Our feathered friends are the only reason insects aren't 3ft wide
Birds may have helped to keep flying insects to a manageable size - and prevented them growing into fluttering monsters with wingspans up to two feet wide.
Faced with an airborne predatory threat - the first birds, which evolved from dinosaurs - the creatures stayed small to be more manoeuvrable, it is claimed.
Throughout earth's history, giant insects evolved during periods when the atmosphere was rich in oxygen - and 300 million years ago, had wingspans up to 30 inches wide.
They reached their largest size about 300 million years ago during the late Carboniferous and early Permian periods.
At this time huge dragonfly-like creatures called griffinflies, with wingspans up to 28 inches wide, ruled the sky.
High atmospheric oxygen levels are believed to have triggered insect growth.
It allowed insects to obtain enough oxygen through the tiny breathing tubes they use instead of lungs. Maximum insect size has tracked oxygen levels at other times too, moving up and down according to how breathable the air has been, according to US researchers.
Then, during the reign of the dinosaurs, something happened: birds appeared.
Study leader Dr Matthew Clapham, from the University of California at Santa Cruz in the US, said: ‘Right around the end of the Jurassic and beginning of the Cretaceous period, about 150 million years ago, all of a sudden oxygen goes up but insect size goes down. And this coincides really strikingly with the evolution of birds.’
With predatory birds on the wing, the need to be nimble in the air became a driving force in insect evolution, he believes. This favoured a smaller body size.
The research is reported in the latest online edition of the journal Proceedings of the National Academy of Sciences.
Some support for the theory comes from studying the evolution of pterosaurs, flying reptiles that appeared around 230 million years ago. Like birds, they would have posed a threat to flying insects.
Limited evidence suggests that insects reduced in size after pterosaurs took to the skies in the late Triassic period.
Insects again became smaller at the end of the Cretaceous period, between 90 and 65 million years ago.
Several factors could be responsible, including the mass extinction at the end of the Cretaceous that wiped out the dinosaurs.
Dr Clapham said: ‘I suspect it's from the continuing specialisation of birds. The early birds were not very good at flying. But by the end of the Cretaceous, birds did look quite a lot like modern birds.’
Source: Daily Mail