Here in the United States, before agricultural activities destroyed the natural balance, there were great migrations of Rocky Mountain locusts(蝗虫). Great migrating hordes of these insectsonce darken the skies on the plains east of the Rockies where crops 1were often destroyed; the worst years were those between 1874 to 21877. One of these migrating swarms was estimated to contain 1. 24billion locusts. During the another migration in Nebraska it was 3estimated that the swarm of locusts averaged half a mile high and was 100 miles wide and 300 miles long. Usually, these swarms takeup from the ground against the wind, but, once airborne, they turn 4and fly with it. Warm convection currents(对流)help to lift them,often at great heights. During the great locust plagues the situation 5in Nebraska became so serious that the original state constitutionhad to be rewritten to take care the economic problems. The new 6document was known as "The Grasshopper Constitution". It is now believed that these locusts were a migratory form or phase of thelesser migratory locust, that is still common there. In this respect, 7the North American migratory locusts resemble their Africanrelatives. In both regions the migratory forms rise as a result of 8crowding and climatic factors. Migratory forms are apparently natural adaptations which bring about dispersal when locustpopulations become too crowded. Unfortunately for our farmers, the 9migratory form no longer seems to occur regularly, although there was a serious outbreak as late as 1938 in midwestern United States and Canada. Actually, there is no reason why the destructivemigratoiy form might not again appear if circumstances become 10favorable.