Although the word locust usually calls to mind plagues of biblical proportions, these insects actually start out as shy, solitary creatures. Yet within a matter of hours they can transform into the farmers worst nightmare, joining forces to create a voracious, migrating swarm. Researchers have known for some time that swarm formation results when the locusts become crowded. But exactly what prompts the transition to gregarious conduct has remained unclear. Now new research, described today in the Proceedings of the National Academy of Sciences, indicates that stimulation of the locusts hind legs sparks the behavioral revolution.

To discern which part of the locusts body holds the key to transformation, researchers at the University of Oxford stimulated various body regions of 170 locusts that had been raised in isolation. After the four-hour treatment period, each insect was placed in an observation arenaone end of which contained a group of gregarious locusts behind a clear partitionand their behavior was recorded. The team found that only stroking the locusts back leg evoked a statistically significant behavioral shift.

Why the back leg? The researchers suggest that whereas the locust might self-stimulate other partssuch as its face, antennae and abdomen during feeding, grooming and walkingit would not normally touch its own hind legs. "Our results provide an explanation for field observations that a population of solitarious locusts is more likely to gregarize in vegetation consisting of compact clumps than where vegetation is spread out evenly but sparsely," the team writes, noting that once the habitat brings locusts closer together, they are more likely to make contact. "As they bump into others while moving within and between the resource sites, the process of behavioral gregarization is initiated, leading to the formation of local aggregations, which may in turn seed large-scale swarms."