In the late Carboniferous insects were huge! Insects grew to enormous sizes because there was more oxygen in the air than today. Giants included Meganeuridae, dragonflies with wing spans of 70 centimetres, and the 1 metre long centipede-like Arthropleura.
Why did insects get smaller?
Matthew Clapham and Jered Kerr wanted to know if oxygen was the only reason why flying insects were so big. To find out they compared the wing lengths of 10,500 fossilised insects from the Paleozoic, Mesozoic and Cenozoic eras.
The scientists learnt that oxygen was the primary influence on body size for the first 150 million years of insect evolution, until the end of the Jurassic. Afterwards other factors came in to play.
Flying insects got smaller in the Early Cretaceous despite an increase in atmospheric oxygen. Clapham and Kerr think this was a response to predatory pressure from birds. At this time bird species became more varied and more agile in flight.
Further decreases in the Cenozoic might relate to improvements in bird flight, the evolution of bats, or the Cretaceous mass extinction.
These findings were published in a 2012 research paper by Clapham and Kerr, titled Environmental and biotic controls on the evolutionary history of insect body size.
Importantly, the pair note: “This trend is primarily the result of body-size changes in large flying insects, such as dragonflies (but also in grass-hoppers), and ground-dwelling groups, such as many beetles or cockroaches, may not follow the same pattern because the history of terrestrial predation differs from that of aerial predation.”
Mega-insects: further reading
A glance at the deep past history of insects by André Nel