LOCUST AND GRASSHOPPER PROBLEM 385 
of the area lies in the fact that outbreaks of grasshoppers are 
usually associated with a series of dry years. The effects of dry- 
ness in favoring grasshoppers are, naturally, greater in an envi- 
ronment which is partly arid naturally, or made so by man. A 
feature of mosaic habitats, particularly those created by man, is 
their instability, as the relative extent of their arid and more 
humid parts is likely to vary from year to year. This has an 1m- 
portant effect on grasshoppers, which are very mobile insects. 
Even in most equable conditions, they move from the food-shel- 
ter habitat to the oviposition sites, but when the contrasts are 
very strong such movements extend and become migrations, 
leading to a concentration of the insects in crops. 
The result of migrations is especially striking in the case of 
locusts. A relatively favorable season may cause a great increase 
in a local population of locusts; if this is followed by drought, 
this population would move and concentrate in the most favor- 
able places. This creates crowding, to which locusts respond in a 
most characteristic way—they acquire gregarious habits and 
travel in dense masses. As the direction of flight of locust swarms 
is largely dependent on winds, the swarms arising in one area 
may travel great distances and invade fertile lands far from their 
birthplace. An extreme case of this kind is offered by the desert 
locust of Africa and western Asia. The home of this species is in 
deserts which are generally extremely arid, but are liable to lo- 
calized rainstorms. Such rains bring forth abundant ephemeral 
vegetation, on which locusts can multiply rapidly. In a few weeks, 
however, the plants wither away and the locusts, if they have had 
time to grow up, must migrate elsewhere or perish. 
Swarm flights follow seasonal winds and generally end in an 
area where such winds bring rain, so that locusts can produce a 
new generation many hundreds of miles away from the previous 
one. The survival of such nomadic insects is clearly dependent 
on the chance of swarms reaching an area where they can feed 
and reproduce. In this respect also, man is already beginning to 
make their life less hazardous; some areas in the Sudan and 
Arabia, where natural vegetation is green for only a few months 
in the year and that only if rain happens to fall, have been irri- 
