How did ancient bugs get so big? The prevailing theory may be wrong
About 300 million years ago, giant dragonflylike insects with half-meter wing spans buzzed through hot and swampy forests on the former supercontinent of Pangaea. Scientists have long debated what allowed griffenflies, as they’re known, and similar fearsome flying bugs to grow so big during the Carboniferous period. The atmosphere at the time held more oxygen than it does today, and the textbook hypothesis suggests these giant insects developed more respiratory tubes to deliver that gas to their muscles, enabling them to grow and grow. But a new analysis of the anatomy of insect flight muscles, published last week in Nature, undercuts that idea, suggesting past ferocious fliers didn’t incorporate oxygen into their muscles any more generously than their smaller counterparts do today.
“This study places what may be the final nail in the coffin for the prevailing view that more oxygen made ancient insects bigger,” says Caleb Gordon, a paleontologist at the Florida Museum of Natural History who was not involved with the study. “[It] has been ‘the common wisdom’ among paleontologists for longer than I’ve been alive.”
The new work focuses on tracheoles, a branching system of respiratory tubes that delivers oxygen to insect tissues. Rather than actively pumping blood to the muscles through a complex vertebrate circulatory system, insects channel oxygen through these tubes in a simpler and slower process of diffusion—and the larger the body, the more surface area the oxygen has to travel. To get bigger insects, the idea goes, you need more oxygen in the air and more tracheoles to effectively deliver it to the muscles.
“It’s an old idea that has, until now, been pervasive in the literature,” says Ned Snelling, an experimental physiologist at the University of Pretoria and lead author of the study.
During the Carboniferous, 50-meter-tall mosses and other plant life pumped massive amounts of oxygen into the atmosphere. Back then, oxygen made up about 30% of Earth’s atmospheric air, compared with 21% today.
But did such abundant oxygen really lead to gargantuan bugs? To test the idea, Snelling and his colleagues looked closely at the tracheoles in flight muscles of 44 species of modern flying insects of various sizes across several orders, including beetles, wasps, and grasshoppers. Using high-powered electron microscopy, they scanned and modeled the relationship between body size and the number of tracheoles across the insects, fleshing out how tracheolar volume scaled according to size. They found that regardless of size, tracheoles made up less than 1% of the insects’ muscle volume. Next, they extrapolated this relationship to a 300-million-year-old, 100-gram griffenfly known as Meganeuropsis permina, the largest insect ever documented. Just like modern insects, the researchers found, M. permina’s tracheoles would have constituted less than 1% of its muscles.
The discovery suggests that relative to their size, these ancient behemoths didn’t incorporate much more oxygen into their muscles than their more diminutive, modern relatives, Snelling says. Although larger insects did have a slight uptick in their number of tracheoles, the increase was minimal.
Pablo Schilman, a physiologist at the University of Buenos Aires, says the work is convincing. “It directly tests and challenges a widely accepted hypothesis that has been around since at least the 1960s,” he says.
Warren Burggren, a physiologist at the University of North Texas, agrees. The prevailing hypothesis was a “dogma,” he adds, “which I myself taught until seeing this paper.”
So, if abundant oxygen and more tracheoles didn’t spur the growth of giant insects, what did? Snelling says the simplest solution is that predators grew bigger, too, eventually munching larger insects into obsolescence. “When gigantic insects were around there were no birds or bats,” he says. “Big insects are easier for birds and bats to catch, and this keeps them small.”
Whatever the reason, Snelling continues, “our results cast some pretty serious doubt on the old idea that oxygen levels have constrained, and continue to constrain, the body size of insects.”
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