Sensilla

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]. 

Fig. 5 Single sensillum recordings from the pheromone-detecting sensilla placodea on P. di-versa male antennae. Note a dose-dependent increase in spike frequency after stimulus application for 300 ms (bar)...

Using a specific antibody, Shanbhag and collaborators [34] demonstrated that LUSH is expressed in sensilla trichodea of the Drosophila antennae along with two other putative odorant-binding proteins Obp83a (PBPRP-3, OS-F) and Obp83b (OS-E). When antennal sections of the LUSH-deficient mutant were la-... 

In this model, OBPs participate in the selective transport of pheromone and other semiochemicals to their olfactory receptors. The selectivity of the system is likely to be achieved by layers of filters [ 16], i.e., by the participation of compartmentalized OBPs and olfactory receptors. It seems that OBPs transport only a subset of compounds that reach the pore tubules. Some of these compounds may not bind to the receptors compartmentalized in the particular sensilla. The odorant receptors, on the other hand, are activated by a subset of compounds, as indicated by studies in Drosophila, showing that a single OR is activated by multiple compounds [66]. If some potential receptor ligand reaches the pore tubules but are not transported by OBPs, receptor firing is prevented because the receptors are protected by the sensillar lymph. In other words, even if neither OBPs nor odorant receptors (ORs) are extremely specific, the detectors (olfactory system) can show remarkable selectivity if they function in a two-step filter. 

Hardin There are other drivers available for other subsets of sensillae, and we are testing those to see whether they are insensitive to that particular odorant. The other thing is to use other odorants with these flies and do the converse. 

Hardin Basoconic sensillae are sensitive to a huge array of odours, but other sensillae have more limited sensitivity. 

Are these other sensillae part of the pheromone system, or are they olfactory too ... 

At the same time insects are able to discriminate between host and non-host plant species as they select plants on which feeding ultimately results in growth and reproduction, and on the other hand avoid poisoning or malnutrition on non-host plants. By means of chemosensory sensilla, insects are able to perform the difficult task, being well equipped analytical chemists, of identifying the chemical composition of plants that insects meet in their environment (6). 

Lepidopterous larvae bear on their mouthparts two pair of styloconic sensilla (see Figure 1). The papilla of each sensillum possesses one terminal pore which gives entrance to the dendritic regions of four gustatory receptor cells. Besides, a fifth cell in each sensillum acts as a mechanoreceptor in detecting positional changes of the papilla (7, ) ... 

In addition to the sensilla styloconica, lepidopterous larvae possess gustatory sensilla on the maxillary palps. Eight basiconic sensilla are located on top of each palpus (see Figure 1). Five of them possess a terminal pore, and for that reason these sensilla might be considered as contact chemoreceptors. The remaining three show numerous small perforations all over the cuticle, which indicates an olfactory function (.8). The response spectra of these sensilla are, however, still obscure. 

One pair of epipharyngeal sensilla located in the buccal cavity completes the set of contact chemoreceptors in lepidopterous larvae. In P. brassicae larvae, each of these... 

Figure 1. The head of a Pieris brassicae larva. The chemosensory sensilla are...

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. 

Foelix, R.F. and Chu-Wang, I. W. (1973). The morphology of spider sensilla. II. Chemoreceptors. Tissue Cell 5 451M60. 

Hallberg, E. and Hansson, B. S. (1999). Arthroped sensilla morphology and phylogenetic considerations. Microscopy Research and Technique 47 428 139. 

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. 

The density of antennal sensilla in males rises sharply away from the basal segment for about 1 cm then declines over the next 4 cm to the tip of the antenna (Schaller, 1978 Hosl, 1990). The two receptor cells that are tuned to each of the two periplanones are housed within the same sensillum, the basiconic single-walled type , along with two other cells that respond to terpenes and alcohols (Boeckh and Ernst, 1987). However, unlike the highly specialized receptor cells of male moths, the periplanone-A and periplanone-B cells have overlapping response spectra to these two compounds. Also, it is not known how responsiveness of pheromone-sensitive sensilla to food odorants (terpenes and alcohols) affects behavior of the male cockroach. 

Axons originating in sensilla that are responsive to food odorants and those that originate in sex pheromone-responsive neurons terminate in different glomeruli, the first synaptic relay station within the antennal lobe (Boeckh and Ernst,... 

A number of chemo- and mechanoreceptors participate in the male behaviors. The female contact pheromone is detected by chemosensilla on the antennae and labial and maxillary palps (Ramaswamy and Gupta, 1981). The number of these sensilla increases dramatically during the metamorphic molt, and much more so in males than in females. Unfortunately, no electrophysiological recordings have been conducted, and the specific sensillum type that responds to the contact pheromone... 

Ramaswamy, S. B. and Gupta, A. P. (1981). Sensilla of the antennae and the labial and maxillary palps of Blattella germanica (L.) (Dictyoptera Blattellidae) their classification and distribution. Journal of Morphology 168 269-279. 

Schaller, D. (1978). Antennal sensory system of Periplaneta americana L. Distribution and frequency of morphologic types of sensilla and their sex-specific changes during postembryonic development. Cell and Tissue Research 191 121-139. 

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