Think: eight-foot-long millipedes and dragonflies with two-foot wingspans.
Imagine a giant millipede crawling along the jungle floor with a leggy, eight-foot-long body. Next, consider an oversized dragonfly that can stretch its wings to an incredible 28 inches across. This may have been what life looked like in an oxygen-rich environment during a period of the early Paleozoic Era (541 million years ago to 252 million years ago), showcasing the largest creepy crawlies Earth has ever known.
Along with an absence of predatory birds, an atmosphere of potentially up to 35 percent oxygen concentration (versus modern-day Earth’s 21 percent) allowed these giant insects and terrestrial arthropods to grow enormous, both in the air and on the ground.
Arthropods are a group of invertebrate animals that include insects, millipedes, and spiders; they collect oxygen through openings in their bodies called spiracules, which branch into a network of tubes that diffuse the gas throughout their bodies. The high oxygen concentration allowed the arthropods to continue growing because it remained easy for them to imbibe the gas at ever-increasing sizes.
Researchers believe that the rise in the insect population started during the early periods of the Paleozoic Era, from the Cambrian Period through to the Silurian Period. But the giant insects really got going during the Devonian Period and Carboniferous Period more than 300 million years ago, when Earth hosted many more trees and plants that released oxygen.
During that time, fossil records show that a giant dragonfly from the genus Meganeura Monyi had a wingspan of up to 28 inches and fed off smaller insects. Likely living in open areas that allowed the wings to stretch, growing oversized eyes helped it “hawk” its prey and then use its spine-like legs to capture it.
The Mazothairos insect from the Paleodictyoptera group may not have been quite as large—likely around a 22-inch wingspan—but interestingly, it had developed a beak-like mouth.
On the ground, the eight-foot millipede Arthropleura Armata crawled around; it’s believed to be the largest invertebrate in the history of Earth. Likely a true forager of fruits and seeds, the bug’s massive size could be explained thanks in part to both a lack of predators and the oxygen-rich environment.
Of course, it wasn’t just friendly millipedes growing jumbo-sized. An early scorpion, known as the Pulmonoscorpius kirktonensis, could have been as large as 27 inches.
But as the Carboniferous Period started to fade, so did the high oxygen levels, bringing with it a lowering of global temperatures. The change in climate started the decline of the superbugs. And even when oxygen levels spiked again, the introduction of predatory birds made oversized insects a thing of the prehistoric past.