（a）5对,肛后毛（pa）3对。通过对腐食酪螨成螨外部形态结构的观察为其科学分类提供了实验依据。     

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     

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.     

Are the funnel-canal organs the ‘campaniform sensilla’ of the shore crab,Carcinus maenas (Decapoda,Crustacea)?

Summary The funnel-canal organs on the dactyls of the shore crab, Carcinus maenas, are innervated by 3–24 sensory cells with unbranched dendrites, which attain a length of 500–1400 m. The outer dendritic segments are enclosed in a dendritic sheath and pass through the cuticle within a canal. Two dendrite types can be distinguished according to ultrastructural criteria: Type I has a long ciliary segment, A-tubules with an osmiophilic core and arms, and a thick ciliary rootlet. Type II possesses only a short ciliary segment and a thin ciliary rootlet. Each funnel-canal organ contains two type-I dendrites. Their ciliary bases appear a few m distal to those of the type-II dendrites (1 to 22 in number). Two inner and two to eight outer enveloping cells belong to a sensillum. The innermost enveloping cell contains a large scolopale. In the second enveloping cell single scolopale rods are present. Thus, the funnel-canal organs are characterized by structural features typical for mechano-sensitive scolopidia, on the one hand, and for chemoreceptors, on the other. Therefore, the funnel-canal organs are very likely bimodal sensilla (contact chemoreceptors). A comparison with other arthropod sensilla shows that cuticular mechanoreceptors of aquatic crustaceans generally exhibit a scolopidial organization.     

维螨属2新种记述(蜱螨亚纲:中气门目:维螨科)

记述维螨属2新种:河南维螨Veigaia henanensis sp.nov.和福建维螨Veigaia fujianensis sp.nov.     

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.     

冬虫夏草寄主蒲氏钩蝠蛾幼虫的毛序

蒲氏钩蝠蛾Thitarodes pui(Zhang et al.)幼虫是冬虫夏草的寄主昆虫,本文首次对其幼虫毛序进行观察。结果表明,蒲氏钩蝠蛾幼虫头壳刚毛数量、相对位置与多数蝠蛾昆虫一致,但比拉脊钩蝠蛾少了2对刚毛;蒲氏钩蝠蛾幼虫前胸节也比拉脊钩蝠蛾少1对微腹毛;第2、第3胸节毛序相似,均缺失前背毛而多了微背毛、微亚背毛;除第9、第10节外,所有腹节的背毛、侧毛、亚背毛排列是一致的,但是在有足腹节中,亚腹毛为3对,其它腹节亚腹毛仅有2对;除第1和第2腹节具2对微腹毛外,其它腹节微腹毛仅1对,这些微腹毛在排列、长度上均存在一定的差异。     

The ultrastructure of the so-called olfactory setae on the antennula ofDaphnia magna Straus (Crustacea,Cladocera)

A group of nine sensory setae is found on the tip of the antennula ofDaphnia magna in both sexes. Inside a seta four dendrites are situated, each with one receptor cilium. The receptor cilia extend through a liquor space into the exterior part of the seta. The exterior part of the liquor space is divided from the interior part by a knob-like thickening of the innermost layer of the epicuticle, the basal bead. The basal bead narrows the liquor space and the receptor cilia. The interior part of the liquor space is surrounded by five sheath cells, the exterior part by a thin cuticle. In the exterior part the receptor cilia branch partly and reach a terminal pellet on the tip of the seta. The terminal pellet is a thickened part of the epicuticle. It is permeable to several dissolved substances. It is the exterior part of the receptor that projects over the tip of the antennula and seems to be the entire seta. During the premoult the fifth sheath cell builds up the articulation of the seta, the fourth the basal bead, and the third the shaft of the seta. The first sheath cell forms the cuticular sheath. The organ seems to be a chemoreceptor, but the adequate stimulus is as yet unknown.     

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.     

Ultrastructure of mediodorsal setae in biting midge larvae of the genus Atrichopogon Kieffer with notes on their biological significance (Diptera: Ceratopogonidae)

The ultrastructure of the strong mediodorsal setae in terrestrial stage IV larvae of Atrichopogon (Meloehelea) oedemerarum and A. (M.) meloesugans was examined using light, scanning and transmission electron microscopy. Serrated setae placed on prominent processes are distributed in pairs on all thoracic and abdominal segments. Setae are innervated by a single dendrite and their surface has no pores. The trichogen cell is not retracted from the setal lumen on completion of the hair-forming process but fills the mediodorsal seta also when the larval cuticle is fully sclerotised. Such a phenomenon was previously reported in terrestrial larvae of the genus Forcipomyia. We suggest that the mediodorsal setae described in Atrichopogon are plesiotypic mechanoreceptors for the subfamily Forcipomyiinae. They are preserved in the truly terrestrial larvae of Atrichopogon, but modified to secretory setae in the genus Forcipomyia. Both genera bearing distinct mediodorsal setae have developed functional tracheal gills, unknown in other biting midges.     

中国陕西省新异蚖属一新种记述(原尾纲,古蚖目,古蚖科)

记述了采自陕西省翠华山的新异蚖属1新种,即陕新异蚖Neanisentomonshaanicum sp.nov.。新种主要特征为:前足跗节感器b′-1和c′缺失,感器d极长大;腹部第Ⅴ～Ⅵ节背板缺少前排刚毛A3,毛序为8/16,第Ⅶ节背板缺少前排刚毛A1和A3,毛序为6/16;雌性外生殖器具有明显的鸭头状的头片。该新种可以通过前足跗节感器d的长度和雌性外生殖器等特征与已知种类区分。文中同时列出了新异蚖属的世界种类检索表。     

Structure of dactyl sensilla in the kelp crab,Pugettia producta

In the kelp crab, Pugettia producta, flat plate setae cover all but the ventral surfaces of the walking leg dactyls. Dendrites enter the setal shaft located inside the plate superstructure, and extend to a region of the setal tip that contains a system of minute pores resembling the pore systems found in chemosensory sensilla of insects. Presumably, much of the chemosensitivity of the dactyls in the kelp crab is mediated by the plate setae. In the interior of the dactyl, supporting cells and the neurons innervating plate setae, other types of setae, and other presumptive sensilla form scolopidia. Large scolopidia, containing as many as 12 dendrites, appear to innervate some of the plate setae and also large ventral rodlike setae that might be chemosensory. Two of the dendrites of large scolopidia usually have more densely packed microtubules, longer ciliary axonemes, slightly larger rootlets, and dark A fibers with arms, characteristics indicative of mechanosensory function. Some dactyl setae, therefore, could be both mechanosensory and chemosensory. Small scolopidia containing two or three dendrites that exhibit mechanosensory characteristics appear to innervate small, rodlike setae, which presumably are strictly mechanosensory. The two types of structures located on the epicuticular cap, elliptical structures resembling campaniform sensilla and small cones in pits resembling CAP organs, appear to be dually innervated and presumably are mechanosensory, although other functions are possible. The internal positions of the scolopidia, together with the support afforded by an extracellular dendritic sheath, by the scolopale, and by desmosomelike and septate junctions, may serve to protect internal portions of setal dendrites, some of which appear to remain functional in nonmolting adults that have abraded setae.