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Mechanisms of Olfaction in Insects - ResearchSpace@Auckland ...

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General Introduction 6<br />

Cactoblastic cactorum (Poph<strong>of</strong> et al., 2005) <strong>in</strong> electrophysiological studies. Sensilla<br />

auricillica is a shoehorn shaped, porous sensilla that is <strong>in</strong>nervated by the branched<br />

dendrites <strong>of</strong> three neurons. They are <strong>in</strong>volved <strong>in</strong> detect<strong>in</strong>g plant volatiles (Anderson et<br />

al., 2000) and sex pheromone components <strong>in</strong> Cydia pomonella (Ebb<strong>in</strong>ghaus et al.,<br />

1998; Ansebo et al., 2005). Sensilla chaetica is similar to the trichoid sensilla except<br />

that it has thicker walls and the base has a flexible circular membrane (Schneider,<br />

1964). This sensilla type are either devoid <strong>of</strong> cuticular pores with a function <strong>in</strong><br />

mechano-reception, or have only one pore at their tip and function <strong>in</strong> gustatory<br />

reception (Keil, 1999). Sensilla styloconica are short, devoid <strong>of</strong> pores with a peg-like<br />

shape and function as thermo– or hygro– receptors (Shields and Hildebrand, 2001).<br />

The last sensilla type, sensilla coeloconica are very short, double walled, shaped like<br />

peg and are located <strong>in</strong> pits below the antennae cuticle. The sensilla are aporous but the<br />

double wall is not fused hence odours can move through it. This sensilla type is<br />

<strong>in</strong>nervated by dendrites <strong>of</strong> five neurons (Altner et al., 1977) and are <strong>in</strong>volved <strong>in</strong><br />

detection <strong>of</strong> aliphatic acids and aldehydes <strong>in</strong> B. mori and C. cactorum (Poph<strong>of</strong>, 1997;<br />

Poph<strong>of</strong> et al., 2005).<br />

The cascade <strong>of</strong> events that leads from the detection <strong>of</strong> an odorant present <strong>in</strong> the<br />

environment and its conversion <strong>in</strong>to a neuronal signal <strong>in</strong> the antennal lobe <strong>of</strong> the moth<br />

is still unclear. A proposed mechanism <strong>of</strong> the perireceptor events that follow detection<br />

<strong>of</strong> an odorant is as follows (Kaissl<strong>in</strong>g, 2009). The hydrophobic odorant molecule<br />

enters the sensillum lymph via waxed pores <strong>in</strong> the cuticular wall <strong>of</strong> the sensilla. This<br />

hydrophobic molecule has to travel through the aqueous layer to the membrane bound<br />

receptors <strong>in</strong> order for signal transduction to occur. Soluble prote<strong>in</strong>s present <strong>in</strong><br />

abundance <strong>in</strong> the lymph may function <strong>in</strong> transport<strong>in</strong>g the odorants by form<strong>in</strong>g<br />

complexes. The b<strong>in</strong>d<strong>in</strong>g <strong>of</strong> the odorant to the membrane receptor activates a series <strong>of</strong><br />

signall<strong>in</strong>g cascades <strong>in</strong> the ORN caus<strong>in</strong>g action potentials to be generated. This results<br />

<strong>in</strong> the conveyance <strong>of</strong> the signal to the antennal lobe and process<strong>in</strong>g <strong>in</strong> higher bra<strong>in</strong><br />

centres. Odorants need to be cleared from the olfactory system once the signal has<br />

been relayed to the ORNs. This ensures the sensitivity and specificity <strong>of</strong> the system<br />

and the detection <strong>of</strong> new odorants or different concentrations <strong>of</strong> odorant plumes that<br />

direct the moths‟ flight (towards or away from attractants and repellents) (Kaissl<strong>in</strong>g,<br />

1974; Vogt, 1987; Prestwich et al., 1989; Kaissl<strong>in</strong>g, 2009).

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