300 million years ago, long before the era of dinosaurs, our planet vibrated with the sounds of colossal insects.
One of the most remarkable among these species is the griffin fly, a dragonfly-like predator boasting a wingspan of up to 70 cm (28 inches). With its formidable jaws, even today’s creepiest insects pale in comparison.
While many of us are relieved not to share the planet with these giants, a question lingers: where did these massive insects go, and could they return?
For decades, scientists believed they had the answer. During their reign, the atmosphere contained about 35% oxygen, compared to today’s 21%. This abundance allowed larger flying insects to thrive. As atmospheric oxygen levels declined, these giants dwindled in size, as it became energetically prohibitive to sustain them in flight.
However, a groundbreaking study published in Nature challenges this long-held view, suggesting that oxygen isn’t the primary limiting factor for insect size.
With this barrier potentially lifted, what stands in the way of the resurgence of giant insects?
How Insects Breathe
Insects have a unique respiratory system that differs significantly from ours. Lacking lungs, they transport oxygen through a network of air-filled tubes.
This system begins at spiracles (small openings on the body) and channels air through progressively smaller tubes. The tiniest of these tubes, known as ‘tracheae,’ penetrate deeply into tissues, delivering oxygen directly to cells.
For many years, scientists believed this process relied mainly on diffusion—the passive movement of oxygen from areas of high to low concentration.
However, diffusion alone becomes inefficient over long distances. The larger the insect, the greater the challenge of supplying adequate oxygen to its cells. Hence, if insects were solely dependent on diffusion, their size would be severely constrained.
Ancient Earth’s rich oxygen levels may have lifted these size limitations. “Previously, giant insects roamed the Earth,” said Edward Snelling, a professor at South Africa’s University of Pretoria and lead author of the new Nature study.
“The previous argument suggested that the tracheal system primarily relies on diffusion,” Snelling states. However, recent studies indicate that diffusion isn’t the sole driver of insect respiration.
“In addition to tracheae, insects possess large air sacs functioning as bellows to ventilate their tracheal systems, similar to how we breathe into our lungs,” explains Professor Snelling. “This ventilation significantly enhances diffusion and compensates for its limitations.”
This revelation sparked an idea in Snelling: If diffusion is no longer a constraint, oxygen might not be the reason we aren’t plagued by giant dragonflies today. But now, he and his colleagues had to validate this.
Examining the Evidence
To explore whether oxygen limits the size of modern insects, Snelling embarked on a quest to collect them.
“I roamed the campus with a net, resembling a somewhat eccentric scientist,” he admits. “I captured insects of various sizes and analyzed samples of their flight muscles under a microscope to measure the density of their tracheae.”
The hypothesis was straightforward: If flying insects were limited by oxygen, their flight muscles would require a higher density of tracheae. Given the energy demand of flight, more tracheae would be needed to transport sufficient oxygen.
“If the oxygen limitation hypothesis held true, we’d expect tracheae to occupy more than 10% of the relative space,” Snelling notes.
However, the research team discovered that tracheae typically occupy less than 1% of the space within an insect’s flight muscle. Despite an incredible size variation across 44 species—ranging over 10,000 times—the occupied space increased only by a factor of 1.8.
This indicates that even at sizes akin to griffin flies, oxygen supply doesn’t demand a significant spatial allocation.
“Even among the largest insects, the increase is minuscule, casting serious doubt on the notion that the tracheal system constrains insect body size,” Snelling concluded.
The Fate of Giant Insects: Expiration or Return?
Dr. Snelling’s study provides strong evidence for the argument that oxygen may not be the constraining factor for size, yet it leaves other reasons for the diminishment of large insects unexplored.
One compelling alternative, suggests Snelling, is environmental pressures.
“300 million years ago, there were no birds or bats to hunt flying insects; those predators had yet to evolve. Nowadays, birds and bats excel at capturing airborne insects,” he notes.
“The larger the insect, the more manageable it becomes for warm-blooded creatures.” This concept resonates; small flies challenge our ability to catch them, but larger beetles and moths are considerably easier targets.
However, this remains a theory. The ultimate reasons for the extinction of giant insects—and the possibility of their return—are still enigmas.
“Historically, gigantism tends to surface when environmental conditions are stable,” Snelling comments, implying that the resurgence of griffin flies is unlikely under current conditions.
“Very large animals are typically unadaptable to rapidly changing environments. Given human-induced environmental shifts, it may take humanity’s extinction before giant insects can return,” he speculated.
“However, if humans were to disappear and the environment stabilized, insects could potentially evolve back into the sizes they occupied 300 million years ago. Contrary to common belief, they don’t require a high-oxygen atmosphere for such a transformation.”
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Source: www.sciencefocus.com