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Once upon a time, hundreds of millions of years ago, many insects were much larger than they are today.
很久以前，大約幾億年前，許多昆蟲的體型要比現在的昆蟲大得多。
Scientists have found fossilized remains of dragonflies with wingspans several feet in diameter.
科學家已經發現翼展直徑長達幾英尺的蜻蜓化石。
Other insects were equally monstrous due largely to the abundance of oxygen in the atmosphere.
其它昆蟲的體型也不可小覷，究其原因是因為當時的大氣中氧氣含量充足所致。
Oxygen is the fuel that powers insects’ flight muscles. So the more oxygen in the atmosphere, the larger insects tend to grow.
氧氣是昆蟲鍛煉飛行肌的動力來源。因此，大氣中的氧氣越充足，昆蟲的體型就越龐大。
Hundreds of millions of years ago, oxygen made up around 30% of the earth’s atmosphere, compared to about 20% today.
幾億年前，地球大氣的含氧量高達30%，如今大氣的含氧量只有20%。
But around 150 million years ago, even though oxygen saturated the atmosphere, giant insects began to disappear.
大約一億五千萬年前，雖然大氣中的含氧量仍然趨于飽和，但巨型昆蟲還是相繼滅絕。
Why? Scientists suspect it was due mainly to the evolution of insect-eating birds.
究竟是何原因？科學家懷疑主要原因可能是食蟲鳥類的進化所致的。
Bird like creatures existed earlier, but their ability to fly and maneuver was limited.
現在的鳥類與遠古時期的鳥類相差無幾，只是當時鳥類的飛行能力和控制力十分有限。
Their offspring eventually evolved smaller tails, specialized wing bones, and larger breast bones.
它們的后代最終進化成尾巴更小，胸骨更大，翼骨更加發達的鳥類。
