A TEST OF IDENTIFIED RESPONSE CLASSES AMONG OLFACTORY RECEPTOR NEURONS IN THE HONEYBEE WORKER

Two compounds were selected from each of three previously defined olfactory receptor neuron response classes in worker honeybees, viz: citral and geraniol, limonene, and undecanoic and dodecanoic acids. These odorants were presented at a standard dose to 70 sensilla placodea. The responses of these sensilla (i.e. rates of firing) to each of the six odorants were compared to responses to odorant-free solvent controls and were analysed in three ways. First, the summed responses across all placodes, as might occur in an electroantennogram, was significantly different from the controls only in the case of citral, geraniol and linalool. Secondly, principal component and cluster analyses of the responses of individual whole placodes revealed a few specialized placodes, but most placodes responded to more than one of the compounds and no neat categorization emerged. Thirdly using spike height and, to some extent, spike shape to separate the responses of individual placodes into units that more closely represented the activity of single neurons revealed a greater level of specialization, although many generalist subplacode units still remained. A comparison of the distribution of interspike intervals revealed no difference between generalist and specialist units. However. a comparison of these distributions at the placode versus subplacode level suggest that receptor neurons within placodes may pace one another so as to not fire simultaneously. Our results do not support the previous categorization of receptor types, but do agree with behavioral observations in olfactory learning contexts regarding the special role that citral plays in worker honey-bee olfactory perception and also the dominance of linalool over limonene.