The migration of the Desert Locust (Schistocerca gregariaForsk.) I. The behaviour of swarms. II. A theory of long-range migrations

Abstract

Part I. The behaviour of swarms Summary New evidence is given on the behaviour of migrating Desert Locust swarms, based upon some sixty encounters by various observers with swarms in Iran in 1943 and in Kenya in 1944. After describing the process by which a swarm launches on migration in the morning, the observations are used to illustrate the separate influence of locust condition, wind, gregariousness, the sun and landscape on the behaviour of swarms. Migratory activity increased gradually after the last moult, but the tendency to fly reached full development when hardening of the cuticle was still not quite complete, whereas flight vigour, as shown by the ability to resist carriage by the wind, did not reach full development till later. Migratory activity became reduced again around the time of mating and egg-laying but continued until death. Special attention was devoted to behaviour in relation to wind, with recording of wind speed and height of flight as well as wind direction and the courses and tracks of the flying locusts. Close, continuous observations of single swarms and the numerical data accumulated suggested that low fliers tend to stabilize their ground speed by varying their air speed according to the wind speed, and showed that they tend to come down and even settle when the wind is too strong for them to make headway against it, to take off and rise in the air when the wind speed is lower, to orient into the wind as long as they can make headway but to turn aside and commonly to turn right round and fly with the wind when they can no longer make headway and do not settle. Rather higher fliers behaved in a parallel manner, but were distinctly less responsive to wind changes than lower fliers. When parts of the same swarm were observed over land and over water, the latter failed to respond to wind changes when the former did so. Mutual stimulation among swarm members was observed to reinforce flight activity, to make the locusts keep together and fly in parallel, thereby stabilizing the direction of the swarm as a whole (gregarious inertia), and to accelerate mass changes of direction initiated by external agencies such as wind. There was no consistent relation between the direction of the sun’s rays and the locusts’ orientation, but there was evidently some direction-stabilizing agency at work other than gregarious inertia, and reasons are given for supposing this to be a light-compass-reaction to the sun. Evidence was obtained that the avoidance of upstanding obstacles is merely a special case of a general effect, a ‘compensatory’ change of direction being caused by any gross movement across the eyes. This type of optomotor reaction is regarded as contributing to the observed tendency to fly along lanes, through trees, along shore-lines and valleys. There follows a critical review and further analysis of the foregoing and previously published observations, temperature and moisture being considered as well as the influences already mentioned. The extremely confused picture presented by the data accumulated hitherto on flight direction in relation to wind direction, is attributed to a tendency to seek some universal relationship, without sufficient regard to the varying circumstances at the time of observation, especially the wind speed and the height of flight. When these are taken into account, there are indications of a regular system of relations between flight direction and wind direction. The wind appears to be a most important direction-determining agency which acts, not only by carrying the insects, but through the optomotor reactions of the insects to the apparent movement of the visible substrate. Migratory flight occurs as the normal, ‘spontaneous’ activity of gregarious locusts when at or near their preferred temperature. The suddenness of themass departureof a swarm in the morning is attributed primarily to gregariousness. As flight activity increases with rising temperature, mutual stimulation among the locusts also increases and finally transforms the disorderly mob of insects into a coherent swarm, for the first time able, and now obliged to move offen bloc. Finally, all the available information is used in an attempt to distinguish what aspects of its behaviour are peculiar to the migrant locust, hopper or adult, and to find their underlying cause. The primary characteristic of the migrant is its locomotion, which is carried on with extraordinary persistency through each day and day after day, while other forms of activity are relegated for the time being to a quite subsidiary role. The tendency to go straight has often been regarded as an equally fundamental characteristic of large insect migrants, but it appears rather to be secondary, an outcome of the locomotory ‘drive’. Locust migrants have no ‘goal’ and no special 'directional sense’. Their orientations are governed not by any single one but by a number of reactions, mainly of the optomotor type, to fellow-individuals, the sun, the wind and to other gross features of their everchanging environment. What is peculiar about the migrant in this connexion is the unfamiliarpatternin which familiar reactions are arranged, resulting in some changes of direction, but also in an unusual amount of moving steadily in one direction. This peculiar pattern seems to be due to the combination of the migrant’s peculiar neuro-physiological state in which reactions to stimuli which would arrest it are suppressed, with the peculiar pattern of its sensory experience during continuous locomotion. Thus external features which are the most constant concomitants of the moving insect, such as wind and sun and fellow-locusts, become especially effective as sources of stimulation when their relation to the insect is changed by them or by it, its reactions to them having thus a certain stabilizing influence on its direction....