Physiological responses to stimulations with various behaviourally relevant odours (Obeng-Ofori et al. 1993;Torto et al. 1994) were recorded from olfactory receptor neurons in sensilla basiconica and s. coeloconica on the antennae of adult solitarious phase of the desert locust, Schistocerca gregaria. In the sensillum basiconicum, a large number of neurons were excited by odourous stimulation and hence no single neuron could be identified within the physiological response necessitating the entire receptor neuron assembly to be treated as an entity. However, the multiple receptor neurons showed excitatory response in a dose dependent manner to single gregarization pheromone components as well as to the full blend. A general plant odour, E-2-hexenal elicited responses that were much lower. Moreover, an organic acid that has been identified in the odour bouquet of nymphal desert locusts elicited inhibitory responses from neurons in this sensillum type at high concentrations. Receptor neurons housed in a second olfactory sensillum type, s. coeloconica, demonstrated a reversed activity pattern as compared to the one in s. basiconica. The general plant odour and organic acid elicited strong excitatory responses at iow threshold values in a dose dependent relationship, while the pheromone compounds were either non-active or inhibited the spontaneous.activity of these neurons. It was shown that in S. gregaria, antagonistic interactions between behaviourall y relevant semiochemicals take place at the peripheral olfactory receptor level and emphasizes the importance of olfactory sensory input in the mediation of aggregation behaviour and the phase dynamics of this species (Mordue, A.J.1977; Gillett 1983 ).
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