Sensilla Contact chemoreception

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 matching dichotomy of sensilla construction and neuroanatomical organization of sensory neuropils suggests that in crustaceans chemical information is received and processed in two fundamentally different modes. One mode is Olfaction which we define as chemoreception mediated by the aesthetasc - OL pathway the second mode is Distributed Chemoreception, which we define as chemoreception mediated by bimodal sensilla on all appendages and the associated striated neuropils that serve as local motor centers. Distributed chemoreception not only comprises taste, which we define as contact chemoreception in the context of... 

Mclver et al. (1980) have recently observed a unique contact chemoreceptive sensillum, the bifurcate sensilla on the tarsi of the female black fly (Simulidae). This type of sensillum has characteristics of a contact chemoreceptor, although a sieve-like structure in the pore region, which increases the absorptive surface area at the tip, suggests a secondarily acquired olfactory function. 

Contact chemoreceptive sensilla have been identified on the major parts of the body including the wings (Wolbarsht and Dethier, 1958) and the ovipositor of flies (Rice, 1976 Wallis, 1962 Wolbarsht and Dethier, 1958). This redundancy of receptors is probably a way to avoid unsuitable environments. Most sensilla are located on the proboscis, the tarsi and palpi. Contact chemoreceptors in the food channel have been described for flies (Stocker and Schorderet, 1981),... 

Fig. 1.2 Scanning electron micrographs of contact chemoreceptive organs and sensilla. 

Some of the antennal contact chemoreceptive sensilla of Periplaneta ameri-cana contain cells sensitive to alcohols and fatty acids (Riith, 1976) fruits stimulated the sugar and alcohol cell, whereas meat was perceived by the fatty acid cell. 

In contrast to the leaf surfaces much more is known about the chemistry of the leaf interior, which is probably considerably more complex. These compounds representing nutrients, toxic allelochemicals and allelochemicals with a host plant sign character, influence feeding behavior (see Scriber, Chapter 7). As can be seen from Table 1.1, contact chemoreceptors for all types of compounds have been identified in a variety of insect species. With the exception of the two beetle larvae and the locust, all of the examples are lepidopteran larvae mainly because the two pairs of sensilla styloconica on the galea (maxilla) of these larvae can be investigated easily (Schoonhoven and Dethier, 1966). The relatively small number of contact chemoreceptors (four sensilla with four neurons) and their importance for food plant discrimination (Hanson and Dethier, 1973) make them ideal for the study of contact chemoreception in relation to host plant selection. 

Non-volatile pheromones exist, but their chemical isolation and identification has largely just begun. Consequently, few specific receptor cells have been characterized. Using behaviorally active extracts, contact chemoreceptive sensilla with sensitive cells have been identified (Table 1.1). In Fig. 1.5, representative recordings from a tarsal D-hair of Rhagoletis cerasi are presented. Only artificial fruits which were contacted by females can stimulate a receptor cells, and thus activity due to food residues can be excluded. Similar recordings were ailso obtained from D-hairs of Ceratitis capitata (Stadler and Boiler, unpublished). 

Since the contact pheromones are spread as small droplets over the fruit surface, the contact chemoreception process is similar to the earlier mentioned perception of leaf surfaces. Females have also been observed running over the fruit surface for some time before oviposition occurs (Prokopy, 1981 see also Chapter 11). It can be assumed that this sampling of the dry surface does increase the sensory input to the CNS by the multiple contacts of the tarsal hairs during these inspection runs. Presumably, this type of multiple stimulation over time is comparable to the simultaneous perception of volatile pheromones with multiple sensory hairs on the antennae. Both the multiple stimulation of a limited number of sensilla and the multitude of sensitive sensilla will result in a low perception threshold for chemical signals occurring at a low concentration in the environment. 

The generated spike activity of the-different sensory cells in response to stimuli is decoded by the central nervous system (CNS). To investigate the sensory code of the receptor cells of a sensillum or several sensilla, different approaches can be used. The most direct method is to record from central neurons. In contrast to the investigation of central neurons responding to olfactory stimulations (Chapter 2), such recordings have been performed only recently from interneurons of the thoracic ganglion of the blowfly in response to the stimulation of tarsal contact chemoreceptive sensilla (Rook et al., 1980). 

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