Plymouth scientists breathe new life into oxygen theory behind giant dragonfly extinction

Plymouth scientists breathe new life into oxygen theory behind giant dragonfly extinction

Now a study of more diminutive modern day insects may have provided answers to questions that have occupied scientists for over 100 years – why did these giant insects become extinct, and how did they evolve in the first place?

The oxygen-rich atmosphere of the Carboniferous period has long-been suspected to have played a part in this gigantism, and ecologists at Plymouth University have proposed a novel explanation for how drove gigantism, after investigating the responses of modern-day aquatic insect larvae to 02 levels in water.

In their article, published in Public Library of Science, Dr Wilco Verberk and Dr David Bilton, of the School of Marine Science and Engineering, set out their findings on how the thermal tolerance of aquatic larvae is more sensitive to differences in oxygen than that of terrestrial adults.

Dr Verberk said: “To date, attempts to understand insect gigantism in the past have been mainly approached from the perspective of (fossilized) terrestrial adults.

“Our work suggests that oxygen limitation at thermal extremes operates differently for aquatic larvae and terrestrial adults, and that approaching the problem of historical gigantism from a larval perspective may shed new light on the way in which oxygen sets insect body size limits.”

The study shows that aquatic insect larvae are more sensitive to fluctuations in oxygen levels than terrestrial adults. Aquatic larvae, such as those of dragonflies, stoneflies and mayflies, extract their oxygen directly from the water, where far less is available than in air.

In addition, whereas terrestrial can just open their trachea to take up the oxygen they need, that option is not available in water. As a result, oxygen’s role in shaping insect body size may be particularly important in aquatic larvae, setting an upper size limit since larger larvae need to transport oxygen further to get it to their tissues.

Dr Bilton added: “In prehistoric times, higher levels of oxygen may have favoured the evolution of giant insects largely through their effects on larvae, and it is perhaps no accident that many extinct giants had aquatic juvenile stages.”

Explore further

Raising giant insects to unravel ancient oxygen

More information: Can oxygen set thermal limits and drive gigantism? PLoS One 6(7): e22610. doi: 10.1371/journal.pone.0022610
Provided by University of Plymouth
Citation: Plymouth scientists breathe new life into oxygen theory behind giant dragonfly extinction (2011, August 4) retrieved 16 October 2019 from
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Aug 04, 2011
I can see that more oxygen would allow bigger larvae that could form bigger adults. But it is not clear to me why they keep throwing the word "thermal" in there. Any ideas?

Aug 04, 2011
Thermal is in there because the maximum oxygen content of water varies with the temperature. Higher temperatures drive oxygen out of the water while lower temperatures allow water to carry more oxygen.

You have to learn this stuff if you want to be successful at trout fishing.


Aug 04, 2011
But what does surviving at higher temperatures have to do with gigantism? It implies that bugs can get bigger if they grow where it is warmer, but the article doesn't mention that.

Aug 04, 2011
You went the wrong way.

Bugs get more oxygen in COLDER water thus they can grow larger. Insects do not have blood or a heart or lungs. They have air passages which under water would be water passages. With more oxygen in the water the O2 can reach farther in the larvae thus allowing the larger larvae to stay oxygenated.

The usual theory for the big insects is that they had more oxygen in the air. Which should allow them to function even when they are bigger BUT these guys think the larvae are the key to the increased size and for them the water temperature has more effect on the O2 content than the O2 content of the air.

Thus it wouldn't matter for water based insects if the air had more O2 UNLESS the water was still cold enough to allow the water to have more O2 as well.


Aug 04, 2011
Brief correction to Eth...
"They have air passages which under water would be water passages"
The species they mention have external gills as larva.

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