Die Ontogenie der basiconischen Riechsensillen auf der Antenne von Necrophorus (Coleoptera)

Nanopore Formation in the Cuticle of an Insect Olfactory Sensillum

Nanometer-level patterned surface structures form the basis of biological functions, including superhydrophobicity, structural coloration, and light absorption [1-3]. In insects, the cuticle overlying the olfactory sensilla has multiple small (50- to 200-nm diameter) pores [4-8], which are supposed to function as a filter that admits odorant molecules, while preventing the entry of larger airborne particles and limiting water loss. However, the cellular processes underlying the patterning of extracellular matrices into functional nano-structures remain unknown. Here, we show that cuticular nanopores in Drosophila olfactory sensilla originate from a curved ultrathin film that is formed in the outermost envelope layer of the cuticle and secreted from specialized protrusions in the plasma membrane of the hair forming (trichogen) cell. The envelope curvature coincides with plasma membrane undulations associated with endocytic structures. The gore-tex/Osiris23 gene encodes an endosomal protein that is essential for envelope curvature, nanopore formation, and odor receptivity and is expressed specifically in developing olfactory trichogen cells. The 24-member Osiris gene family is expressed in cuticle-secreting cells and is found only in insect genomes. These results reveal an essential requirement for nanopores for odor reception and identify Osiris genes as a platform for investigating the evolution of surface nano-fabrication in insects.

Sensory cilia in arthropods

In arthropods, the modified primary cilium is a structure common to all peripheral sensory neurons other than photoreceptors. Since its first description in 1958, it has been investigated in great detail in numerous sense organs (sensilla) of many insect species by means of electron microscopy and electrophysiology. The perfection of molecular biological methods has led to an enormous advance in our knowledge about development and function of sensory cilia in the fruitfly since the end of the last century. The cilia show a wealth of adaptations according to their different physiological roles: chemoreception, mechanoreception, hygroreception, and thermoreception. Divergent types of receptors and channels have evolved fulfilling these tasks. The number of olfactory receptor genes can be close to 300 in ants, whereas in crickets slightest mechanical stimuli are detected by the interaction of extremely sophisticated biomechanical devices with mechanosensory cilia. Despite their enormous morphological and physiological divergence, sensilla and sensory cilia develop according to a stereotyped pattern. Intraflagellar transport genes have been found to be decisive for proper development and function.

The Antennal sensilla of Oxelytrum erythrurum (Blanchard) and Oxelytrum apicale (Brullé) (Coleoptera: Silphidae)

The typology and placement of antennal sensilla of the carrion beetles Oxelytrum erythrurum (Blanchard) and Oxelytrum apicalis (Brullé) (Coleoptera: Silphidae) were studied using scanning electron microscopy. Two types of sensilla chaetica, two types of sensilla trichodea, four types of sensilla basiconica, one type of sensilla coeloconica, and an unidentified type of sensillum were found in both species. Sensilla chaetica type 1 are found on the antennomeres proximal to antennal club (A1-A8); chaetica type 2 are found on the club (A9-A11). Sensilla trichodea are found on A9-A11; one type (T1) is found on the proximal portion of the club, the other type (T2) on the apical portion. Basiconica type 1 are found on the dorsal surface of A9-A11; they are much denser on the apical portion of the antennal club than on the proximal. In O. erythrurum, a nocturnal species of the Chaco-Pampean plain, T2 two are found on A10 and A11. In Oxelytrum apicale, a mountain species, probably diurnal, only A11 bears T2, but they are denser than in the other species. It is suggested that O. apicale depends more on contact chemoreception than O. erythrurum. The ventral surface of the antennal clubs shows no remarkable difference between species.

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.

Arthropod sensilla: morphology and phylogenetic considerations

The structures of different types of arthropod sensilla are compared and theories regarding the evolution of these sensory organs are presented. Arthropod sensilla are built according to a common plan, and are probably homologous to scolopidia. Certain similarities in the structure of sensilla in different arthropod groups can be the result of adaptations to specific environments. The structure of sensilla in insect groups, which are regarded to be ancestral, do not appear to be less sophisticated than in groups considered to be more advanced. The different types of pore systems, as well as the structural differentiations of insect olfactory sensillar types remain unexplained. Olfactory sensilla display a large degree of similarity among terrestrial arthropods, whereas crustacean sensilla diverge in structure. In holometabolous insects larval sensilla appear to be structurally quite advanced, and more complex than in the adult. During the ontogeny of both sensilla and scolopidia, these are differentiated in an epithelial layer, resulting in the formation of both sensory and enveloping cells. The developmental patterns of sensilla in the studied insect groups are similar. During the development of sensilla apoptotic process are usually active.

Functional morphology of insect mechanoreceptors

This paper reviews the structure and function of insect mechanoreceptors with respect to their cellular, subcellular, and cuticular organization. Four types are described and their function is discussed: 1, the bristles; 2, the trichobothria; 3, the campaniform sensilla; and 4, the scolopidia. Usually, bristles respond to touch, trichobothria to air currents and sound, campaniform sensilla to deformation of the cuticle, and scolopidia to stretch. Mechanoreceptors are composed of four cells: a bipolar sensory neuron, which is enveloped by the thecogen, the trichogen, and the tormogen cells. Apically, the neuron gives off a ciliary dendrite which is attached to the stimulus-transmitting cuticular structures. In types 1-3, the tip of the dendrite contains a highly organized cytoskeletal complex of microtubules, the "tubular body," which is connected to the dendritic membrane via short rods, the "membrane-integrated cones" (MICs). The dendritic membrane is attached to the cuticle via fine attachment fibers. The hair-type sensilla (types 1, 2) are constructed as first-order levers, which transmit deflection of the hair directly to the dendrite tip. In campaniform sensilla (type 3), there is a cuticular dome instead of a hair and the dendrite is stimulated by deformation of the cuticle. In these three types, a slight lateral compression of the dendrite tip is most probably the effective stimulus. In scolopidia, the dendritic membrane is most probably stimulated by stretch. On the subcellular level, connectors between the cytoskeleton, the dendritic membrane, and extracellular (cuticular) structures are present in all four types and are thought to be engaged in membrane depolarization.

Fine structure of a developing insect olfactory organ: morphogenesis of the silkmoth antenna

The olfactory organ of the silkmoth Antheraea polyphemus is the feathered antenna which carries about 70,000 olfactory sensilla in the male. It develops within 3 weeks from a leaf-shaped epidermal sac by means of segmental primary and secondary indentations which proceed from the periphery towards the centerline. During the first day post-apolysis, the antennal epidermis differentiates into segmentally arranged, alternating sensillogenic and non-sensillogenic regions. Within the first 2 days post-apolysis, the anlagen of olfactory sensilla arise from electron-dense mother cells in the sensillogenic epidermis. The axons of the developing sensilla begin to form the primary innervation pattern during the second day. The sensilla develop approximately within the first 10 days to their final shape, while the indentations are completed during the same period of time. The indentations are most probably driven by long basal extensions of epidermal cells, the epidermal feet. Primary indentations follow the course of segmentally arranged tracheal bundles and form the segments of the antenna. The secondary indentations follow the course of the primary segmental nerves which are reconstructed by this process. During the remaining time of development, the cuticle of the antenna and the sensory hairs is secreted by the epidermal and the hair-forming cells.

Development of Drosophila larval sensory organs: spatiotemporal pattern of sensory neurones, peripheral axonal pathways and sensilla differentiation

The sensilla of Drosophila larval thoracic and abdominal segments appear in a constant temporal sequence during stage 13/14 (9·5–11·5 h) of embryonic development. Those sensilla innervated by more than one dendrite (basiconical sensilla, chordotonal organs, some of the trichoid sensilla and campaniform sensilla) appear earlier than sensilla innervated by a single dendrite (majority of trichoid sensilla and campaniform sensilla). Furthermore, a dorsoventrally directed gradient underlies the sequence in which sensilla of a given type appear. Sensory axons are emitted in the same sequence. Thus, axons of the polyinnervated sensilla appear first. Together with a distinct set of efferent axons they form ‘pioneer tracts’ of the two fascicles of the segmental nerves. Cytodifferentiation of the sensillum cells resembles the development of larval epidermal cells in many aspects. Thus, the sheath processes formed by sensillum accessory cells and the axons formed by sensory neurones develop from processes transiently formed by all cells. During the phase of cuticle secretion, apical portions of the presumptive accessory cells are modified to form the cuticular apparatus responsible for receiving the sensory stimuli. Finally, two sets of subepidermally located cells which differ with respect to their morphology and, probably, their origin envelop somata and axons of the sensory neurones.

The in vitro development of the pupal integument and the effects of ecdysteroids in Tenebrio molitor (Insecta, Coleoptera)

In order to study the pupal-adult metamorphosis of Tenebrio in vitro, pupal sternites of different ages were cultured in Landureau's medium and their development systematically observed by electron microscopy. In hormone-free medium, explants taken from young pupae do not secrete pupal postecdysial cuticle in vitro, and the epidermis spontaneously detaches from the pupal cuticle. On the contrary, explants taken from pharate adults continue to secrete adult preecdysial cuticle in vitro, and the epidermis never detaches from the cuticle. Ecdysterone in physiological concentrations (0.2 to 4 micrograms/ml) induces the secretion of a new cuticle in explants from young pupae but the epidermis remains undifferentiated. Ecdysone is necessary for the induction of some adult differentiation. Moreover, the quality of the cuticle secreted in vitro is increased by the addition of 2% foetal calf serum; the best results have thus far been obtained in a medium containing 0.2 microgram/ml ecdysone, 1 microgram/ml ecdysterone, and 2% foetal calf serum.

Fine structure of the antennal receptors of the bed bug, Cimex lectularius L

Sensilla on the antenna of the bed bug, Cimex lectularius, were studied with the scanning and transmission electron microscope. Those which display a tubular body in the dendrite ending are presumed to have a mechanoreceptor function (bristles of type A, flat plate of type B). Bristles of type A1 contain additional dendrites which terminate at the tip of the bristle and may be gustatory receptors. Sensilla with pores in the hair wall are supposed to have an offactory, humidity and/or temperature receptor function (pegs and hairs of types C, D, E). Hairs of type E contain receptors for the alarm pheromones of the bed bug. Special attention has been paid to the pore structures and epicuticular layers of these sensilla. Possible differences in stimulus conduction are discussed between (i) sensilla with a simple wall and pores with pore tubules (types D and E) and (ii) the ribbed pegs (type C), which have a complex wall structure and spoke channels. The immersed cones of type F have a peculiar innervation, which has not been described previously. Two dendrites are held closely together by a third flat dendrite which wraps around them in the region of the outer segment. Coupling structures were found between the central dendrites, and between these and the third enveloping dendrite. Possible functions of this unique innervation are discussed. The dendrites innervating type D are grouped in three to eight bundles by multiple sheaths. The term thecogen cell is introduced to denote the innermost of the three sheath cells of a sensillum (the outer being the tormogen and the trichogen cell) which builds the dendrite sheath during ontogeny. Comparative morphometry revealed type-specific differences in the length and diameter of the dendrites. Some axons were found to lack any glial or perineurial sheath. Microorganisms were observed in the antennal tissue of several animals.