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雅思考试 雅思阅读真经介绍(二)
2012-10-22     美国留学网(www.usaliuxue.com)  发布者:博文留学



  You should spend about 20 minutes on Questions 14 – 26 which are based on Reading Passage 2 below.


  Questions 14 - 18


  Reading Passage 2 has 9 paragraphs A – I

  From the list of headings below choose the 5 most suitable headings for paragraphs B, C, E, G and H. Write the appropriate numbers (ⅰ – ⅹ)

  NB There are more headings than paragraphs, so you will not use them all.


  14. Paragraph B

  15. Paragraph C

  16. Paragraph E

  17. Paragraph G

  18. Paragraph H


  Evolution of Insect Flight


  A. Pterosaurs, birds and bats took to the air from evolutionary runways that scientists believe they understand fairly well, but insects began flying so much longer ago that details of their stepwise conquest of flight remain obscure. Scientists at Pennsylvania State University hypothesize, however, that a present-day flightless insect called the stonefly may be closely related to ancestral insects that first learned to fly more than 330 million years ago.


  B. Last February, Dr. James H. Marden, a biologist at Pennsylvania State University, and Melissa G. Kramer, his student, began studying the behavior and biology of stoneflies - the immature nymphs of which are familiar to many fishermen as delicacies for trout. The nymphs begin life in river or pond water and then develop primitive wings enabling them to skim across water at high speed without actually taking to the air. Marden and Ms. Kramer have concluded that the humble ancestor of such expert fliers as mosquitoes and wasps may have been very much like the stonefly.


  C. The stoneflies living in Canada and the northern United States, which belong to a primitive species called Taeniopteryx burksi, breed and mature in cold water and come to the surface for their skimming trip to shore in February and March. To study them, a scientist must work quickly, since the life span of a stonefly is only about two weeks. The adult stonefly has waterproof hair on its feet, and after reaching the surface of the water, it supports itself by coasting on the water's surface meniscus layer. To hasten its trip to the shore, the insect spreads its four feeble wings and flaps vigorously, using aerodynamic thrust to scoot across the water at speeds up to 2 feet per second. This, Marden said, appears to be the only time in its life the stonefly normally uses its wings.


  D. In a series of experiments Marden described in a report published in the current issue of the journal Science, he found that although stoneflies in the wild, where ambient temperatures were recorded as ranging between 32 degrees and 53.6 degrees Fahrenheit, are completely flightless, their flying ability improves when they are warmed up in a laboratory. Even when warm, the insects never voluntarily take flight from a horizontal surface, but if they crawl to the edge of a table and drop over the side they will fly for a few yards before settling to the ground. Several specimens tested by the Penn State scientists actually gained a little altitude under their own power after being launched by hand, but none remained in the air for more than a few seconds.


  E. Stoneflies are interesting, Marden said in an interview, because so little is known of the specific changes insects underwent in the remote past as they gained the ability to fly. The stonefly's faltering efforts to use its wings may approximate a transitional stage of evolution that occurred some 350 million years ago, when swimming insects first became fliers.


  F. The study of insect evolution is hampered by a gigantic gap in the fossil record. Although fossils of early nonflying insects have been found in sediments dating from the Devonian period nearly 400 million years ago, no insect fossils have turned up from the following 75-million-year period. Marden said that fossil insects reappear in strata 325 million years old, but by then they had evolved greatly, and their increased diversity suggests that at least some species had left the water to colonize land. Many of the fossils of that period look like present-day insects, including grasshoppers.


  G. Stoneflies lack some features that are important for true fliers, They have relatively weak wing muscles, and their thoracic cuticle plates are not fused together to create a rigid external skeleton. Rigidity is needed to provide strong, inflexible attachment points for an insect's wing muscles if it is to be capable of powered flight - a much more demanding activity than skimming or gliding. If the stonefly is similar to the first protofliers, this would argue against a widely held hypothesis that animal flight begins with gliding, from which powered flight eventually develops. Stoneflies never glide, even though they are on the verge of flying.


  H. Although the stonefly may have evolved to its present form in a progressive direction from primitive swimming insects, it is possible, Marden believes, that its evolution was digressive - that its ancestors were true fliers that evolved into nonflying skimmers. Skimming requires much less energy than true flight, as demonstrated by a new family of skimming "wing-in-ground-effect" flightless aircraft developed during the last decade in Russia, China and Germany. These aircraft never rise more than a few feet above the ground or water, but their stubby wings support them on an air cushion that eliminates the drag of surface friction.


  I. "Stoneflies seem to have found an ecological niche in any case," Marden said. Whether the evolutionary pathway of the stonefly was progressive or digressive makes little difference to the insect, he said, but to an entomologist, the direction is important. "By mapping behavioral characters and morphology 1 of stoneflies, we hope eventually to infer the direction by which evolution carried them to their present stage of development," Marden said.



  1 morphology The branch of biology that deals with the form and structure of organisms


  Questions 19 – 22

  Using NO MORE THAN THREE WORDS from the passage, answer the following questions.

  19. How long ago did stoneflies first use their wings?

  20. How wide is the fossil gap?

  21. Where is the only place that stoneflies actually fly?

  22. What time of the year do stoneflies use their wings?


  Questions 23



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