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Migration of Monarch Butterflies
One of the greatest mysteries in biology —how the monarch butterfly travels thousands of
kilometers on its yearly migration —has just been solved.
(1)No other butterfly in the world migrates like the orange and black monarchs of North
America.(2)Each autumn,thousands travel up to 3,000 km to spend the winter in the mountains of Mexico.There are so many,they almost block out the sky,and you can hear their wings beating.
It usually takes the butterflies two months to reach Mexico.After staying five months,they head back up north in the spring.But not one butterfly finishes the whole round-trip.(3)Three generations of butterflies will live and die during the spring journey alone.The fourth and final generation of the year is born in early autumn and will reach the north.But it hardly gets to rest before starting the long journey south to Mexico.
The most amazing thing about these butterflies is that they return to the very same tree in Mexico that their great-great-grandparents used the winter before.Past studies have shown that the insects use the sun as a compass to show them the way.(4)This new study now shows that the insects use a "circadian(生理节奏的)" clock in their bodies as part of their sun compass.In laboratory tests,monarch butterflies were found to fly in the right direction under normal daylight hours.But those flying in
24-hour light headed straight towards the sun.(5)Scientist Steven Reppert,who led the research,said: "We have shown the need for the circadian clock for monarch butterfly migration.When the clock is interrupted,monarchs are unable to fly towards Mexico."
A.That is,they no longer had any sense of time.
B.They lay eggs along the way and die.
C.The circadian clock is useful for monarch butterfly migration.
D.Monarch butterflies use the sun to set their body clocks and make their journeys,according to US scientists.
E.But experts are unsure how the butterflies change their direction as the sun moves in the sky.
F.They cannot live for long periods in cold weather.
解析：
1.D ，前一句提到有关蝶王的谜已经解开。本句详细阐述这个谜底是什么，即美国科学家发现蝶王根据太阳设定自己的生物钟去旅行
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2.F ，后一句提到每年秋天成千只蝴蝶长途跋涉到墨西哥山脉过冬。因此本句是个衔接句，说明在寒冷的天气这些蝴蝶生活的时间不会很长。
3.B ，前一句提到没有一只蝴蝶可以完成全程旅行。本句更进一步说明蝴蝶的旅行途中发生的事情。
4.E ，前一句说过去的研究表明有些昆虫利用太阳指引方向。后一句详细介绍有关蝴蝶如何利用太阳指引方向。所以本句是转折句，表明专家们不确定蝴蝶是如何利用太阳指引方向的。
5.A ，本句进一步解释前一句话的内容。前一句话意为“蝴蝶在24小时都有灯光的情况下，会直接飞向太阳”。本句意为“这说明这种情况下蝴蝶没有时间概念”。That is,意为“也就是说”。
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