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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.
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Q34:
Which of the following, if true, would most weaken the author’s conclusion in lines 25-30?
New research reveals that the number of species of birds and parasites that prey on lepidoptera has dropped significantly in recent years.
New experiments in which the habitats of lepidoptera are altered in previously untried ways result in the shortening of lepidoptera population cycles.
Recent experiments have revealed that the nuclear polyhedrosis virus is present in a number of predators and parasites of lepidoptera.
Differences among the habitats of lepidoptera species make it difficult to assess the effects of weather on lepidoptera population cycles.
Viral disease is typically observed in a large proportion of the lepidoptera population.
Answer: c我b
不用看我的答案了但答案我不明白为什么选c 。 我就不会作阅读里的逻辑，每做每错。  请高手给些指点，为什么一作阅读逻辑就错？？  | 
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发表于 2004-6-5 19:45:00
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