英语听力文摘 English Digest 596、以前的昆虫是很庞大的！

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　　Once upon a time, hundreds of millions of years ago, many insects were much larger than they are today. Scientists have found fossilized remains¹ of dragonflies with wingspans several feet in diameter.

 

　　Other insects were equally monstrous² due largely to the abundance of oxygen in the atmosphere. Oxygen is the fuel that powers insects’ flight muscles. So the more oxygen in the atmosphere, the larger insects tend to grow. Hundreds of millions of years ago, oxygen made up around 30% of the earth’s atmosphere, compared to about 20% today.

 

　　But around 150 million years ago, even though oxygen saturated³ the atmosphere, giant insects began to disappear. Why? Scientists suspect it was due mainly to the evolution of insect eating birds. Bird like creatures existed earlier, but their ability to fly and maneuver⁴ was limited. Their offspring eventually evolved smaller tails, specialized⁵ wing bones, and larger breast bones.

 

　　These changes, which gave birds greater facility for flight, made them more effective predators⁶. It’s not difficult to imagine that they feasted on giant insects.

 

　　That’s one theory, at least. Another holds that rather than eat giant insects, prehistoric⁷ birds simply out competed them. Both birds and giant insects ate smaller insects that existed alongside their super sized cousins. Evolution may have enabled birds not only to dine on giant insects but also to consume smaller insects and deplete⁸ their competitors’food source.

 

 

 

 

　　以前的昆虫是很庞大的！

 

　　很久以前，大约几亿年前，许多昆虫的体型要比现在的昆虫大得多。科学家已经发现翼展直径达长达几英尺的蜻蜓化石。

 

　　其它昆虫体型也十分庞大，这主要是由于当时大气中充足的含氧量所致。氧气是昆虫飞行肌强壮的动力来源。因此，大气中氧气越充足，昆虫的体型就越大。相比于今天只有20%含氧量的大气来说，几亿年前，地球大气的含氧量高达30%。

 

　　但大约一亿五千万年前，虽然大气中的含氧量仍然趋于饱和，巨型昆虫还是相继灭绝。为什么呢？科学家猜想可能主要由于食虫鸟类的进化所致。现在的鸟类与早期 的鸟类没有太大不同，但是当时鸟类的飞行能力和控制力却非常有限。经过进化，它们的后代最终变得尾巴更小，胸骨更大，翼骨更加发达。

 

　　这些变化让鸟类的飞行能力显著增强，也因而大大提高了捕食效率。因此不难想象他们会以大型昆虫为食。

 

　　至少，这是其中的一种理论。另一种理论认为史前鸟类并不以巨型昆虫为主要食物来源，而是与巨型昆虫竞争，鸟类和巨型昆虫均以较小昆虫为食，而小型昆虫的栖 息地又离大型昆虫很近，进化使得鸟类不仅可以以巨型昆虫为食，同时也可以捕捉小型昆虫来吃，由此便消耗了竞争对手，也就是巨型昆虫的食物来源。

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