to each other, this being seen as early as the second instar. They soon form bands of many thousands of individuals. These groups behave like cohesive units and move across the landscape, mostly downhill, but making their way around barriers and merging with other bands. The attraction between the insects involves visual and olfactory cues. The bands seem to navigate using the sun. They pause to feed at intervals before continuing on, and may cover tens of kilometres over a few weeks. Also, differences in morphology and development are seen. In the desert locust and the migratory locust, for example, the gregaria nymphs become darker with strongly contrasting yellow and black markings, they grow larger, and have a longer nymphal period; the adults are larger with different body proportions, less sexual dimorphism, and higher metabolic rates; they mature more rapidly and start reproducing earlier, but have lower levels of fecundity. The mutual attraction between individual insects continues into adulthood, and they continue to act as a cohesive group. Individuals that get detached from a swarm fly back into the mass. Others that get left behind after feeding take off to rejoin the swarm when it passes overhead. When individuals at the front of the swarm settle to feed, others fly past overhead and settle in their turn, the whole swarm acting like a rolling unit with an ever-changing leading edge. The locusts spend much time on the ground feeding and resting, moving on when the vegetation is exhausted. They may then fly a considerable distance before settling in a location where transitory rainfall has caused a green flush of new growth. Distribution and diversity Several species of grasshoppers swarm as locusts in different parts of the world, on all continents except Antarctica: For example, the Australian plague locust (Chortoicetes terminifera) swarms across Australia. The desert locust (Schistocerca gregaria) is probably the best known species owing to its wide distribution (North Africa, Middle East, and Indian subcontinent) and its ability to migrate over long distances. A major infestation covered much of western Africa from 2003 to 2005, after unusually heavy rain set up favourable ecological conditions for swarming. The first outbreaks occurred in Mauritania, Mali, Niger, and Sudan in 2003. The rain allowed swarms to develop and move north to Morocco and Algeria, threatening croplands. Swarms crossed Africa, appearing in Egypt, Jordan and Israel, the first time in those countries for