填空题. 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 insects once darken the skies on the plains east of the Rockies where crops 21 were often destroyed; the worst years were those between 1874 to 22 1877. One of these migrating swarms was estimated to contain 1.24 billion locusts. During the another migration in Nebraska it was 23 estimated that the swarm of locusts averaged half a mile high and was 100 miles wide and 300 miles long. Usually, these swarms take up from the ground against the wind, but, once airborne, they turn 24 and fly with it. Warm convection currents (对流) help to lift them, often at great heights. During the great locust plagues the situation 25 in Nebraska became so serious that the original state constitution had to be rewritten to take care the economic problems. The new 26 document was known as "The Grasshopper Constitution". It is now believed that these locusts were a migratory form or phase of the lesser migratory locust, that is still common there. In this respect, 27 the North American migratory locusts resemble their African relatives. In both regions the migratory forms rise as a result of 28 crowding and climatic factors. Migratory forms are apparently natural adaptations which bring about dispersal when locust populations become too crowded. Unfortunately for our farmers, the 29 migratory 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 destructive migratory form might not again appear if circumstances become 30 favorable.