The gregarisation pheromone of locusts

Abstract

Swarming locusts show three physical criteria, i.e. the phase changes of melanisation of the nymphal stages or hoppers, of the proportions of certain body parts (morphometric ratios), and increased genetic recombination (meiotic chiasma frequencies) in the adult. The control of these changes, initiated by aggregation into swarms, i.e. gregarisation, seems to be vested in a pheromone which is produced by all hoppers in both the solitaria and gregaria phases, also by hoppers of the albino strain. Such a pheromone can be extracted from the locust room air and from the locust, these extracts showing high activity in bioassays, primarily in increased chiasma frequencies but also in hopper colour. The extract in risella oil is more efficient than that in petroleum ether and can be distilled to yield an active distillate. The pheromone is secreted in the faeces of hoppers but not of adults. There is evidence in faeces bioassays that all three physical criteria are affected; the pheromone may be called locustone. It is manufactured or secreted in a specific section of the alimentary canal, i.e. the crop. Reception is not through the antennae but through the stigmata. Preliminary chemical analysis of a risella oil air extract distilled into various other solvents showed the presence of a relatively simple saturated aliphatic chain with a carbonyl function, perhaps a ketone or an ester.