在大约3亿年前,如现代海鸥一般大小的掠食性蜻蜓统治着地球天空。长久以来,这些家伙以及其他一些虫子怎么会长得这么大一直是一个谜。最流行的理论是,这些远古昆虫之所以长得这么大,是因为它们得益于当时地球大气层中比今天高许多的氧含量。但2011年的一项研究却暗示,太多的氧可能反而不是好事:幼虫必须长大个头,以避免氧中毒。化石记录显示,巨型蜻蜓和巨型蟑螂在石炭纪(大约3.59亿年~2.99亿年前)很常见。在这一时期, About 300 million years ago, predators such as modern seagull-sized dragonflies ruled the earth’s sky. It has always been a mystery how these guys and other bugs have grown so long for a long time. The most popular theory is that these ancient insects grew so much because they benefited from much higher levels of oxygen in the earth’s atmosphere than they were today. But a 2011 study suggests that too much oxygen may not be good on the flipside: larvae must grow taller to avoid oxygen poisoning. Fossil records show that giant dragonflies and giant cockroaches are common in the Carboniferous (about 359 million to 299 million years ago). During this period,