The Gregarious Behaviour of MarchingLocusta Migratoria Migratorioides(R. & F.) Hoppers

Abstract

1. The amount of marching by 4th-instar hoppers of Locusta migratoria migratorioides (R. & F.) which have been reared in crowds, was studied in relation to the total number of individuals and to the size of the cage. 2. Below 50 hoppers per cage, marching depended partly on the total number of hoppers present and partly on the number in relation to the area of the cage floor. Above 50 hoppers per cage, marching probably depended on the total number present, rather than the area of the cage floor. 3. In all cages, marching was greatly reduced with fewer than 30 hoppers, although even single individuals marched for part of the time. 4. An investigation of the hopper interactions which lead to an increase in marching activity in groups showed that the two most important were visual and mechanical ones. 5. Test hoppers followed moving hopper bodies alive or dead, but only gave a partial response to moving vertical stripes. 6. Mechanical interactions between hoppers greatly increased the proportion of reacting hoppers, but did not increase the distance travelled by the active individuals, beyond those travelled in response to optical stimuli alone. 7. Maximal marching activity, by the maximal proportion of hoppers in a band, depended on an opto-motor response to a moving background of hopper shapes and to the physical contact between hoppers. 8. Air and floor vibrations from other marching hoppers played only a small part in marching, whilst stimuli received via the antennae tended to inhibit marching when total activity was low. 9. The importance of the experimental results in explaining the behaviour of locust hoppers in the field was discussed.