Three hundred million years ago, the Earth did not look like it does today. The continents were joined together into a giant supercontinent called Pangaea. Near the equator, vast coal-swamp forests extend across the landscape. Oxygen levels in the atmosphere were much higher than today, and forest fires occurred frequently.
Life was abundant in every environment. The oceans were teeming with fish, while the land was home to amphibians, early reptiles, crawling arthropods, and even giant cockroaches. The sky above was full of insects, some of which had reached astonishing sizes.
giant winged dragonflies
These prehistoric insects included mayfly-like species with 17-inch (45-cm) wingspans and dragonfly-like creatures that reached 27 inches (70-cm). These giant insects, often called “griffinflies”, were first identified from fossilized impressions found in fine-grained sedimentary rock in Kansas about a century ago.
For decades, scientists believed that these giant insects could only exist because oxygen levels in the atmosphere were about 45% higher than today. A new study now challenges that long-held idea.
The Oxygen Theory Behind Giant Insects
In the 1980s, researchers developed techniques that allowed them to reconstruct the composition of the ancient atmosphere. Their results showed that oxygen levels peaked about 300 million years ago.
Based on this discovery, a study published in Nature in 1995 linked that oxygen-rich period to the presence of giant insects. Scientists proposed that larger insects required more oxygen, and that their size was made possible by higher atmospheric oxygen.
This explanation was based on how insects breathe. Instead of lungs, insects rely on a network of air-filled tubes called a tracheal system. These tubes branch throughout the body and end in smaller structures called tracheoles. Oxygen moves through these tracheae by diffusion, traveling down concentration gradients to reach the flight muscles.
Because dispersal becomes less efficient over long distances, the researchers concluded that modern oxygen levels would not be sufficient to meet the energy demands of extremely large flying insects.
New study challenges long-held beliefs
A new study was published in Nature Offers a different perspective. Led by Edward (Ned) Snelling of the University of Pretoria, the research team used high-power electron microscopy to examine how insect body size relates to the number of tracheoles in the flight muscles.
Their findings showed that tracheoles typically occupy only 1% or less of the flight muscles in most insect species. Even when this relationship is applied to giant griffinflies that lived 300 million years ago, the ratio remains small.
This suggests that insects’ flight muscles are not limited by oxygen availability. Because the trachea takes up very little space, insects can theoretically increase their numbers without major structural constraints.
Evidence from modern insects
“If atmospheric oxygen indeed sets a limit on the maximum body size of insects, there should be evidence of compensation at the level of the tracheoles,” said lead author Edward (Ned) Snelling, Associate Professor and Faculty of Veterinary Sciences at the University of Pretoria. “There is some compensation in larger insects, but it is trivial in the grand scheme of things.”
Further supporting this idea, researchers compared insects to vertebrates. In birds and mammals, capillaries in the heart muscle occupy about ten times more space than the trachea in the flight muscles of insects.
“By comparison, the capillaries in the cardiac muscles of birds and mammals occupy about ten times more relative space than the tracheoles present in the flight muscles of insects, so if oxygen transport is indeed limiting body size, there must be great evolutionary potential for increasing the investment of tracheoles,” said Professor Roger Seymour of the University of Adelaide.
Giant insect’s size remains a mystery
Some scientists caution that oxygen may still play a role in limiting the size of insects, particularly in other parts of the body or in the early stages of oxygen transport. As a result, the idea that oxygen limits the size of insects has not been completely rejected.
However, new findings clearly show that oxygen diffusion within the trachea of flight muscles is not the limiting factor. This means that researchers must explore other explanations for why insects once grew so large.
Possible factors include increased vertebrate predation or physical limitations of insect exoskeletons. At the moment, the real reason for the rise and disappearance of giant insects remains an open question.