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.     

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 of Magicicada cassini (fisher) (Homoptera : Cicadidae): Fine structure and electrophysiological evidence for olfaction

The antennae of Magicicada cassini (Homoptera : Cicadidae) (3–4 mm long) look similar in both sexes and consist of scape, pedicel, and a 5-segmented flagellum. The length of flagellar segment 1 varies independently in relation to head size and is slightly longer in females (0.96 mm) than in males (0.89 mm). The ventral side of flagellar segment 1 is covered with sensilla coeloconica comprising about 60 large, 10 medium-sized, and 35 small sensilla with pit diameters of 8–24, 6–10, and 2 μm, respectively. The large and the medium-sized sensilla coeloconica are multiporous single-walled sensilla with pore tubules, containing branched entangled dendrites from 3 receptor cells. The small sensilla coeloconica, situated primarily at the outer border of the sensillum field, are no-pore sensilla with inflexible sockets. They contain 2 unbranched dendrites extending to the tip of the peg, and 1 dendrite reaching to its base and wrapping around the other 2 dendrites. Small sensilla campaniformia (cap diameter 3 μm) are aligned at the outer border of the sensillum field and continue all along the flagellum. Up to 3 olfactory receptor cells were distinguished on the basis of their nerve impulse amplitudes through extracellular electrophysiological recordings from sensilla coeloconica, presumably large ones. They respond to stimulation by cyclic terpenoids with different but highly overlapping reaction spectra, and react selectively to structural variations of the molecules. No responses to CO₂, temperature or moisture were recorded.     

Fine structure and molting of aesthetasc sense organs on the antennules of the isopod,Asellus aquaticus (Crustacea)

In Asellus aquaticus certain distal antennular segments bear single sensilla referred to as aesthetascs. These show a proximal stem and a distal bulbous region. Depending on its position, each aesthetasc is innervated by either 50-60 or 70-80 bipolar sensory cells, the perikarya of which are situated within the pedunculus. Within the antennular segment the dendrites develop unbranched cilia (9 X 2 + 0 structure). The sensory cells are unusual in that mono- as well as biciliary dendrites are present within a single aesthetasc, the ratio of both types being correlated with the number of sensory cells. Cilia and receptor lymph cavity are enveloped by a set of 3-4 inner and 13-14 outer sheath cells, which terminate at the base of the sensillum, so that the delicate and poreless cuticle of the bulbous region encloses only outer segments within the receptor lymph fluid. A new molting type in arthropods is described in which the outer sheath cells alone build the new cuticle, whereas the inner sheath cells most probably have a protective function. A definition of aesthetascs is proposed based on fine-structural criteria. Functionally the sensilla are considered to be chemoreceptors. This assumption is confirmed by experiments with diluted vital dye as well as lanthanum showing that dissolved substances penetrate the poreless cuticle instantaneously.     

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.     

Morphology and distribution of antennal sensilla of the tarnished plant bug,Lygus lineolaris (Palisot de beauvois) (Hemiptera: Miridae)

Sensilla on the antennae of adult and last-instar nymphs of the tarnished plant bug, Lygus lineolaris (Hemiptera: Miridae), were examined with light, scanning and transmission electron microscopy. Six different types were identified in adult females and males and 5 types in last-instar nymphs: types 1 and 4 are sensilla trichodea, 2 and 3 are sensilla chaetica, and 5 and 6 are sensilla basiconica. Type 1 are located at distal region of terminal segment and type 2 are located at distal regions of proximal 3 segments in both adults and nymphs. Type 3 is present on all segments, more numerous on scape and pedicel and less abundant on distal third and fourth segments in both adult and nymphal stages. Types 4 and 6 are absent on the scape and present on the distal 3 antennal segments in adults, but they are present only on the distal-most antennal segment in nymphs. Type 5 sensilla are present only on second antennal segments in adults and are absent in nymphs. Sexual dimorphism is observed in total numbers: there are significantly more type(s) 3, 4, 5 and 6 sensilla in adult males than adult females. Types 1, 4 and 5 are multiporous with thin cuticle, branched dendrites and pore tubules which suggests an olfactory function. These sensilla have 3, 3 and 2 neurons, respectively. The type 6 sensillum has an apical pore and pores in the cuticular wall, and is innervated by 5 nerve cells with unbranched dendrites. Sensillar types 2 and 3 have thick cuticle, a single apical pore and nerve cells with unbranched dendrites. Type 2 has 1 neuron and type 3 has 2 chambers and 2 nerve cells.     

Intraflagellar transport

Eukaryotic cilia and flagella, including primary cilia and sensory cilia, are highly conserved organelles that project from the surfaces of many cells. The assembly and maintenance of these nearly ubiquitous structures are dependent on a transport system--known as 'intraflagellar transport' (IFT)--which moves non-membrane-bound particles from the cell body out to the tip of the cilium or flagellum, and then returns them to the cell body. Recent results indicate that defects in IFT might be a primary cause of some human diseases.     

Hygro- and thermoreceptors in tip-pore sensilla of the tarsal organ of the spider Cupiennius salei: innervation and central projection

The hygro- and thermoreceptive tarsal organ in the wandering spider Cupiennius salei is located on the tarsus of each walking leg and pedipalp, and consists of a tiny air-filled capsule in the cuticle. This capsule communicates with the outside world through a small aperture and contains seven nipple-shaped sensilla, each with a pore at its tip. In both their external morphology and internal structure, the sensilla are indistinguishable, although one sensillum is innervated by only two sensory cells, whereas the other six sensilla contain three sensory cells. Their dendrites are unbranched and terminate at the tip-pore, where they are enveloped by amorphous material that appears to limit their exposure to the atmosphere. Cobalt fillings reveal that each tarsal organ projects to three different areas within the suboesophageal ganglionic mass: (1) the sensory longitudinal tract 3 and 4; (2) the corresponding pedipalpal or leg ganglion; (3) a structured neuropil (here termed the Blumenthal neuropil) beneath the oesophagus. The multiple representation of sensory afferents from each tarsal organ in different regions of the suboesophageal ganglionic mass suggests parallel processing of hygro-/thermoreceptive information.     

Ultrastructure of chemosensory tarsal tip-pore sensilla of Argiope spp. Audouin, 1826 (Chelicerata: Araneae: Araneidae)

While chemical communication has been investigated intensively in vertebrates and insects, relatively little is known about the sensory world of spiders despite the fact that chemical cues play a key role in natural and sexual selection in this group. In insects, olfaction is performed with wall–pore and gustation with tip-pore sensilla. Since spiders possess tip-pore sensilla only, it is unclear how they accomplish olfaction. We scrutinized the ultrastructure of the trichoid tip-pore sensilla of the orb weaving spider Argiope bruennichi—a common Palearctic species the males of which are known to be attracted by female sex pheromone. We also investigated the congener Argiope blanda. We examined whether the tip-pore sensilla differ in ultrastructure depending on sex and their position on the tarsi of walking legs of which only the distal parts are in contact with the substrate. We hypothesized as yet undetected differences in ultrastructure that suggest gustatory versus olfactory functions. All tarsal tip-pore sensilla of both species exhibit characters typical of contact-chemoreceptors, such as (a) the presence of a pore at the tip of the sensillum shaft, (b) 2–22 uniciliated chemoreceptive cells with elongated and unbranched dendrites reaching up to the tip-pore, (c) two integrated mechanoreceptive cells with short dendrites and large tubular bodies attached to the sensillum shaft's base, and (d) a socket structure with suspension fibres that render the sensillum shaft flexible. The newly found third mechanoreceptive cell attached to the proximal end of the peridendritic shaft cylinder by a small tubular body was likely overlooked in previous studies. The organization of tarsal tip-pore sensilla did not differ depending on the position on the tarsus nor between the sexes. As no wall-pore sensilla were detected, we discuss the probability that a single type of sensillum performs both gustation and olfaction in spiders.     

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.     

Antennal contact chemosensilla in Psylliodes chrysocephala responding to cruciferous allelochemicals

Sensilla chaetica, which protrude above all other sensilla on the antenna of Psylliodes chrysocephala L., the cabbage stem flea beetle, were investigated ultrastructurally and found to be innervated by five to six sensory neurones. A dendrite from one of these neurones terminates in a tubular body at the shaft base, whereas dendrites from the others run unbranched to a pore at the shaft tip. Such a structure typifies a sensillum with a combined gustatory/mechanosensory function. Electrophysiological recordings using the tip-recording technique confirmed that this sensillum contains one mechanosensory cell and several chemosensory cells. The chemosensory cells were responsive to host plant chemicals. Sensilla chaetica were also found to be responsive to glucosinolates. One of the sensilla chaetica emerges from a domed area of cuticle on antennomere six. This was found to be relatively less responsive to the chemical stimuli tested and more responsive to mechanical stimulation. It is suggested that the sensilla chaetica are contact chemosensilla, that respond to chemicals present in plant surface waxes when P. chrysocephala antennates a leaf.     

Morphology and distribution of the external labial sensilla in Fulgoromorpha (Insecta: Hemiptera)

The present paper describes the sensory structures on the apical segment of the labium in fifteen fulgoromorphan families (Hemiptera: Fulgoromorpha), using the scanning electron microscope. Thirteen morphologically distinct types of sensilla are identified: five types of multiporous sensilla, four types of uniporous sensilla and four types of nonporous sensilla. Three subapical sensory organ types are also recognized, formed from one to several sensilla, each characteristic of a family group. Sensilla chaetica (mechanoreceptive sensilla) fall into three categories dependent on length and are numerous and evenly distributed on the surface of the labium except where they occur on specialized sensory fields. The planthopper morphological ground plan is represented by two apical pair of sensory fields (dorsal and ventral) on which 11 dorsal pairs of sensilla (10 peg-like pairs + 1 specialized pair dome or cupola-like) and 2 ventral pairs of sensilla basiconica occur. Two main patterns (cixiid and issid) together with more specialized ones (derbid, lophopid, flatid and fulgorid) are reported. Disparity and diversity of the sensory structures are analyzed from a taxonomic and functional perspective. A gustatory function is provided for several chemoreceptive labial sensilla, as in the antennal flagellum sensilla in some other Hemiptera. This represents a more recently evolved function for the planthopper labium. Finally, further lines of study are suggested for future work on the phylogeny of the group based on the studied characters.     

A re-evaluation of the cytology of cat Pacinian corpuscles

Summary The aesthetascs of the spiny lobster, Panulirus argus, are hair sensilla located on the lateral filaments of the antennules. Each hair is about 0.8 mm long and innervated by about 320 bipolar sensory neurons, the dendrites of which project as a bundle into the hair shaft. Each of the dendrites develops two cilia. Within a very short distance each of these cilia branches repetitively and dichotomously resulting in 8000 to 10000 outer dendritic segments per hair, or about 20 to 30 branches per neuron. The branches intertwine frequently before running to the tip of the hair. Each hair also possesses inner and outer auxiliary cells. The inner auxiliary cells surround the bundle of dendrites, extending distally to the origin of the ciliary segments. Extensions of these cells project into the bundle of dendrites, separating groups of dendrites into discrete clusters. Outer auxiliary cells wrap the inner ones, but do not extend beyond the base of the hair.     

A cryofixation study of presumptive hygroreceptors on the antennule of a terrestrial isopod

The structure of the apical sensilla on the antennule of the terrestrial isopod Porcellio scaber was examined in cryofixed and freeze substituted (CRF) and chemically fixed and dehydrated (CHF) material. CRF specimens generally showed a preservation superior to CHF material. Only in deeper regions did the tissue show damage from freezing. Each of the 13–22 sensilla contains two sensory cells. In contrast to earlier reports, it was observed that the dendritic segments of these cells are arranged in a unique, concentric manner. In CRF specimens the dendrites reach the tip of the sensilla and border upon the innermost layer of the complicated wall of the peg which is not pierced by pores. Silver-protein and lanthanum failed to penetrate the wall of the sensilla and also did not reach the dendrites via an apical pore, which therefore is regarded as a molting pore. The lymph spaces which, in CHF specimens, are observed around the dendrites and beneath the cuticle within the antennal tip are regarded as artefactual. From similarities in the dendritic structures to insect hygroreceptors and their relationship to the adjoining cuticle it can be assumed that the antennular sensilla in Porcellio are sensitive to humidity changes. Mechanoreception and chemoreception, however, cannot entirely be excluded as possible functions.     

An aurora kinase is essential for flagellar disassembly in Chlamydomonas

Cilia and flagella play key roles in development and sensory transduction, and several human disorders, including polycystic kidney disease, are associated with the failure to assemble cilia. Here, we show that the aurora protein kinase CALK in the biflagellated alga Chlamydomonas has a central role in two pathways for eliminating flagella. Cells rendered deficient in CALK were defective in regulated flagellar excision and regulated flagellar disassembly. Exposure of cells to altered ionic conditions, the absence of a centriole/basal body for nucleating flagellar assembly, cessation of delivery of flagellar components to their tip assembly site, and formation of zygotes all led to activation of the regulated disassembly pathway as indicated by phosphorylation of CALK and the absence of flagella. We propose that cells have a sensory pathway that detects conditions that are inappropriate for possession of a flagellum, and that CALK is a key effector of flagellar disassembly in that pathway.     

The antennal sensilla of the adult of Libellula depressa (Odonata: Libellulidae)

An ultrastructural investigation (SEM, TEM) on the antennal flagellum of the adult of the dragonfly Libellula depressa (Odonata:Libellulidae) revealed sensilla located in pits on the lateral-ventral side of the antenna. These sensilla are represented by sensilla coeloconica and by deeply sunken sensilla. The sensilla coeloconica are innervated by three unbranched dendrites, which enter the peg and show a dendrite sheath ending at the base of the peg. The peg has no socket and its cuticle is irregular with wide pore-like structures at the base of which actual pores are visible. The structure of these coeloconic sensilla is in agreement with that reported for single-walled insect chemoreceptors. The deeply sunken sensilla are represented by two kinds of sensilla styloconica, named type-1 and type-2, located at the bottom of deep cavities appearing as simple openings on the antennal surface. These sensilla are no-pore sensilla with inflexible socket and unbranched dendrites and, notwithstanding their structural differences, share common features typical of thermo/hygroreceptors. The presence of chemoreceptors in adult dragonflies sheds light on evolutionary trends in insect perception; the previously unknown occurrence of thermo/hygroreceptors in dragonflies is very important in view of the reported ability of Odonata to thermoregulate heliothermically.     

Basal body positioning is controlled by flagellum formation in Trypanosoma brucei

To perform their multiple functions, cilia and flagella are precisely positioned at the cell surface by mechanisms that remain poorly understood. The protist Trypanosoma brucei possesses a single flagellum that adheres to the cell body where a specific cytoskeletal structure is localised, the flagellum attachment zone (FAZ). Trypanosomes build a new flagellum whose distal tip is connected to the side of the old flagellum by a discrete structure, the flagella connector. During this process, the basal body of the new flagellum migrates towards the posterior end of the cell. We show that separate inhibition of flagellum assembly, base-to-tip motility or flagella connection leads to reduced basal body migration, demonstrating that the flagellum contributes to its own positioning. We propose a model where pressure applied by movements of the growing new flagellum on the flagella connector leads to a reacting force that in turn contributes to migration of the basal body at the proximal end of the flagellum.     

Fine structure and distribution of antennal sensilla of the desert locust, Schistocerca gregaria (Orthoptera: Acrididae)

The fine structure and distribution of various types of antennal sensilla in three nymphal stages and in adults of both solitary-reared (solitary) and crowd-reared (gregarious) phases of the desert locust, Schistocerca gregaria, were investigated by scanning and transmission electron microscopy. Four types of sensilla were identified: sensilla basiconica, s. trichodea, s. coeloconica and s. chaetica. S. basiconica contain up to 50 sensory neurons, each of which displays massive dendritic branching. The sensillar wall is penetrated by a large number of pores. In contrast, s. trichodea contain one to three sensory neurons that branch to give five or six dendrites in the sensillar lumen; the sensillum wall is penetrated by relatively few pores. The s. coeloconica are situated in spherical cuticular pits on the antennal surface. The s. coeloconica are of two types: one type contains one to three sensory neurons with double sensillar walls penetrated by slit-like pores, whereas the second type contains four sensory neurons with non-porous double sensillar walls. The s. chaetica have a flexible socket and a thick non-porous sensillum wall and contain four sensory neurons that send unbranched dendrites to a terminal pore. A fifth sensory neuron of the s. chaetica terminates in a tubular body at the base of the hair. S. basiconica and coeloconica are normally distributed over the entire antennal flagellum, with a concentration in the middle segments; s. trichodea have three areas of concentration on the 5th, 10th and 14th flagellar segments. Sensilla chaetica are most abundant on the terminal segment. Locusts raised in solitary conditions have more olfactory sensilla (s. basiconica and s. coeloconica) than crowd-reared locusts. The difference in sensillar numbers is more evident in adults than in nymphs. These results suggest that differences in the odour-mediated behaviour of nymphs and adults, and between the phases of S. gregaria, may be attributable to differences at the sensory input level.     

Fine structure of antennal sensilla basiconica and their detection of plant volatiles in the eucalyptus woodborer, Phoracantha semipunctata Fabricius (Coleoptera: Cerambycidae)

The ultrastructure and distribution pattern of two types of basiconic sensilla (I and II) on the antennal flagellum of both sexes of Phoracantha semipunctata (Coleoptera: Cerambycidae) was investigated by scanning and transmission electron microscope. Both types are thin-walled multiporous sensilla and occur mostly along the anterior border of the Fl1-Fl6 flagellomeres, while on the distal flagellomeres (Fl7-Fl9) they are more evenly distributed on both surfaces. Clusters of sensilla basiconica II are found on the distal half of the anterior border of the Fl1-Fl6 flagellomeres. Sensilla basiconica I have one bipolar sensory cell with a branched distal dendrite, whereas the sensilla basiconica II contain two bipolar sensory cells with branched distal dendrites. No sexual dimorphism was found in the fine structure and distribution pattern of both types of sensilla basiconica. Responses from single sensory cells to host and non-host plant odors were examined, using gas chromatography linked with electrophysiological recordings. Most cells associated with each sensillum type were narrowly tuned, each specialized for the detection of one or two chemically related compounds. No clear functional distinction between the two morphological types of sensilla was found, although the few cells that responded specifically to non-host volatiles were associated with sensilla basiconica II.     

Trypanosoma,Paramecium and Tetrahymena: From genomics to flagellar and ciliary structures and cytoskeleton dynamics

Cilia and flagella play an important role in motility, sensory perception, and the life cycles of eukaryotes, from protists to humans. However, much critical information concerning cilia structure and function remains elusive. The vast majority of ciliary and flagellar proteins analyzed so far are evolutionarily conserved and play a similar role in protozoa and vertebrates. This makes protozoa attractive biological models for studying cilia biology. Research conducted on ciliated or flagellated protists may improve our general understanding of cilia protein composition, of cilia beating, and can shed light on the molecular basis of the human disorders caused by motile cilia dysfunction. The Symposium “From genomics to flagellar and ciliary structures and cytoskeleton dynamics” at ECOP2019 in Rome presented the latest discoveries about cilia biogenesis and the molecular mechanisms of ciliary and flagellum motility based on studies in Paramecium, Tetrahymena, and Trypanosoma. Here, we review the most relevant aspects presented and discussed during the symposium and add our perspectives for future research.