In 1995, a Nature paper presented a hypothesis that periods of high atmospheric oxygen helped the insects grow so large. This remained the consensus for about 30 years, until coincidentally, an international team of researchers, also in Nature, discovered strong evidence that insects’ flight muscles are not inhibited by atmospheric oxygen levels. The latest paper, published yesterday, potentially overturns this “textbook” theory on giant ancient insects – meaning insect gigantism now returns to the basket of unsolved mysteries about ancient creatures.
If the new study is valid, “there is no physical reason why insects the size of griffinflies cannot fly in today’s environments,” the researchers wrote in a column about the work for The Conversation. “And yet they do not exist today.”
Giant Bug-O-Spherical
According to the new paper, it is “the widely accepted paradigm that oxygen enabled the evolution of complex life.” This led researchers to consider whether oxygen levels in the atmosphere, which have changed over Earth’s history, would effectively “constrain” the evolution of body size of different species.
During the 20th century, researchers discovered many fossils of giant insects with incomparably wide wings. One of these was the griffinfly, which was later found to exist at a time when Earth’s atmospheric oxygen levels were 9% higher than today.
At the time, it was very logical to believe that the two variables — the griffinfly’s size and high oxygen levels — were linked, because the giant bugs “need these high external oxygen levels to burn energy faster during flight,” the team wrote in their column. Staying in the air requires that a person flying defy gravity, and “the rate of oxygen consumption increases approximately in proportion to the flyer’s weight,” the researchers said.
unused flight capacity
But the team wondered whether the insects could meet that oxygen demand themselves, given that they have a unique biological, tree-like mechanism called a tracheal system. The paper says this structure delivers oxygen to the insects’ flight muscles through a network of air-filled tubes called tracheoles, the development of which was confirmed in previous research as “heritable” and “highly plastic.”
The team arrived at this hypothesis during a separate investigation on the flight muscles of locusts, which revealed that tracheoles took up only 1% of the muscle fibers. The researchers then measured 44 species of flying insects of varying sizes and took 1,320 microscopic photographs over five years.
Their results showed that this strangely low investment in tracheoles was quite common in flying insects. For context, a different organ with similar functions in birds and mammals “occupies about ten times more space than the relative one,” Roger Seymour, the study’s senior author and a biologist at the University of Adelaide in Australia, said in a statement.
“This suggests that there is considerable scope for increasing the number and volume of tracheae without weakening the muscles,” the team wrote in the column. “The conclusion is that the body size of flying insects is never limited by the structure or function of their tracheal systems.”
Reopening a closed case?
If the findings are confirmed, it means that, theoretically speaking, there is no reason why griffinflies “could not survive in today’s environment,” the team wrote. Given the physiological capabilities of flying insects, ginormous flappers can compensate for less atmospheric oxygen by developing more tracheoles.
But the team said in the statement that the theory of oxygen deprivation being limited to the insects’ size has not been “debunked yet,” as it is still possible that other physiological factors may be limiting oxygen levels. However, the findings strongly suggest that researchers “should look elsewhere for why these giants existed”.
“The simple reason may be that larger animal species are more at risk of extinction than smaller species,” the team wrote. “300 million years ago, there were no bird or mammal predators to keep an eye on the griffinfly.”
The griffinfly and its extra-large contemporaries may be long gone, but their legacy continues to reveal some fascinating insights into the versatility of insect biology.
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