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  3. 1 The number of insect species on Earth exceeds that of all other life forms combined. Entomologists estimate that there are wel

1 The number of insect species on Earth exceeds that of all other life forms combined. Entomologists estimate that there are wel

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问题 1 The number of insect species on Earth exceeds that of all other life forms combined. Entomologists estimate that there are well over one million different known species of insects, or possibly as many as 10 million. Insects live in almost every habitat on land and in fresh water, as well as in the air. Even in the frozen extremes of the Arctic and Antarctic some insects are active during the summer months. The oldest insect fossils date from the Devonian period, which began 400 million years ago. Later, when insect flight evolved during the Carboniferous and Permian periods, there was a rapid expansion in insect variety.
2 The adult insect body has three pairs of walking legs and a body segmented into three regions: head, thorax, and abdomen. Many species have one or two pairs of wings emerging from the thorax. The wings are not true legs; therefore, insects can fly without sacrificing any walking legs. In contrast, the wings of birds are one of their two pairs of walking legs modified for flight. Consequently, birds are relatively clumsy on the ground.
3 The evolution of flight is one key to the tremendous success of insects. Flying enables insects to escape from predators, find food and mates, and move quickly to new habitats. Some entomologists believe that wings first evolved to help the insect body absorb heat and later became organs for flight. Others think that wings allowed the insects to glide from plants to the ground, or possibly served as gills for aquatic insects to breathe. Still others hypothesize that wings first evolved not for flying but for swimming. Insects called stoneflies still use their tiny wings to skip across the surface of streams. The fossil record shows that dragonflies, with two pairs of wings, were among the first insects to fly. Several insects that evolved later have modified flight equipment. For example, bees and wasps hook their wings together and move them as a single pair. Beetles have rear wings that are used for flight and front wings modified as covers that protect the flight wings when the beetle is on the ground or burrowing.
4 Because insects are so numerous, diverse, and widespread, they affect the lives of all other terrestrial organisms, including humans. A On the one hand, insects have an essential role in pollinating many of our crops and orchards. B On the other hand, they are carriers for many diseases, including malaria, Lyme disease, and African sleeping sickness. Insects that eat plant leaves have defoliated forests, killed trees, and created nuisances in urban areas. C In some parts of the world, insects claim over half of the grain crops. D In the United States, farmers who try to minimize their losses spend billions of dollars each year on pesticides, spraying crops with massive doses of poison.
5 Despite all of our efforts, we are not able to challenge the preeminence of insects on Earth. In fact, global warming will lead to an increase in the number of insects worldwide, with serious consequences for humans. New research shows that insect species in warmer regions are more likely to undergo rapid population growth because they have high metabolic rates and they reproduce frequently. A surge in populations of beetles, locusts, and other crop eaters will require the use of more pesticides, a solution that will be not only very costly but also hazardous to other organisms. Rising temperatures will cause a widening of areas habitable for mosquitoes, and a surge in mosquito populations will mean more infections of mosquito-borne diseases such as malaria. Researchers have already observed that new cases of malaria have appeared in previously unaffected areas.
Look at the four squares, A, B, C, and D, which indicate where the following sentence could be added to paragraph 4. Where would the sentence best fit? Moreover, insects that eat crops compete with us for food.
选项 A、 
B、 
C、 
D、 
答案C
解析 In the added sentence, moreover is a transition showing addition between insects that eat plant leaves in the previous sentence and insects that eat crops in the added sentence. The following sentence further develops the topic of crops by stating that in some parts of the world, insects claim over half of the grain crops.
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