About 300 million years ago, the Earth looked very different from what it looks like today. At that time, the continents were connected into one continent called Pangea. There were vast coal bog forests near the equator. The oxygen content in the atmosphere was significantly higher than today's levels. Frequent wildfires raged across this ancient land. Fish flourished in the ocean, and amphibians, early reptiles and various arthropods occupied their place on land, including giant cockroaches. And in the air, insects ruled the skies, with some species growing to enormous sizes, far exceeding their modern counterparts.
Among these flying insects, there are both mayfly-like insects with a wingspan of about 45 centimeters, and giant "dragonfly-like" insects with a wingspan of up to 70 centimeters. These giant insects, often collectively referred to as "griffins," were first identified from well-preserved fossil impressions in fine-grained sedimentary rocks in Kansas and have been studied for nearly a hundred years. For a long time, the mainstream view has been that these huge insects could exist because the oxygen content of the atmosphere at that time was about 45% higher than now, providing the necessary conditions to support giant insects. However, a recent study published in Nature challenges this classic explanation of "high oxygen creates giant insects".
In the 1980s, scientists began to develop methods for reconstructing the composition of the ancient atmosphere. Related technologies showed that there was a period of significant increase in atmospheric oxygen content about 300 million years ago. In 1995, a study published in Nature formally linked this period of high oxygen to the existence of giant insects, proposing the hypothesis that "giant insects need more oxygen, and a high-oxygen environment makes this size possible." This idea is based on the unique breathing method of insects: insects do not have lungs, but rely on the tracheal system to transport oxygen - a network of branched tracheae throughout the body, forming tiny tracheoles at the end, and oxygen diffuses along the concentration gradient into the flight muscles. Due to the limited efficiency of diffusion over long distances, the researchers concluded that it would be difficult to maintain such huge flying insects under today's lower atmospheric oxygen conditions, so giant insects are considered "unachievable" in modern atmospheric environments.
New research gives a different picture. A team led by Edward (Ned) Snelling of the University of Pretoria used high-resolution electron microscopy to systematically analyze the relationship between insect body size and the number of tracheal tubules in the flight muscles. They found that in most insect species, tracheal tubules typically account for no more than 1% of the volume of the flight muscles. This rule can also be extrapolated to giant "gryphon flies" 300 million years ago, including those with wingspans exceeding 60 centimeters or even approaching 2 feet. This means that the oxygen supply structures within the flight muscles do not take up much space, and insects have "evolutionary scope" to increase the number of tracheal tubules when needed without having to pay a drastic structural cost.
Based on this, the research team pointed out that the oxygen supply to insect flight muscles is not fundamentally limited by the oxygen level in the atmosphere. If atmospheric oxygen levels truly are a "hard upper limit" on the maximum size of insects, then in larger insects we should see a clear "compensatory increase" in the tracheal tubules of the flight muscles. Snelling said that although a certain degree of compensation is indeed observed in large insects, when viewed in the overall structure, this compensation is very limited and is far from enough to indicate that atmospheric oxygen content alone determines the upper limit of body size.
To further demonstrate, the researchers also compared insects with birds and mammals. In the heart muscle tissue of birds and mammals, the capillaries used to transport oxygen occupy about ten times the proportion of space than the tracheal tubules in the flight muscles of insects. Roger Seymour, a professor at the University of Adelaide who participated in the study, pointed out that if oxygen transport is really the key constraint on insect body size, then insects have the potential to "significantly increase" the investment in tracheal tubules like vertebrates in order to break through the upper body size limit. This comparison further weakens the single causal explanation that high oxygen determines giant insect body size.
Of course, some scientists have warned that atmospheric oxygen levels have not completely “cleared the suspicion.” Oxygen may still limit body size in other parts of the insect's body, or in early stages of the oxygen transport chain. Therefore, the hypothesis that "oxygen limits the maximum body size of insects" is still difficult to say has been completely overturned. However, new research clearly shows that, at least in the diffusion of tracheal tubules within flight muscles, oxygen is not a critical factor in determining the existence of giant insects. This has forced researchers to look at other possible explanations to answer the open question of how insects once grew so large and why they eventually disappeared.
In the current discussion, some alternative factors mentioned include: as evolution progresses, vertebrate predators increase, and predation pressure from birds, reptiles, etc. may have a profound impact on the evolution of insect body size; at the same time, the upper limit of the mechanical strength of insect exoskeletons may also become a structural "ceiling" at a certain body size scale, limiting the feasibility of further increasing body size. However, these hypotheses currently lack quantitative evidence that is as widely accepted as the "hyperoxia theory" and still need to be verified by future research. What is certain is that this new analysis of tracheal tubules and flight muscles makes the mystery of the origin of ancient giant insects even more confusing.