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Physiological and morphological characteristics of antennal lobe interneurons in male and female Schistocerca gregaria were studied by using intracellular recording and staining techniques. For the first time, the responses of projection neurons to behaviourally active and potential aggregation pheromone compounds and plant odour compounds were recorded in young adult locusts. Excitatory, inhibitory, or combined excitatory/inhibitory responses to the presented odours were detected. The stained neurons had their cell bodies in the frontal cell group, arborized in 10 to 25 glomeruli at equal distances from the central fibre core, and sent their axons to the calyces of the mushroom body and to the lateral protocerebrum. The projection neurons responded to the set of different stimuli with varying specificity. In females, more neurons responded specifically to single plant or aggregation pheromone compounds than in males, where more generalist responses were found. ’’Blend specialist’’ neurons, responding only to mixtures of behaviourally active aggregation pheromone compounds, but not to the single compounds, were present in both males and females. Most neurons responded to the behaviourally active aggregation pheromone mixtures and to single compounds present in these mixtures, as well as to plant odours. Fewer neurons responded to the potential aggregation pheromone compounds tested. In several experiments, two spike sizes in the recording were correlated with two stained neurons in the antennal lobe, suggesting electrical coupling of the neurons. No response to any of the stimuli was found in antennal lobe interneurons in old adults. The morphological and physiological features of the projection neurons in S. gregaria are compared with projection neuron characteristics in other insects. (C) 1996 Wiley-Liss, Inc.