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Q34 to Q37: A small number of the forest species of lepidoptera (moths and butterflies, which exist as caterpillars Line during most of their life cycle) exhibit (5) regularly recurring patterns of popu- lation growth and decline—such fluctuations in population are known as population cycles. Although many different variables influence popula- (10) tion levels, a regular pattern such as a population cycle seems to imply a dominant, driving force. Identification of that driving force, however, has proved surprisingly elusive despite (15) considerable research. The com- mon approach of studying causes of population cycles by measuring the mortality caused by different agents, such as predatory birds or parasites, (20) has been unproductive in the case of lepidoptera. Moreover, population ecologists’ attempts to alter cycles by changing the caterpillars’ habitat and by reducing caterpillar popula- (25) tions have not succeeded. In short, the evidence implies that these insect populations, if not self-regulating, may at least be regulated by an agent more intimately connected with the insect than (30) are predatory birds or parasites. Recent work suggests that this agent may be a virus. For many years, viral disease had been reported in declining populations (35)
of caterpillars, but population ecolo- gists had usually considered viral disease to have contributed to the decline once it was underway rather than to have initiated it. The recent (40) work has been made possible by new techniques of molecular biology that allow viral DNA to be detected at low concentrations in the environ- ment. Nuclear polyhedrosis viruses (45) are hypothesized to be the driving force behind population cycles in lepidoptera in part because the viruses themselves follow an infec- tious cycle in which, if protected from (50) direct sun light, they may remain virulent for many years in the envi- ronment, embedded in durable crystals of polyhedrin protein. Once ingested by a caterpillar, (55) the crystals dissolve, releasing the virus to infect the insect’s cells. Late in the course of the infection, millions of new virus particles are formed and enclosed in polyhedrin (60) crystals. These crystals reenter the environment after the insect dies and decomposes, thus becoming avail- able to infect other caterpillars. One of the attractions of this (65) hypothesis is its broad applicability. Remarkably, despite significant differ- ences in habitat and behavior, many species of lepidoptera have population cycles of similar length, between eight (70) and eleven years. Nuclear polyhe- drosis viral infection is one factor these disparate species share. Q34: Which of the following, if true, would most weaken the author’s conclusion in lines 25-30? A. New research reveals that the number of species of birds and parasites that prey on lepidoptera has dropped significantly in recent years. B. New experiments in which the habitats of lepidoptera are altered in previously untried ways result in the shortening of lepidoptera population cycles. C. Recent experiments have revealed that the nuclear polyhedrosis virus is present in a number of predators and parasites of lepidoptera. D. Differences among the habitats of lepidoptera species make it difficult to assess the effects of weather on lepidoptera population cycles. E. Viral disease is typically observed in a large proportion of the lepidoptera population. Answer: b WHY B? Answer: b WHY B?
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发表于 2006-7-17 10:11:00
