The emergence and decline of gigantic flying insects millions of years ago may have been linked to the amount of oxygen available to their water-breathing young, according to a study.
Scientists studying the smaller modern-day descendants of the huge creatures, which included dragonflies with wingspans of almost a metre, believe they have solved a question which has puzzled experts for more than 100 years by looking at their larvae, which live in water.
They believe that larvae 300million years ago took advantage of the higher oxygen levels available, using it to help fuel their growth to the size shown in fossilised remains found by palaeontologists today.
When the climate later changed and the oxygen level dropped, the larger species' larvae could not take in enough of the gas to survive and the species went extinct, leaving only their smaller relatives alive.
Dr David Bilton, of Plymouth University's School of Marine Science and Engineering, who carried out the research, said: '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.'
In their paper, published in the Public Library of Science, Dr Bilton and co-author Dr Wilco Verberk show that aquatic insect larvae are more sensitive to fluctuations in oxygen levels than the air-breathing land-based adults.
Though a link between oxygen levels and giant insects has been suggested before, no one has provided firm evidence of how they are linked.
The research, which looked at the stonefly (Dinocras cephalotes), said 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.
The larvae were also less efficient at extracting oxygen from the water than their air-breathing elders.
This, the scientists suggest, would make them more sensitive to changes in available oxygen and therefore the gas's role in shaping insect body size may be particularly important in aquatic larvae, setting an upper size limit.
Giant insects have featured largely in science-fiction stories involving the far distant past, with dragonflies featuring in Michael Crichton's novel Jurassic Park, which spawned the multi-million dollar film franchise.
Scientists believe that the creatures, with wingspans of up to 75cm recorded, lived during the Carboniferous period from about 354 to 290 million years ago.
Dr Verberk added: 'To date, attempts to understand insect gigantism in the past have been mainly approached from the perspective of (fossilised) terrestrial adults.'
'Our work suggests 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.'
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