300 million years ago, Earth looked nothing like it does today. The continents were combined into a giant supercontinent called Pangea. Near the equator, vast coal swamp forests stretched across the landscape. Oxygen levels in the atmosphere were much higher than they are today, and wildfires were common.
Life was abundant in all environments. The seas were filled with fish, and the land was home to amphibians, early reptiles, crawling arthropods, and even giant cockroaches. The sky above us was filled with insects, some of which reached surprising sizes.
Dragonfly with huge wings spread
These prehistoric insects included mayfly-like species with wingspans of 17 inches (45 cm) and dragonfly-like creatures that reached up to 27 inches (70 cm). These giant insects, often called “griffin flies,” were first identified nearly a century ago from fossil impressions found in fine-grained sedimentary rocks in Kansas.
For decades, scientists believed that these giant insects could only exist because oxygen concentrations in the atmosphere were about 45% higher than they are now. New research challenges that long-held belief.
The oxygen theory behind giant insects
In the 1980s, researchers developed a technique 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 1995 study published in the journal Nature linked periods of abundant oxygen to the presence of giant insects. Scientists proposed that larger insects require more oxygen, and that their size may be enabled by more oxygen in the atmosphere.
This explanation is based on how insects breathe. Instead of lungs, insects rely on a network of air-filled tubes called the tracheal system. These tubes branch throughout the body and end in small structures called tracheas. Oxygen moves through these tracheas by diffusion and travels down the concentration gradient to the flight muscles.
Because diffusion efficiency decreases over long distances, the researchers concluded that modern oxygen levels are not sufficient to support the energy needs of very large flying insects.
New research questions long-held assumptions
New research published in nature provides another perspective. A research team led by Edward (Ned) Snelling at the University of Pretoria used high-magnification electron microscopy to examine how the insect’s body size is related to the number of flight muscle tracheae.
Their findings showed that the trachea typically accounts for only about 1% or less of the flight muscles in most insect species. Even if we apply this relationship to the giant griffins that lived 300 million years ago, the proportion remains small.
This suggests that insect flight muscles are not limited by oxygen availability. Because the trachea occupies so little space, insects could theoretically increase their numbers without major structural constraints.
Evidence from modern insects
“If atmospheric oxygen really imposes a limit on maximum body size in insects, there should be evidence of compensation at the level of the trachea,” said lead author Edward (Ned) Snelling, associate professor in the School of Veterinary Medicine at the University of Pretoria. “There is some compensation occurring in larger insects, but overall it is negligible.”
Further supporting this idea, researchers compared insects and vertebrates. In birds and mammals, the capillaries of the heart muscle occupy about 10 times more space than the trachea of the insect flight muscles.
Professor Roger Seymour from the University of Adelaide said: “By comparison, capillaries in the heart muscle of birds and mammals occupy about 10 times the relative space occupied by the trachea in insect flight muscles. So if oxygen transport is indeed limiting body size, there must be significant evolutionary potential to increase investment in the trachea.”
The mystery of the size of giant insects remains
Some scientists warn that oxygen may still play a size-limiting role in insects, especially in other parts of the body and during early stages of oxygen transport. As a result, the idea that oxygen limits insect size has not been completely ruled out.
However, new findings clearly demonstrate that oxygen diffusion within the flight myotrachea is not the limiting factor. This means researchers must look for alternative explanations for why insects once grew so large.
Possible factors include increased predation by vertebrates and the physical limitations of insect exoskeletons. For now, the real reasons behind the emergence and disappearance of giant insects remain an open question.
