Three-hundred-million years ago the Earth was very different. The continents had coalesced into Pangaea, which was dominated in its equatorial regions by vast coal-swamp forests. With high atmospheric oxygen levels, wildfires were common.
The waters teemed with fishes, the land was dominated by amphibians, reptiles, crawling arthropods and giant cockroaches, and the skies were ruled by flying insects, some of which took on truly gigantic proportions.
Among them were giant mayfly-like species with 17 inch (45-cm) wingspans and enormous dragonfly-like species with 27 inch (70-cm) wingspans. These "griffinflies" were first discovered as fossilized impressions in fine-grained sedimentary rock in Kansas nearly a century ago.
Now, a new study by an international team of researchers, including ASU School of Life Science professor Jon Harrison, has provided strong evidence refuting the long-held theory that gigantic dragonfly-like insects could only have existed 300 million years ago because atmospheric oxygen levels were about 45% higher than they are today.
In the 1980s, and a new technique emerged that allowed geochemists to reconstruct the gas composition of past atmospheres. Their striking finding was that a period of high atmospheric oxygen occurred 300 million years ago.
In a 1995 study published in Nature, scientists pointed out that this period of high atmospheric oxygen coincided with the occurrence of giant insects. They proposed that a higher demand for oxygen and larger body sizes of giant sized Palaeozoic insects ought to require a higher atmospheric oxygen concentration. This made sense, because insects obtain oxygen through their unique tracheal system, which is a branching tree-like system of airways leading to their ends, the tracheoles. Oxygen must move by diffusion down concentration gradients through the tracheoles to fuel the flight muscle cells. Scientists reasoned that a flying insect of such gigantic proportions could not exist now, because the level of oxygen in the present atmosphere is too low to support the high demand for oxygen in the flight muscles.
In a new study, published in the latest issue of Nature, a team led by Edward (Ned) Snelling of the University of Pretoria used high-power electron microscopy to assess how body size affects the number of tracheoles in flight muscle. They found that the space occupied by tracheoles in the flight muscle is typically only 1% or less in most species, and that this observation holds when the relationship is extended to the 300-million-year-old, gigantic 2-feet and larger griffinflies. This suggests that the flight muscles of insects are not constrained by atmospheric oxygen levels as they could easily add tracheoles in the muscle, since they take up so little space.
"If atmospheric oxygen really sets a limit on the maximum body size of insects, then there ought to be evidence of compensation at the level of the tracheoles", said lead author Edward (Ned) Snelling, associate professor, and Faculty of Veterinary Science at the University of Pretoria. "There is some compensation occurring in larger insects, but it is trivial in the grand scheme of things."
"By comparison, capillaries in the cardiac muscle of birds and mammals occupy about ten-times the relative space than tracheoles occupy in the flight muscle of insects, so there must be great evolutionary potential to ramp up investment of tracheoles if oxygen transport were really limiting body size," said professor Roger Seymour of Adelaide University.
Some scientists counter argue that oxygen flow upstream of the tracheoles, or in other parts of the body, could still limit body size, so the theory of oxygen-constrained insect maximal size may not be dead yet.
Regardless, these new data definitively show that diffusion in the flight muscle tracheoles cannot provide such a limit. Scientists will have to look elsewhere for why these giants existed.
If oxygen does not limit maximal insect size, then perhaps other culprits are responsible for the small size of insects, such as predation from vertebrates, or biomechanical support limits on the exoskeleton itself.
