Answer: Atmospheric Oxygen Levels

The largest living odonate species (order Odonata, containing dragonflies and damselflies) is the Megaloprepus caerulatus, found in Central and South America, with a wingspan of up to 7.5 inches (19 centimeters) across. The largest odonate species to have ever lived, however, was the Meganeura—a prototypical dragonfly species from the Carboniferous period, approximately 300 million years ago—with a wingspan of 25.6 inches (65 centimeters). To put that into perspective, the average dragonfly would scarcely take up the area of your palm, the biggest living dragonfly-type insect would scarcely cover a small plate, but a Meganeura would cover a garbage can lid.

What allowed the Meganeura to grow so much larger than the biggest of its distant descendants? More food? Better food? A sturdier exoskeleton? The most promising hypothesis, now backed up with laboratory studies conducted with modern beetles, is that atmospheric oxygen was the key.

Unlike mammals, who have a centralized respiratory system with a trachea and lungs, insects breath through a series of external openings called spiracles, which lead to the internal respiratory system, a densely networked array of tubes called tracheae. The physiological structure of the spiracles (and tracheae) as well as limitations on their method of gas transportation impose a limit on the size of the insect. There is simply an upper threshold to how large spiracles can be. Because the insects can’t simply “grow” bigger spiracles to get more oxygen, the actual level of oxygen in the surrounding atmosphere has to be higher to support mega growth.

This was exactly the situation during the Carboniferous period where oxygen levels were at their highest in all of geological history—the atmosphere was 35 percent oxygen compared to the 21 percent oxygen we have today. Thanks to the oxygen-rich air, insects could grow much larger since their spiracles could deliver more oxygen to their tissue. As the atmospheric oxygen levels dropped towards the end of the age, the super-sized dragonflies, beetles, and other invertebrates vanished, unable to support their large bodies on less oxygen.