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How large insects may have come about




http://news.nationalgeographic.com/news/2011/08/110808-ancient-insects-bugs-giants-oxygen-animals-science/
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The leading theory is that ancient bugs got big because they benefited from a surplus of oxygen in Earth's atmosphere. But a new study suggests it's possible to get too much of a good thing: Young insects had to grow larger to avoid oxygen poisoning.

"We think it's not just because oxygen affects the adults but because oxygen has a bigger effect on larvae," said study co-author Wilco Verberk of Plymouth University in the U.K.

"So a larval perspective might lead to a better understanding of why these animals existed in the first place, and maybe why they disappeared."
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For the new study, Verberk and colleague David Bilton instead focused on how varying oxygen levels affect stonefly larvae, which, like dragonflies, live in water before becoming terrestrial adults. Higher concentrations of oxygen in air would have meant higher concentrations dissolved in water.

The results showed that juvenile stoneflies are more sensitive to oxygen fluctuations than their adult counterparts living on land.

This may be because insect larvae typically absorb oxygen directly through their skin, so they have little or no control over exactly how much of the gas they take in. By contrast, adult insects can regulate their oxygen intake by opening or closing valve-like holes in their bodies called spiracles.
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One way to decrease the risk of oxygen toxicity would have been to grow bigger, since large larvae would absorb lower percentages of the gas, relative to their body sizes, than small larvae.

"If you grow larger, your surface area decreases relative to your volume," Verberk explained.
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