Odour detection in the desert locust, Schistocerca gregaria: antennal structure and function

Abstract: The desert locust, Schistocerca gregaria, is a polymorphic insect that occurs in two morphological and physiological distinct phases, solitary and gregarious, depending on population density. The two phases are reversible at any developmental stage. The gregarious phase produces and uses stage-specific pheromones, that keep the swarm together. In my thesis, I investigated the functional morphology of olfactory receptor neurones on the desert locust antennae that are responsible for the detection of aggregation pheromones. First I characterized the different antennal receptor types using electron microscopic methods. Four structurally different types could be distinguished, three bearing olfactory features: sensilla basiconica, s. trichodea, and s. coeloconica, and one belonging to a mechano-/contact-chemoreceptor type, s. chaetica. The distribution of olfactory sensilla was different between the phases, with solitary phase individuals having significantly more sensilla than the gregarious phase locusts. I then investigated the physiological characteristics of receptor neurones present in the various olfactory sensillum types by electrophysiological techniques, stimulating them with synthetic compounds of behavioural significance to the locust. Olfactory receptor neurones housed in s. basiconica were found to be excited in a dose-dependent manner by aggregation pheromone compounds, produced both by nymphal stages and mature adults. Receptor neurones housed in s. coeloconica were found to be excited by a general plant odour and nymphal-produced aggregation pheromones, while the adult-produced pheromones inhibited their activity. Receptor neurones in s. trichodea were excited only by a possible sex pheromone compound of S. gregaria. Receptor neurone sensitivity was found to be higher in solitary phase locusts than in gregarious individuals. These studies show that the desert locust is endowed with a specific and sensitive olfactory system for monitoring its environment. The results presented lay a firm foundation for future short- and long-term studies of semiochemicals that modulate desert locust behaviour.