Unit 1The Perfect SwarmNarrator:Damage from swarms of locusts can reach disastrous proportions. A single swarmof desert locusts can consume over 70,000 metric tons of vegetation a day. There is, however,one continent that’s locust-free: North America.旁白：蝗虫群的破坏可以达到灾难性的程度。一堆沙漠蝗虫每天可以消耗超过70000 公吨的植被。然而，有一个大陆没有蝗虫：美国北部。Interestingly enough, this wasn’t always true. For hundreds of years, the Rocky Mountain locustwas a common pest in the American West. Back in the mid-1800s, thousands of pioneersjourneyed across the U.S. in search of free land and new opportunities. They settled on thefrontier of the western states, and began to farm the land intensively, growing corn and othercrops.有趣的是，这并不总是真的。几百年来，落基山蝗虫是美国西部常见的害虫。早在19 世纪中期，成千上万的拓荒者就穿越了这座城市。 美国在寻找自由的土地和新的机会。他们定居在西部各州的边境，开始集中耕种土地，种植玉米和其他作物。Then, in 1875, out of nowhere, a rare combination of air currents, drought, and basic biologyproduced the right conditions for an unthinkable event, the worst storm ever recorded, the“perfect swarm.” It came over the horizon like a strange, dark cloud. Not millions, not billions, buttrillions of insects, sweeping through the land like a living tornado. Those who saw the incredibleevent and survived never forgot what they witnessed.然后，1875 年，突然间，罕见的气流、干旱和基本生物学的结合为一场不可想象的事件创造了合适的条件，这是有史以来最严重的风暴“完美群”。 “它像一朵奇怪的乌云般从地平线上掠过。不是几百万，不是数十亿，而是数万亿的昆虫，像活生生的龙卷风一样横扫大地。那些看到了令人难以置信的事件并幸存下来的人从来没有忘记他们见证了什么。The swarm came together over the state of Texas, and soon moved quickly across the frontier in ahuge destru...
