Recent models from theoretical physics have predicted that mass-migrating animal groups may share group-level properties, irrespective of the type of animals in the group. One key prediction is that as the density of animals in the group increases, a rapid transition occurs from disordered movement of individuals within the group to highly aligned collective motion. Understanding such a transition is crucial to the control of mobile swarming insect pests such as the desert locust. We confirmed the prediction of a rapid transition from disordered to ordered movement and identified a critical density for the onset of coordinated marching in locust nymphs. We also demonstrated a dynamic instability in motion at densities typical of locusts in the field, in which groups can switch direction without external perturbation, potentially facilitating the rapid transfer of directional information.
Desert locusts in the solitarious phase were repeatedly touched on various body regions to identify the site of mechanosensory input that elicits the transition to gregarious phase behavior. The phase state of individual insects was measured after a 4-h period of localized mechanical stimulation, by using a behavioral assay based on multiple logistic regression analysis. A significant switch from solitarious to gregarious behavior occurred when the outer face of a hind femur had been stimulated, but mechanical stimulation of 10 other body regions did not result in significant behavioral change. We conclude that a primary cause of the switch in behavior that seeds the formation of locust swarms is individuals regularly touching others on the hind legs within populations that have become concentrated by the environment.
Central to swarm formation in migratory locusts is a crowding-induced change from a ''solitarious'' to a ''gregarious'' phenotype. This change can occur within the lifetime of a single locust and accrues across generations. It represents an extreme example of phenotypic plasticity. We present computer simulations and a laboratory experiment that show how differences in resource distributions, conspicuous only at small spatial scales, can have significant effects on phase change at the population level; local spatial concentration of resource induces gregarization. Simulations also show that populations inhabiting a locally concentrated resource tend to change phase rapidly and synchronously in response to altered population densities. Our results show why information about the structure of resource at small spatial scales should become key components in monitoring and control strategies.The desert locust, Schistocerca gregaria, is one of the world's most notorious insect pests. Most of the time, it exists at low densities across sub-Saharan Africa and into India. At unpredictable intervals, plagues occur; swarms leave this recession zone and invade neighboring areas of Africa, Asia, and Europe. The crowding-induced phase transition between the ''solitarious'' and ''gregarious'' forms involves a suite of changes in behavior, morphometry, color, development, fecundity, and endocrine physiology (1-3). Gregarious individuals become more active and are attracted, instead of repelled, by other locusts. Most phase characters change between instars or accrue across generations through maternal inheritance (4-7), but a solitarious individual's behavior becomes gregarious after only 4 h of crowding and reverts to solitariousness after only 4 h of reisolation (4,5,8). The detailed time course of behavioral-phase change has been quantified only recently and has important implications for selecting the appropriate temporal and spatial scales required to understand locust swarming.An experimental study has shown that locusts confined in an experimental arena for 8 h are more gregarious if the arena contains a lower density of resource (e.g. food, perches, or favorable microclimatic sites; ref. 8). In the present study, we examine the effects of resource distribution and locust density on gregarization, while keeping the overall density of resource constant. MATERIALS AND METHODSFractal Dimension of Resource. Fractal surfaces used in the experiment and simulation were created on a 64 by 64 grid by using an algorithm with midpoint displacement (9). Food patches were located on the highest 20 points. The fractal dimension of the resulting distribution is then described by using a box-counting algorithm. The arena is divided into boxes of length r grid squares, and the number of boxes containing food patches is counted. This process is carried out four times with the origin of the boxes successively shifted diagonally across one grid square. By using the mean number of nonempty boxes N(r), the fractal dimension is calcula...
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