A recent international study has challenged a long-standing theory that high atmospheric oxygen levels were necessary for the existence and flight of giant insects during the Carboniferous period. The research, published in Nature, concludes that the insect tracheal system, responsible for oxygen delivery to flight muscles, does not physiologically limit body size, suggesting that ancient giant insects like griffinflies could theoretically exist in Earth's current atmosphere.
Historical Giants and Ancient Atmospheres
Insects developed flight approximately 350 million years ago, predating birds by two centuries. By the end of the Carboniferous period, around 300 million years ago, some flying insects evolved to significant sizes. Notable among these were griffinflies, which featured wingspans up to 70 centimeters (27 inches). Other giant insects, such as mayfly-like species, are reported to have had wingspans up to 45 centimeters (17 inches). Fossilized impressions of these insects were first discovered in Kansas.
During this ancient period, Earth's atmosphere contained higher oxygen levels, estimated at about 30%, compared to the modern 21%. Some analyses suggest oxygen levels were approximately 45% higher than present-day concentrations.
The Prevailing Oxygen Limitation Hypothesis
For approximately 30 years, a prevailing scientific hypothesis proposed that these elevated external oxygen levels were crucial to power the high energy demands of flight for such large insects.
This theory suggested that the structure and function of the insect tracheal system—a branching network of air-filled tubes delivering oxygen primarily via diffusion to flight muscles—limited body size.
The hypothesis, which is included in educational textbooks, posited that larger insects would require higher atmospheric oxygen concentrations to facilitate sufficient oxygen diffusion to their muscles. This theory gained support from geochemical reconstructions in the 1980s indicating a period of high atmospheric oxygen and a 1995 study in Nature correlating this with the presence of giant insects.
Groundbreaking New Research
Recent research, led by Edward (Ned) Snelling of the University of Pretoria and including Jon Harrison from Arizona State University's School of Life Science, utilized high-power electron microscopy to investigate how insect body size impacts the number of tracheoles in flight muscle. The five-year project involved studying 44 species of modern flying insects and analyzing 1,320 transmission electron micrographs.
Unpacking the FindingsThe study's findings indicate that tracheoles consistently occupy about 1% or less of the cross-sectional area of flight muscles, regardless of the insect's body size or metabolic rate.
This observation stands in contrast to the approximately 10% area occupied by blood-filled capillaries in the flight and cardiac tissue of some birds and mammals.
Researchers concluded that insect flight muscles possess ample capacity to increase the number and volume of tracheoles without compromising muscle integrity, suggesting that oxygen transport at the tracheole level is not a limiting factor for body size.
Further evidence from developing insects demonstrated their ability to generate more tracheoles in flight muscle when exposed to lower oxygen conditions, a trait that can be inherited by offspring. These findings led researchers to conclude that the body size of flying insects has not been constrained by the structure or function of their tracheal systems.
Implications and Unanswered Questions
The research suggests that there is no physiological impediment preventing insects the size of ancient griffinflies from flying in Earth's current atmosphere. The absence of such giant insects today may be attributed to other factors.
Proposed alternative explanations include the greater susceptibility of larger animal species to extinction. Additionally, 300 million years ago, giant insects did not face modern predators such as birds or mammals. Other potential factors suggested for the smaller size of modern insects include vertebrate predation or biomechanical limitations of the exoskeleton.
While the new data indicate that diffusion within flight muscle tracheoles does not limit maximal insect size, some scientists propose that oxygen flow upstream of the tracheoles or in other body parts might still impose size constraints. Therefore, the theory of oxygen-constrained insect maximal size may remain subject to further investigation.