About 300 million years ago, prior to the age of dinosaurs, our planet resonated with the sounds of enormous insect life.

The most iconic of these species was the griffin fly, a predatory dragonfly-like creature boasting a wingspan of up to 70 cm (28 inches) and formidable jaws for hunting prey. In comparison, even today’s creepiest crawlies seem rather charming.

While many are relieved that we no longer share the Earth with these giants, curiosity remains about their disappearance and the possibility of their return.

For decades, scientists believed they had the answer: the atmosphere once contained about 35 percent oxygen, while today it holds only 21 percent. This high oxygen level allowed flying insects to grow larger as they could efficiently breathe enough to sustain their enormous bodies.

However, as oxygen levels fell, these massive creatures shrank, as it became energetically challenging to keep them airborne.

Recent groundbreaking research published in Nature questions this long-held belief, revealing that oxygen may not be the true limiting factor for insect size.

With this obstacle removed, what’s stopping giant insects from making a comeback?

How Insects Breathe

Insects have a unique respiratory system that differs significantly from ours. Lacking lungs, they do not transport oxygen through blood cells.

Instead, they utilize a complex network of air-filled tubes. Air enters through spiracles (valves on the sides of their bodies) and flows through progressively smaller tubes. The tiniest of these, called tracheae, penetrate deep into tissues, delivering oxygen directly to cells.

Initially, scientists believed this system relied largely on diffusion, the passive movement of oxygen from regions of high concentration to those of low.

However, diffusion is inefficient over long distances. When more oxygen is needed, it becomes increasingly difficult to supply oxygen to cells. Hence, if insects relied solely on diffusion, their growth would be severely restricted.

Ancient Earth’s high oxygen levels allowed for the growth of massive insects. “Back then, giant insects roamed the earth,” says Edward Snelling, a professor of veterinary medicine at the University of Pretoria and lead author of the Nature study.

“Previously, it was thought that the tracheal system operated mainly through diffusion,” Snelling explains. However, recent discoveries show that ventilation also plays a crucial role in insect respiration.

“In addition to tracheae, insects possess large air sacs that function as bellows, enhancing the ventilation of the tracheal system,” Professor Snelling adds. “This ventilation significantly boosts diffusion, mitigating its limitations.”

This insight led Snelling to ponder if the absence of a diffusion limit could imply that oxygen isn’t the reason giant carnivorous dragonflies don’t invade our picnics.

Research Findings

To determine if oxygen constraints limit the size of modern insects, Snelling set out to capture various specimens.

“I ran around campus with a net, looking like a mad scientist,” he recalls. “I gathered insects across a wide range of sizes and analyzed their flight muscles under a microscope to assess tracheal density.”

The underlying theory was straightforward: if oxygen limits the size of winged insects, one would expect a high tracheal density within flight muscles. Flight demands energy, and if muscles struggle to remain airborne, more tracheae would be needed to supply sufficient oxygen.

“If the oxygen limitation hypothesis held true, tracheae would likely occupy over 10 percent of the relative space,” Snelling states.

However, their findings revealed that tracheae occupy less than 1 percent of the space in an insect’s flight muscle. Despite body sizes varying over 10,000 times from tiny insects to giant beetles, the increase in occupied space across 44 species was only 1.8 times.

This implies that even at griffin fly size, the demand for oxygen does not require a significant amount of space.

“Even in the largest insects, the increase was minimal, casting significant doubt on the tracheal system’s potential limitations on insect body size,” Snelling concluded.

What Happened to Them? (Will They Ever Return?)

Dr. Snelling’s research provides compelling evidence against oxygen being a limiting factor but reveals little about other reasons for insects shrinking in size.

A notable alternative reason proposed by Snelling is environmental pressure.

“300 million years ago, there were no birds or bats, which are proficient at catching flying insects. Larger insects might have been easier prey for these warm-blooded animals,” he speculates.

This complexity makes sense; small flies are notoriously tricky to catch by hand, while larger beetles and moths are generally easier targets for both predators and humans. However, this remains a theory, as the reasons behind the extinction of giant insects and their potential return are still largely unknown.

“Historically, gigantism tends to emerge under stable environmental conditions,” Snelling notes, suggesting another reason for the unlikely resurgence of the griffin fly and its kin.

“Large animals typically struggle to adapt to shifting environments. With human activities dramatically altering ecosystems, it may take humanity’s absence before giant insects can re-evolve,” he adds.

“However, if we can stabilize our environment, it’s conceivable that insects could return to sizes last seen 300 million years ago. Contrary to popular belief, high-oxygen atmospheres may not be a necessity for this re-emergence.”

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