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.

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.

thanks a lot !!!