Prehistoric bugs grew to sizes that seem almost impossible today. The largest arthropod ever discovered stretched about 2.5 meters (over 8 feet) long, while dragonfly relatives had wingspans wider than a hawk’s. These creatures thrived hundreds of millions of years ago, when conditions on Earth were radically different from what we see now.
Giant Millipedes the Size of Alligators
The biggest land-dwelling bug in Earth’s history was Arthropleura, a millipede-like creature that roamed the swampy forests of the Carboniferous period, roughly 300 million years ago. The largest known specimen, discovered at Howick Bay near Newcastle, England in 2018, may have measured 2.7 meters (nearly 9 feet) long and weighed around 50 kilograms (110 pounds). That’s roughly the width of a school bus, or about as long as an adult female American alligator. These animals looked similar to modern millipedes but were built on a completely different scale.
Sea Scorpions Longer Than a Person
If you include aquatic arthropods, the record-holder for largest bug of all time is a sea scorpion called Jaekelopterus, which lived during the Early Devonian period, about 390 million years ago. Scientists determined its size from a single fossilized claw measuring 46 centimeters long. Based on the proportions of related species, the animal that owned that claw had a body length of roughly 2.5 meters (about 8 feet). With its claws fully extended, it would have added nearly another meter to its reach. These were apex predators in ancient waterways, and nothing else with a jointed exoskeleton has come close to matching them.
Dragonflies With Hawk-Sized Wings
The giant dragonfly relatives of the late Carboniferous and Permian periods are some of the most iconic prehistoric insects. The best-studied species, Meganeura, had a wingspan of roughly 32 centimeters (about 12.5 inches), and closely related species likely reached even larger sizes. For comparison, the largest dragonfly alive today lives in Costa Rica and has a wingspan of just 19 centimeters. These ancient insects were active aerial predators, hunting other insects on the wing much like modern dragonflies do, just at a much larger scale.
Scorpions as Long as Your Arm
Pulmonoscorpius was a land scorpion that lived in what is now Scotland during the Carboniferous period. The largest known individual reached an estimated 70 centimeters (28 inches), making it roughly as long as a baseball bat. Unlike modern scorpions, which tend to be nocturnal burrowers, Pulmonoscorpius had large compound lateral eyes with 40 to 60 individual lenses on each side, suggesting it was an active daytime hunter. It breathed through book lungs, the same basic respiratory structures scorpions use today, and was fully terrestrial. It had the familiar scorpion body plan: clawed pincers up front, a segmented tail ending in a bulbous stinger.
Ants the Size of Hummingbirds
Not all giant insects lived in the Carboniferous. Titanomyrma was a genus of enormous ants from the Eocene epoch, about 50 million years ago. Queen specimens had wingspans up to 14 centimeters and body lengths around 7 centimeters, making them the largest ants ever known, living or extinct. A single forewing on the largest species measured 4 to 6 centimeters. To put that in perspective, a queen Titanomyrma was roughly the size of a hummingbird.
Why Oxygen Made Giant Bugs Possible
Insects don’t have lungs. Instead, they breathe through a network of tiny tubes called tracheae that carry air directly to their cells. Oxygen simply diffuses through these tubes and across a small gap of tissue to reach the energy-producing structures inside each cell. That tissue gap is the bottleneck. Because oxygen doesn’t dissolve well in watery tissue, there’s a hard limit on how far it can travel from the nearest air tube to the cells that need it. The bigger the insect, the harder it is to deliver oxygen deep into its body.
During the late Carboniferous, about 300 million years ago, atmospheric oxygen levels peaked at around 35%, compared to the 21% we breathe today. With so much more oxygen in each breath of air, the diffusion limit stretched further, allowing insects and other arthropods to grow far larger than their modern relatives. Vast swampy forests dominated the landscape during this period, pumping out oxygen through photosynthesis while burying enormous amounts of carbon in what would eventually become coal deposits.
Why Bugs Shrank and Stayed Small
If high oxygen enabled giant insects, you might expect them to return whenever oxygen levels rose again. But that’s not what happened. A comprehensive analysis of more than 10,500 fossil insect specimens published in the Proceedings of the National Academy of Sciences found that the relationship between oxygen and insect size broke down permanently after a key biological event: the evolution of birds.
During the Early Cretaceous period, maximum insect size decreased even as atmospheric oxygen was rising. This timing coincides with the radiation of early birds and their increasing specialization for agile flight. Birds were faster, more maneuverable aerial predators than anything insects had faced before. The research suggests that competition and predation from flying vertebrates replaced oxygen as the dominant constraint on how large insects could get. A further size decrease during the Cenozoic era may be linked to the evolution of bats, which added yet another group of agile, insect-hunting fliers to the skies.
In short, giant bugs were a product of a unique window in Earth’s history: high oxygen, lush vegetation, and no flying vertebrates to compete with or flee from. Once birds and later bats took to the air, being large became a liability rather than an advantage, and the age of giant insects was permanently over.