How large insects may have come about

["Richard W. Travsky" <rtravsky@uwyo.edu>]

http://news.nationalgeographic.com/news/2011/08/110808-ancient-insects-bugs-giants-oxygen-animals-science/
...
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."
...
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.
...
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.
...