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Experiments on aggregations of Phymateus purpurascens (Orthoptera, Acrididae, Pyrgomorphinae)



Phymateus purpurascens Karsch has been studied in the wild and in captivity in Uganda. Its habits, colouration, behaviour and life cycle are described.

All individuals are attracted towards each other by visual stimuli, and a moving individual is more attractive than a stationary one. This behaviour is unchanged by several instars in isolation. The aggregated larvae tend to move in synchronous bursts. and it is suggested that this is due to retinal image stabilization by an optomotor reflex.

In an open field all instars are attracted towards isolated vertical objects in the visual field. This, together with the interindividual visual responses described, produces columns of moving insects. No olfactory communication could be demonstrated.

Adults and larvae behave similarly in experimental situations; this contrasts with the observed dispersion in the wild with increasing age. It is suggested that dispersion results from loss of visual contact, caused by (a) loss of the conspicuous larval colouration, (b) chance isolation, and (c) increased mobility of fledged adults. The possible biological function of the gregariou habit is discussed, and the significance of the various colour patterns of the life history as inter-and intraspecific signals.

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