英語聽力文摘596 以前的昆蟲是很龐大的！

In The Past, Insects Were Huge!

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

Other insects were equally monstrous 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 saturated 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 maneuver was limited. Their offspring eventually evolved smaller tails, specialized wing bones, and larger breast bones.

These changes, which gave birds greater facility for flight, made them more effective predators. 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, prehistoric 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 deplete their competitors’food source.

　　以前的昆蟲是很龐大的!

　　很久以前，大約幾億年前，許多昆蟲的體型要比現(xiàn)在的昆蟲大得多?？茖W家已經(jīng)發(fā)現(xiàn)翼展直徑達長達幾英尺的蜻蜓化石。

　　其它昆蟲體型也十分龐大，這主要是由于當時大氣中充足的含氧量所致。氧氣是昆蟲飛行肌強壯的動力來源。因此，大氣中氧氣越充足，昆蟲的體型就越大。相比于今天只有20%含氧量的大氣來說，幾億年前，地球大氣的含氧量高達30%。

　　但大約一億五千萬年前，雖然大氣中的含氧量仍然趨于飽和，巨型昆蟲還是相繼滅絕。為什么呢?科學家猜想可能主要由于食蟲鳥類的進化所致?，F(xiàn)在的鳥類與早期 的鳥類沒有太大不同，但是當時鳥類的飛行能力和控制力卻非常有限。經(jīng)過進化，它們的后代最終變得尾巴更小，胸骨更大，翼骨更加發(fā)達。

　　這些變化讓鳥類的飛行能力顯著增強，也因而大大提高了捕食效率。因此不難想象他們會以大型昆蟲為食。

　　至少，這是其中的一種理論。另一種理論認為史前鳥類并不以巨型昆蟲為主要食物來源，而是與巨型昆蟲競爭，鳥類和巨型昆蟲均以較小昆蟲為食，而小型昆蟲的棲 息地又離大型昆蟲很近，進化使得鳥類不僅可以以巨型昆蟲為食，同時也可以捕捉小型昆蟲來吃，由此便消耗了競爭對手，也就是巨型昆蟲的食物來源。