Three hundred million years ago, before the age of the dinosaurs, our planet was abuzz with the sounds of giant insect life.
The most iconic of these species were perhaps the predatory dragonfly-like creatures known as griffinflies. With wingspans of 70cm (28in) and powerful, grasping jaws for hunting prey, they would make even the creepiest of our modern-day crawlies seem cutesy and quaint by comparison.
But while most of us are just grateful that we don’t have to share Earth with them, others may wonder where all the giant insects went – and whether they can come back.
For decades, scientists thought they knew. Back then, oxygen made up around 35 per cent of the atmosphere, compared to just 21 per cent today. This meant flying insects could grow to larger sizes and still respire efficiently enough to get their hulking bodies airborne.
As oxygen levels subsequently dropped, sustaining a giant body in flight became too energetically costly, so they shrank.
Except that may not be true. A landmark new study published in Nature has cast doubt on this established narrative, showing that oxygen is not a limiting factor on size.
And with that barrier gone, what’s standing in the way of giant bugs returning?
Breathing like an insect
Insects don’t breathe like we do. They don’t have lungs and they don’t move oxygen around their bodies through blood cells.
Instead, they have a network of air-filled tubes to do the job. Starting with spiracles – valves on the side of the body – air travels into progressively smaller and smaller tubes. The finest of these are known as ‘tracheoles’, which penetrate deep into tissues and deliver oxygen directly to cells.
For years, it was believed this system was driven mainly by diffusion, the passive movement of oxygen from an area of high concentration to one of low concentration.
But diffusion is inefficient over long distances. The more oxygen has to travel, the harder it becomes to deliver enough to hungry cells. So, if insects relied on diffusion alone, there would be a strict limit to how large they could grow.
Ancient Earth’s high oxygen level pushed up this size limit. “So hey presto, we had gigantic insects – really big ones,” says Edward Snelling, a professor of veterinary science at the University of Pretoria, South Africa, and lead author of the new Nature study.
“The argument was that the tracheal system is predominantly diffusion-based, especially in the tracheoles,” says Snelling. However, in recent years, scientists have discovered that diffusion is not, in fact, solely responsible for insect respiration.
“Along with these tracheal tubes inside the animal, there are also these large air sacs that act as bellows and they ventilate the tracheal system,” Snelling says. “Just like when you breathe, you ventilate your lungs. They ventilate, and this greatly facilitates and makes up for the limitations of diffusion.”
With this, Snelling had an idea: if insects are not diffusion-limited, then oxygen may not be the reason why carnivorous hawk-sized dragonflies aren’t ruining our picnics. Now, he and his colleagues had to prove it.
The evidence
To determine whether the size of modern insects was limited by oxygen, Snelling first had to catch some.
“I ran around my campus here with a net, looking like a very odd, crazy scientist,” he says. “I managed to catch a very broad body size range of insects, from among the very smallest to the largest. In those insects, I took little samples of the flight muscle and analysed it with a microscope to quantify the density of tracheoles.”
The theory was simple: if winged insects were limited by oxygen, you’d expect the density of tracheoles in the flight muscles to be high. Flying is energetically expensive, and so if the muscles were struggling to keep the insects up in the air, a lot of tracheoles would be needed to deliver the required level of oxygen.
“If the oxygen limitation hypothesis were true, then we would expect the relative space occupied by tracheoles to be more like 10 per cent or higher,” Snelling says.
What the team found, however, was that the space occupied by tracheoles in the flight muscle of insects was typically less than 1 per cent. And over a 10,000-fold body size range – from tiny insects right up to goliath beetles – and across 44 species, the space occupied only increased 1.8-fold.
This suggests that even up to the size of griffinflies, a tremendous amount of space isn’t needed for oxygen delivery.
As Snelling puts it, “Even in the largest insect, the increase was trivial, and so that really cast doubt on the idea that the tracheal system constrains the body size of insects.”
So where did they go? (And will they come back?)
Snelling’s study provides solid evidence against the oxygen-as-a-limiting-factor hypothesis, but it provides little in the way of another reason why insects shrank to much less threatening sizes.
Perhaps the most compelling alternative, Snelling suggests, is ecological pressure.
“One of the ecological reasons is that 300 million years ago, there weren’t birds and bats flying around; they hadn’t evolved yet. Today we have birds and bats, and they’re very good at catching flying insects.
“I imagine the larger the insect, the easier it is for a high-tempo, warm-blooded animal to catch.”
This makes intuitive sense. Catching a small fly in your hand is notoriously difficult, yet snatching a large beetle or moth from mid-air is far more manageable – and that’s for us, creatures with no evolutionary stake in the outcome.
But that’s just a theory. In reality, we have no idea why gigantic bugs went extinct or whether they could come back again.
“The thing about gigantism is that historically it tended to occur when the environmental conditions are very stable,” Snelling says, hinting at another reason why we might, thankfully, not see the return of griffinflies and their ilk anytime soon.
“Very large animals are not so good at responding to changing environmental conditions. With humans around and what we’re doing to the environment, changing it so rapidly, I think we need to go extinct first before we’re going to see gigantic insects re-evolve, because we're too much of a problem.
“But if we get rid of humans and things settle down a bit, then I would say that it’s possible for insects to re-evolve at least to the body size that they occurred 300 million years ago. And they would not need, contrary to popular belief, a hyperoxic atmosphere to do so.”
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