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GRE阅读题目解析:舞毒蛾幼虫

2017-07-06 09:09:00来源：网络

　　【GRE阅读题目解析:舞毒蛾幼虫】GRE考试考什么？GRE阅读是重点。新东方在线GRE频道为大家带来GRE阅读题目解析:舞毒蛾幼虫，答案见下一页，希望对大家GRE备考有所帮助。更多精彩尽请关注新东方在线GRE频道!

　　查看全部：GRE阅读答案解析(汇总)

　　GRE阅读题目解析:舞毒蛾幼虫

　　P2

　　The defoliation of millions of acres of trees by massive infestations of gypsy moth caterpillars is a recurring phenomenon in the northeastern United States. In studying these outbreaks, scientists have discovered that affected trees fight back by releasing toxic chemicals, mainly phenols, into their foliage. These noxious substances limit caterpillars' growth and reduce the number of eggs that female moths lay. Phenols also make the eggs smaller, which reduces the growth of the following year's caterpillars. Because the number of eggs a female moth produces is directly related to her size, and because her size is determined entirely by her feeding success as a caterpillar, the trees' defensive mechanism has an impact on moth fecundity.

　　The gypsy moth is also subject to attack by the nucleopolyhedrosis virus, or wilt disease, a particularly important killer of the caterpillars in outbreak years. Caterpillars contract wilt disease when they eat a leaf to which the virus, encased in a protein globule, has become attached. Once ingested by a caterpillar, the protein globule dissolves, releasing thousands of viruses, or virions, that after about two weeks multiply enough to fill the entire body cavity. When the caterpillar dies, the virions are released to the outside, encased in a new protein globule synthesized from the caterpillar's tissues and ready to be picked up by other caterpillars.

　　Knowing that phenols, including tannins, often act by associating with and altering the activity of proteins, researchers focused on the effects on caterpillars of ingesting the virus and leaves together. They found that on tannin-rich oak leaves, the virus is considerably less effective at killing caterpillars than when it is on aspen leaves, which are lower in phenols. In general, the more concentrated the phenols in tree leaves, the less deadly the virus. Thus, while highly concentrated phenols in tree leaves reduce the caterpillar population by limiting the size of caterpillars and, consequently, the size of the female's egg cluster, these same chemicals also help caterpillars survive by disabling the wilt virus. Forest stands of red oaks, with their tannin-rich foliage, may even provide caterpillars with safe havens from disease. In stands dominated by trees such as aspen, however, incipient gypsy moth outbreaks are quickly suppressed by viral epidemics.

　　Further research has shown that caterpillars become virtually immune to the wilt virus as the trees on which they feed respond to increasing defoliation. The trees' own defenses raise the threshold of caterpillar vulnerability to the disease, allowing populations to grow denser without becoming more susceptible to infection. For these reasons, the benefits to the caterpillars of ingesting phenols appear to outweigh the costs. Given the presence of the virus, the trees' defensive tactic apparently has backfired. (446 words)

　　2. It can be inferred from the passage that wilt disease virions depend for their survival on

　　(A) protein synthesized from the tissues of a host caterpillar

　　(B) aspen leaves with high concentrations of phenols

　　(C) tannin-rich oak leaves

　　(D) nutrients that they synthesize from gypsy moth egg clusters

　　(E) a rising threshold of caterpillar vulnerability to wilt disease

　　3.Which of the following best describes the function of the third paragraph of the passage?

　　(A) It resolves a contradiction between the ideas presented in the first and second paragraphs.

　　(B) It introduces research data to support the theory outlined in the second paragraph.

　　(C) It draws a conclusion from conflicting evidence presented in the first two paragraphs.

　　(D) It shows how phenomena described in the first and second paragraphs act in combination.

　　(E) It elaborates on the thesis introduced in the first paragraph after a digression in the second paragraph.

　　4. Select the sentence in the passage that the author uses as a supporting idea to explicate how gypsy moth caterpillars become immune to the wilt virus.

　　For the following question, consider each of the choices separately and select all that apply

　　5.Which of the following statements about gypsy moth caterpillars is supported by information presented in the passage EXCEPT?

　　A Differing concentrations of phenols in leaves have differing effects on the ability of the wilt virus to kill gypsy moth caterpillars.

　　B Female gypsy moth caterpillars stop growing after they ingest leaves containing phenols.

　　C The longer a gypsy moth population is exposed to wilt disease, the greater the likelihood that the gypsy moth caterpillars will become immune to the virus.

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