Collective orientation by nocturnally migrating Australian plague locusts, Chortoicetes terminifera (Walker) (Orthoptera: Acrididae): a radar study

The collective orientation of Chortoicetes terminifera (Wlk.) migrating at night in New South Wales was studied with a radar. Measurements of the distribution of insect echoes on the radar's plan-position-indicator display provide information about the variation of both the direction and the degree of orientation with both height and time. A quantitative analysis procedure used in a detailed study of one particularly interesting series of observations is described. It was found that collective orientation occurred frequently, and that it sometimes became very well-developed. The direction of orientation remained approximately constant for long periods, but did not appear to be consistently related to any obvious directional cue; changes in direction were observed on occasions, however, and one such change was clearly associated with a change in the direction of the wind. The degree of collective orientation was a rather more variable quantity than the direction, and increases in degree were sometimes associated with increases in the number of locusts arriving at the observation site.