ВУЗ:
Составители:
Рубрика:
IV. Suggest the English for:
составлять значительную часть; культурный картофель; кроме того,
экономическая важность; подобным образом (так же); личинка фруктовой
мухи; экономическая роль; быть не в состоянии сделать что-либо; первые
шаги; основная составная часть.
V. Find topical sentences in each paragraph of the text, make up a model of its
information and sum it up.
VI. Define which details of the text are of particular importance for
characterizing the nutrition of insects.
Text 18
I. Read the text and state what it is about.
THE NATURE 0f METAMORPHOSIS
There are two ways of regarding metamorphosis.
(I) It may be supposed that the insect is undergoing a progressive
development towards the adult forms that it is subject during its larval stages to
some inhibition which prevents the completion of this development until it is
fully grown; that the restraint is then removed and differentiation la completed.
(II) Alternatively, it may be supposed that metamorphosis involves a switch
into a line of development which la qualitatively different from that of the larva.
In other words, that a new system of genes, previously latent in the
chromosomes, la brought into action at a given point in development and it la -
this system that is responsible for the control of the adult form. According to
.this conception, the origin of metamorphosis has consisted In the independent
evolution of two genetic systems, one controlling larval form and the other
controlling adult form. The successive stages of the insect are thus comparable
with the different forms in a polymorphic species, or with the different forms of
the parts of the body, which are likewise believed to result from the activation of
different sets of genes.
Hormonal differences certainly exist in the blood at metamorphosis, and
these are responsible for the morphological changes. If young larvae of
Rhodnius are decapitated and transfused with the blood of 5th-stage larvae,
which are In process of moulting to become adult, they undergo a precocious
metamorphosis. Even let-stage larvae recently emerged from the egg, if treated
in this way, will develop the cuticle and pigmentation rudimentory wings,
abdominal structure and genitalia characteristic of the adult. Pieces of the
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