For decades, the explanation for why we no longer share our forests with dragonflies the size of hawks or millipedes as long as cars has been a staple of biology textbooks. The narrative was simple: during the Carboniferous and early Permian periods, the Earth’s atmosphere was saturated with significantly higher levels of oxygen than we see today. This atmospheric abundance allowed insects to grow to monstrous proportions because their unique respiratory systems could finally push enough oxygen deep into their tissues to support massive bodies.
However, new research is challenging this long-held assumption, suggesting that the answer to warum gibt es heute keine Rieseninsekten mehr (why there are no giant insects today) is far more complex than a simple change in air chemistry. While oxygen levels certainly played a role, a recent study published in Nature indicates that the physiological “ceiling” for insect size may not be where scientists previously thought.
The study, led by researcher Edward Snelling and his team, focuses on the final delivery stage of the insect respiratory system: the tracheoles. These are the microscopic, fluid-filled ends of the tracheal tubes that deliver oxygen directly to the cells via diffusion. For years, the prevailing theory was that as an insect grew larger, the inefficiency of this diffusion process would eventually starve the internal organs of oxygen, unless the surrounding atmosphere was hyper-oxygenated to compensate.
The Tracheole Density Paradox
To test this, Snelling’s team examined the flight muscles of insects across a vast range of sizes. In the high-metabolic environment of a flight muscle, oxygen demand is at its peak. If oxygen diffusion were the primary limiting factor for growth, the researchers reasoned that larger insects would have evolved a way to compensate—specifically by increasing the density of tracheoles within their muscle tissue to ensure enough gas reached the mitochondria.
The data, however, revealed a surprising lack of compensation. When comparing a tiny insect weighing just 0.5 milligrams to one weighing 5 grams—a 10,000-fold increase in body mass—the volume of the muscle tissue occupied by tracheoles barely budged. In the tiny insect, tracheoles took up roughly 0.47 percent of the volume; in the much larger insect, that figure was only 0.83 percent.
This represents only a 1.8-fold increase in volume density despite a 10,000-fold increase in weight. From a physiological standpoint, this suggests that insects are not “pushing the limit” of their delivery system. In fact, they are operating well below their potential capacity.
| Insect Body Mass | Tracheole Volume Percentage | Relative Density Increase |
|---|---|---|
| 0.5 Milligrams | ~0.47% | Baseline |
| 5 Grams | ~0.83% | 1.8x |
Comparing Insects to Vertebrates
To position this in perspective, the researchers looked at the cardiovascular systems of vertebrates. In the heart muscles of birds and mammals, capillaries—which serve a similar delivery function to tracheoles—occupy roughly ten times more relative space than tracheoles do in insect flight muscles. This comparison is critical because it demonstrates that there is ample “architectural room” for insects to increase their oxygen delivery infrastructure without compromising the function of their muscles or organs.
Roger Seymour of the University of Adelaide, a co-author of the study, noted that if tracheoles were truly the limiting factor, evolution would likely have increased their density far beyond the current 1 percent threshold. The fact that it hasn’t suggests that the bottleneck for Körpergröße von Insekten (insect body size) lies elsewhere in the respiratory chain.
The Real Bottleneck: The “Plumbing” Problem
If the fine delivery tubes (tracheoles) aren’t the problem, why did giant insects disappear? The research suggests the limitation is likely found in the larger, primary tracheae—the main “pipes” that transport air from the exterior spiracles into the body.
Unlike the flexible network of tracheoles, these larger tubes must pass through narrow anatomical constraints. To support a massive body, an insect would need either wider primary tracheae or a significantly higher number of them. However, there is very little physical space at the “pinch points” where the torso connects to the head and limbs. This anatomical constraint creates a physical ceiling that no amount of atmospheric oxygen can fully override.
This finding aligns with evidence from a study conducted nearly two decades ago, which suggested that the primary tracheal tubes—rather than the final diffusion step—were the true evolutionary bottleneck. While the 1995 theory regarding atmospheric oxygen levels remains partially relevant, it appears to be a contributing factor rather than the sole definitive cause.
Why This Matters for Evolutionary Biology
This shift in understanding changes how we view the relationship between an organism and its environment. It suggests that while a high-oxygen atmosphere can *enable* giantism, the ultimate limit is dictated by the internal structural “plumbing” of the organism. This proves a reminder that evolution is often a compromise between chemical possibility and physical space.
For those interested in the broader implications of paleo-entomology, the research highlights a recurring theme in biology: the transition from simple diffusion to active transport. While vertebrates evolved a pump (the heart) and a transport medium (hemoglobin) to overcome the limits of diffusion, insects remained tethered to a system of tubes. This architectural choice allowed them to dominate almost every niche on Earth in terms of sheer numbers, even if it cost them the ability to grow to the size of a dog.
The scientific community now looks toward further modeling of tracheal flow and pressure to determine exactly how much larger an insect could have grown if those anatomical pinch points were different. Future updates on this research are expected as teams from Arizona State University and the University of Greifswald continue to refine their models of ancestral respiratory systems.
Disclaimer: This article is for informational purposes and provides an overview of evolutionary biology and physiological research.
Do you think we will ever discover a living “relic” of the giant insect era, or is the atmospheric limit too absolute? Share your thoughts in the comments below.
