Giant Insects

Birds limited giant flying insect size

The late Carboniferous is notable for its giant insects. Thanks to high concentrations of oxygen in the air, insects grew to enormous sizes. Giants of the time included Meganeuridae, dragonflies with wing spans of 70 centimetres, and also the 1 metre long centipede-like Arthropleura.

Scientists Matthew Clapham and Jered Kerr knew that the high oxygen concentrations helped flying insects to grow large. They wanted to find out if it was the only influence on body size. To this end they analysed the wing lengths of 10,500 fossilised insects from the Paleozoic, Mesozoic and Cenozoic eras.

The scientists found that oxygen was the primary influence on body size for only the first 150 million years of insect evolution, until the end of the Jurassic. Afterwards other factors came in to play.

Maximum size decreased in the Early Cretaceous despite an increase in atmospheric oxygen. Clapham and Kerr think that the flying insects grew smaller in response to predatory pressure from birds. At this time bird species became more varied and more agile in flight.

Clapham and Kerr suggest that a further decrease in size in the Cenozoic might relate to further 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