2016-05-18 11:10:08 来源:新东方在线托福资料下载
5月22日托福阅读真题第二篇
题材划分: 生物类
主要内容:该篇文章主要讨论昆虫翅膀的进化,提出第一种理论:翅膀是由审题中的一个部位进化而来的。第二种理论:是由腿部中的肌肉组织进化而来的。提供了各种论据然后又拓展到水里的某种生物,说这种生物的翅膀是怎样一步一步进化来的,进而支撑第二种理论。最后说虽然翅膀的进化比较有争议,但是捕食者是最有可能导致翅膀进化的原因。
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TPO25- The Evolutionary Origin of Plants
相关知识背景:
Insect wings are adult outgrowths of the insect exoskeleton that enable insects to fly. They are found on the second and third thoracic segments (the mesothorax and metathorax), and the two pairs are often referred to as the forewings and hindwings, respectively, though a few insects lack hindwings, even rudiments. The wings are strengthened by a number of longitudinal veins, which often have cross-connections that form closed "cells" in the membrane (extreme examples include Odonata and Neuroptera). The patterns resulting from the fusion and cross-connection of the wing veins are often diagnostic for different evolutionary lineages and can be used for identification to the family or even genus level in many orders of insects.
The physical dynamics of flight are composed of direct and indirect flight. Those species that employ direct flight have wing muscles directly attached to the wing base, so that a small downward movement of the wing base lifts the wing itself upward. However, insects with indirect flight have muscles that attach to the thorax and deform it; since the wings are extensions of the thoracic exoskeleton, the deformations of the thorax cause the wings to move as well.
The wings may be present in only one sex (often the male) in some groups such as velvet ants and Strepsiptera, or selectively lost in "workers" of social insects such as ants and termites. Rarely, the female is winged but the male not, as in fig wasps. In some cases, wings are produced only at particular times in the life cycle, such as in the dispersal phase of aphids. Beyond the mere presence/absence of wings, the structure and colouration will often vary with morphs, such as in the aphids, migratory phases of locusts and in polymorphic butterflies.
At rest, the wings may be held flat, or folded a number of times along specific patterns; most typically, it is the hindwings which are folded, but in a very few groups such as vespid wasps, it is the forewings. How and why insect wings evolved is not well understood. Three main theories on the origins of insect flight are that wings developed from paranotal lobes, extensions of the thoracic terga; that they are modifications of movable abdominal gills as found on aquatic naiads of mayflies; and that insect wings arose from the fusion of pre-existing endite and exite structures each with pre-existing articulation and tracheation.
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