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The relationship between the feeding response and sugar concentra- tion was investigated using the proboscis extension reflex (PER) test, and the feeding sensitivity of a group of flies was indicated as a mean value of the feeding threshold concentration of sucrose. The mean value of the feeding threshold was taken as an indicator of appetite and was determined by half of the maximum concentration of the PER-concentration curve. When the flies increased their appetite, the mean value of the feeding threshold decreased, and vice versa. We carried out the PER test at various concentrations of sucrose in the absence or presence of D-limonene odor. In the presence of D- limonene, the mean value of the feeding threshold increased three- fold. The appetite reducing effect was also observed after the dietary experience with the D-limonene odor. The flies, which were fed on sucrose in the presence of D-limonene for 5 days after eclosion, exhibited obvious appetite reduction to sucrose even in the absence of this compound. Thus, dietary experience with the odor of a toxic substance suppresses the feeding motivation in the flies, reducing the probability of ingesting toxic substances. Effect of mushroom body ablation Considering the experiential effect of the diet in the presence of D- limonene, one may expect that the mushroom body, a neural struc- ture involved in learning in the insect brain, may contribute to the effect. We succeeded in ablating the mushroom body of P. regina by hydroxy urea treatment of the larvae (de Belle and Heisenberg, 1994). When we carried out the same PER test in the mushroom body-ablated flies, they showed appetite reduction in the presence of D-limonene both before and after the dietary experience with that odor. Thus, the fly can integrate olfactory information about D- limonene with the taste information about sucrose without the mushroom body. However, the mushroom body-ablated fly showed normal appetite to sucrose even after the dietary experience with the odor of D-limonene. This suggested that the mushroom body- ablated fly could not learn or remember the dietary experience, through which aversive conditioning between the taste of sucrose and the odor of D-limonene should occur. Effect of antennae or maxillary palps We also carried out the same PER tests with the flies whose antennae or maxillary palps were removed. Removal of antennae influenced the memory of dietary experience with the odor of D-limonene, but removal of maxillary palps did not. Thus, the neural routes from the maxillary palps may not be involved in formation of associative memory between the taste of sucrose and the noxious odor of D- limonene.

Feeding Behavior Regulation in the Fly: Effect of a Noxious Substance through the Taste and Olfactory Neurons