This fossil insect wing (Stephanotypus schneideri) from the period about 300 million years ago when insects reached their greatest sizes, measures 19.5 centimeters (almost eight inches) long. The largest species of that time were even bigger, with wings 30 centimeters long. For comparison, the inset shows the wing of the largest dragonfly of the past 65 million years. (Credit: Photo by Wolfgang Zessin.)
Insects got bigger as oxygen levels rose during the late Carboniferous and early Permian. But 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.
Blame the birds. In PNAS this week:
Environmental and biotic controls on the evolutionary history of insect body size.
Maximum insect size decreased even as atmospheric pO2 rose in the Early Cretaceous following the evolution and radiation of early birds, particularly as birds acquired adaptations that allowed more agile flight. A further decrease in maximum size during the Cenozoic may relate to the evolution of bats, the Cretaceous mass extinction, or further specialization of flying birds. The decoupling of insect size and atmospheric pO2 coincident with the radiation of birds suggests that biotic interactions, such as predation and competition, superseded oxygen as the most important constraint on maximum body size of the largest insects.