Here in the United States, before agricultural activities destroyed the natural balance, t

here were great migrations of Rocky Mountain locusts (Melanoplus spretus). Great migrating hordes of these insects once darkened the skies on the plains east of the Rockies where crops were often destroyed; the worst years were those from 1874 to 1877. One of these migrating swarms was estimated to contain 124 billion locusts. During another migration in Nebraska it was estimated that the swarm of locusts averaged half a mile high and was 100 miles wide and 300 miles long. Usually, these swarms take off from the ground against the wind, but, once airborne, they turn and fly with it. Warm convection (对流) currents help to lift them, often to great heights. During the great locust plagues the situation in Nebraska became so serious that the original state constitution had to be rewritten to take care of the economic problems. The new 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, which is still common there. In this respect, the North American migratory locusts resemble their African relatives. In both regions the migratory forms arise as a result of crowding and climatic factors. Migratory forms are apparently natural adaptations which bring about dispersal when locust populations become too crowded. Fortunately for our farmers, the migratory form—the so called spretus species—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 should become favorable.

Which of the following is the best title for the passage？

A．The Life Cycle of Locusts.

B．Migratory Locusts in the United States.

C．Locust Plagues in Nebraska.

D．The Reproductive Capability of the Locust.