Feeding behavior regulation in the fly: effect of a noxious substance through the taste and olfactory neurons.

The contact chemosensilla on the labellum of the fly are in the form of a hair housing five sensory neurons, i.e. four contact chemoreceptor cells and one mechanoreceptor neuron (Ozaki and Tominaga, 1999). Three of the four chemoreceptor cells are the sugar, salt and water receptor cells, respectively. The remaining chemoreceptor cell, traditionally called the ‘fifth cell’, has recently been proposed to function as a deterrent cell (Liscia and Solari, 2000; Ozaki et al., 2003). Thus, the balance of response between the sugar receptor cell and the deterrent cell could determine feeding behavior of the fly. Previously, Ozaki et al. (1995) found an odorant-binding protein (chemical sense-related lipophilic ligand-binding protein, CRLBP) common to the taste and olfactory sensilla of the blowfly, Phormia regina. This acidic, small molecular protein binds lipophilic noxious compounds like D-limonene, and since aversive behavior is triggered when the fly touches such a noxious compound, this sensory cell may be related to aversive behavior via CRLBP. On the other hand, during food searching by flies, a noxious compound such as D-limonene, which has strong oral toxicity (Ozaki et al., 2003), should carefully be avoided. Phormia regina is not counted as a phytophagous insect but the adult flies are nectar feeders. Hence they could encounter monoterpenes, which are abundantly present in citrus rinds, by chance. The flies may exhibit more effective aversive behavior toward the D-limonene when both the taste and the olfactory receptors are stimulated spontaneously. Thus, the olfactory inputs from the antennae and/or the maxillary palps may be expected to influence the feeding or the aversive behavior in the flies.