Plymouth scientists' oxygen 'key' to gigantic insects

Image caption Scientists believe giant prehistoric ancestors of the dragonfly had wingspans of up to 75cm (30ins)

The level of oxygen in water could be the key to the emergence and decline of prehistoric gigantic flying insects.

Plymouth University scientists have studied the oxygen levels available to the larvae of modern-day insects.

They believe higher oxygen levels 300 million years ago helped fuel the growth of giant dragonflies.

When the climate changed and oxygen levels dropped, the larger species' larvae could not get enough to survive.

The research was carried out by Dr David Bilton, of the university's School of Marine Science and Engineering.

Body size limit

"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 a paper, published in the Public Library of Science, Dr Bilton and co-author Dr Wilco Verberk show aquatic insect larvae is more sensitive to fluctuations in oxygen levels than air-breathing land-based adults.

A link between oxygen levels and giant insects has been suggested before, but no-one has provided firm evidence of how they are linked.

The research, which looked at the stonefly (Dinocras cephalotes), says 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.

This would make them more sensitive to changes in available oxygen, therefore the gas's role in shaping insect body size could be particularly important in aquatic larvae, setting an upper size limit.

Scientists believe that giant dragonflies with recorded wingspans of up to 75cm (30ins) lived during the Carboniferous period - about 354 to 290 million years ago.

Dr Verberk said previous attempts to understand insect gigantism have mainly been 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," he said.

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