Locusts are in essence polymorphic grasshoppers, whose morphology and behavior are affected by population density. Isolated solitarious individuals tend to avoid contact and are more subdued. Gregarious locusts socially interact and are more active (Uvarov, 1966). They march in bands as nymphs and swarm as adults. Behavioral response to changes in population density is rapid and is reversible (e.g., Giliett, 1988; Roessingh and Simpson, 1994; Heifetz et al., 1996).
Locusts communicate chemically in the process of phase transition. The behaviour of solitarious Desert Locust (Schistocerca gregaria) nymphs is shifted towards gregarious behaviour within five hours of exposure to cuticular surface lipids of gregarious nymphs. Surface lipids from the Migratory Locust (Locusta migratoria) do not induce behavioural phase transition in the Desert Locust, indicating the specificity of this response. Hydrocarbon "contact" pheromones in the lipid fraction directly affect antennal chemoreceptors, inducing second messenger in subcellular antennal preparations.
Nothing is known about the events that follow initial signal transduction on exposure of solitarious nymphs to the cuticular lipid extract. We have observed several responses further downstream as a consequence of high population density or exposure to cuticular lipids. High population density induces increases in haemolymph metabolites. The concentration of specific peptides in methanolic extracts of the Desert Locust nymphal brain or corpora cardiaca (CC), including the decapeptide adipokinetic hormone, corresponds with changes in population density. Exposure of solitarious nymphs to the cuticular lipids rapidly adjusts the levels of these specific peptides to those representative of gregarious nymphs. Injection of brain or CC extracts elicits gregarious behaviour.
The integration of these results suggests that the increase in specific peptides, and of subsequent haemolymph energy resources, are part of the downstream pathway leading to behavioural phase transition, initiated by cuticular hydrocarbons.
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