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Lesson 2 Spare that spider不要伤害蜘蛛 Why, you may wonder, should spiders be our friends? Because theydestroy so many insects, and insects include some of the greatest enemies ofthe human race.Insects would make it impossible for us to live in the world;they would devour all our crops and kill our flocks and herds, if it were notfor the protection we get from insect-eating animals.We owe a lot to the birdsand beasts who eat insects but all of them put together kill only a fraction ofthe number destroyed by spiders.Moreover, unlike some of the other insecteaters, spiders never do the least harm to us or our belongings. Spiders are not insects, as many people think, nor even nearlyrelated to them.One can tell the difference almost at a glance, for a spideralways has eight legs and an insect never more than six. How many spiders are engaged in this work on our behalf? Oneauthority on spiders made a census of the spiders in a grass field in the southof England, and he estimated that there were more than 2,250,000 in one acre;that is something like 6,000,000 spiders of different kinds on a footballpitch.Spiders are busy for at least half the year in killing insects.It isimpossible to make more than the wildest guess at how many they kill, but theyare hungry creatures, not content with only three meals a day.It has beenestimated that the weight of all the insects destroyed by spiders in Britain inone year would be greater than the total weight of all the human beings in thecountry. | 
