gwd-13-37

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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.

37。According to the passage, before the discovery of new techniques for detecting viral DNA, population ecologists believed that viral diseases

were not widely prevalent among insect populations generally

affected only the caterpillar life stage of lepidoptera

were the driving force behind lepidoptera population cycles

attacked already declining caterpillar populations

infected birds and parasites that prey on various species of lepidoptera

查了前人的帖子， 没有怎么讨论37， 只是说答案是D

但是我认为答案是B， 理由：

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.

wycg

找的地对拉.不过那就是D选项说的意思.

contributed to the decline once it was underway rather than to have initiated it.

>>>建意LZ再把文章看一便.>

chelsea333

是不是

life stage 和 caterpillar populations

delining是两个基本点不同的概念?

xiaoyinihon

This means virus did not initiate(begin) the destruction process by itself. Some agent(reason) started decline process and the. Virus added to the destruction.
Think of an analogous situation: earthquake caused havoc and then the flooding caused plague to add on to the cause