Antennae olfactory sensilla

Differential sensory sensitivity. The insect s perception of plant odours differs essentially from their discrimination of non-volatile taste substances, as phytophagous insects may already perceive the odour at some distance from the plant. In adult phytophagous insects the antennae bear a large number of olfactory sensilla in order to detect the minute concentrations of the leaf odour components in the air downwind from a plant. The overall sensitivity of the antennal olfactory receptor system can be measured by making use of the electroantennogram technique (17). An electroantennogram (EAG) is the change in potential between the tip of an antenna and its base, in response to stimulation by an odour component. Such an EAG reflects the receptor potentials of the olfactory receptor cell population in the antenna. 

P americana is one of just a few species of insects in which both peripheral and central olfactory processing have been studied. In contrast to many short-lived lepidopterans, in which the male antenna is highly specialized for sex pheromone reception, the antennae of male cockroaches contain numerous food-responsive sensilla. In addition to olfactory sensilla, the antennae also house mechano-, hygro-and thermoreceptors, as well as contact chemoreceptors (Schaller, 1978 review Boeckh et al., 1984). Extensive ultrastructural and electrophysiological evidence has demonstrated that morphologically defined sensillum types house receptor cells of specific functional types (Sass, 1976, 1978, 1983 Schaller, 1978 Selzer, 1981, 1984 review Boeckh and Ernst, 1987). Boeckh and Ernst (1987) defined 25 types of cell according to their odor spectra, but of the 65 500 chemo- and mechanosensory sensilla on the antenna of adult male P. americana, an estimated 37 000 house cells that respond to periplanone-A and periplanone-B. 

Sexual dimorphism of antenna sensillum types does not become morphologically apparent before the adult stage. Antennal segments increase in length approximately three-fold during postembryonic development in both males and females (Schafer and Sanchez, 1976). In the female, the sensillar population increases 7.5-fold, whereas adult males have 12 times more sensilla than first instars the difference results from a significant proliferation of olfactory sensilla in males. 

In S. gregaria, immunocytochemical experiments showed selective CSP labeling of the outer lymph in diverse chemosensory organs, including contact sensilla of tarsi, maxillary palps, and antennae. However, in antennae, only sensilla chaetica were labeled with no labeling observed in olfactory or coeloconic sensilla (Angeli et al., 1999), suggesting a role for CSPs in contact chemoreception in Orthoptera. 

The sensitivity and selectivity of olfaction and contact chemosensation are due (1) in the brain, to the existence of a neuronal network of neurons tuned to a specific chemical stimulus, and (2) in the periphery, to the existence of olfactory/ chemosensory receptor neurons housed in sensory microorgans called sensilla. The sensilla can best be viewed as simple cuticular porous extrusions that increase the surface that captures airborne odorants or chemicals dissolved in water droplets. They contain the receptive olfactory or chemosensory structures (Schneider, 1969). The olfactory sensilla are most numerous on the antennae and mediate the reception of sex pheromones and plant volatiles, as well as other odorants. Low volatility pheromones may also be detected by contact chemoreceptors on... 

Like all Diptera, Drosophila has two paired appendages that carry olfactory sensilla (Figure 23.1A) the antennae carry ca. 1200 ORNs and the maxillary... 

The chemosensory organs of crustaceans are divided into bimodal chemo- and mechanosensory sensilla (found e.g., on mouthparts and legs) and unimodal olfactory sensilla (usually found on the first antenna). Although the general morphology of these structures has often been thoroughly described, less is known about the function of the sensilla in an ecological and behavioral context. 

Upon returning to our home base we analyzed the more detailed structure of the olfactory organs of B. latro. The aesthetascs could be divided into at least two types, where one is more prevalent towards the tip of the antenna, and displays what might be a terminal pore. The placement and the possible pore could indicate a function in taste perception. The major part of the aesthetascs was uniformly scale-like without terminal pore. In fact, no pores at all could be observed, not even the minute cuticular pores observed in almost all insect olfactory sensilla. We have so far not been able to elucidate how molecules in gas phase enter the inside of the B. latro aesthetascs. Each hair was shown to contain a very large number of olfactory receptor neurons, not only similar to what has been found in other crustaceans, but also in e.g., honeybees and locusts (Hansson et al. 1996 Ochieng et al. 1998). 

Insect perception of volatile semiochemicals is mediated through olfactory sensilla, located mainly on the antennae. These sensilla have a porous cuticular surface through which semiochemicals can pass and make contact with the sensillum lymph. Semiochemicals are usually hydrophobic, organic chemicals. For land-living insects, these molecules must be transferred across the aqueous lymph to membrane-bound G-protein-coupled receptors (GPCRs) on the olfactory neurones, from which signal transduction occurs.Transfer across the sensillum lymph is an evolutionary adaptation to terrestrial habitation and is mediated by odorant-binding proteins (OBPs). These are small (14-20 kDa), acidic. 

GC-EAD is gas chromatography method in which a unique biological detector, based on living insect antenna, is used as one of two detection systems. GC-EAD is an analytical method which offers very fast and reliable identification of compoimds in complex natural mixtures that stimulate the olfactory sensilla of insect antennae (Struble Am, 1984). In other words, the GC-EAD helps to discover what specific chemicals in tested mixture an insect can smell, respective, which compound elicits the electric antennal response. 

Fig-1 Schematic view of the overall olfactory processing in insects. Pheromones and other semiochemicals are detected by specialized sensilla on the antennae, where the chemical signal is transduced into nervous activity. The olfactory receptor neurons in the semiochemi-cal-detecting sensilla are connected directly to the antennal lobe. Here the semiochemical-derived electrical signals are processed and sent out (through projection neurons) to the protocerebrum. Olfactory information is then integrated with other stimulus modalities, a decision is made, and the motor system is told what to do... 

Largely, the insect detectors for pheromones and other semiochemicals are arrays of hair-like sensilla distributed over the surface of the antennae and palps. In some species, such as scarab beetles [3, 4] and the honeybee [5], semiochemicals are received by olfactory plates. The more ubiquitous hair-like sensilla typically consist of hollow cuticular hairs (10-400 pm long, 1-5 pm thick) innervated by one or several olfactory receptor cells (neurons) and three auxiliary cells [6]. 

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