2-9. One authority on spiders made a census of the spiders in grass field in the south of 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 football pitch.
authority：学术权威，专家，泰斗（someone who knows a lot about a subject and whose knowledge and opinions are greatly respected）。某方面的authority需用介词on，
an authority on preventive medicine 预防医学的权威
something like：接近，大约（close to but not exactly a large amount），
He is to sell the house for something like $150,000.
census：统计数量（a process of counting something）。
2-10. Spiders are busy for at least half the year in killing insects.
half（形容词）the year是half（名词）of the year的省略形式。
2-11. It is impossible to make more than the wildest guess at how many they kill, but they are hungry creatures, not content with only three meals a day.
it是形式主语，真正的主语是to make more than the wildest guess at how many they kill。It is impossible to…是重点句型，需熟练掌握。not前省略了and they are。
content：adj. 满意，满足（happy and satisfied）。
2-12. It has been estimated that the weight of all the insects destroyed by spiders in Britain in one year would be greater than the total weight of all the human beings in the country.
it是形式主语，真正的主语是the weight of all the insects destroyed…in the country。It has been estimated that…是重点句型，需熟练掌握。