Giant insects could crawl on Earth, or fly above it, if there was just more oxygen in the air, scientists report.
Roughly 300 million years ago, giant insects scuttled and fluttered over the planet, among them dragonflies bearing wingspans comparable to today's hawks — two and a half feet across.
Back then, oxygen made up 35 percent of the lower atmosphere, compared to the 21 percent we breathe now.
Not all the insects back then were giants, but still, "maybe 10 percent were big enough to be considered giant," insect physiologist Alexander Kaiser at Midwestern University in Glendale, Ariz., told LiveScience.
To see if more richly oxygenated air could result in bigger insects, Kaiser and his colleagues investigated whether the present-day atmosphere is limiting insect size.
They compared four species of beetles which ranged in length from about one-tenth of an inch to roughly 1.5 inches.
Specifically, the researchers looked at the size of tubes in the insects known as tracheae, which circulate air in and out of their bodies.
While humans possess one trachea — ours run from the insides of our throats to our lungs — insects have a whole system of tracheae that connect to each other and the atmosphere.
X-rays showed that among the beetles studied, the tracheal systems got larger much faster than did the overall bodily sizes, about 20 percent faster.
This is because as beetles get larger three-dimensionally, their tracheae, whose internal membranes are effectively two-dimensional, have to get even bigger to keep up with the increasing oxygen demand.
Eventually, the tracheae can no longer keep up, placing a physical limit on the size of today's beetles.
Based on their calculations, the researchers figure modern beetles cannot grow larger than about six inches.
This happens to be about the size, Kaiser said, of the largest beetle known — the Titanic longhorn beetle, Titanus giganteus, which lives in South America.
If the atmosphere in the past held more oxygen, tracheae could be narrower and still deliver enough oxygen for a much larger insect. This would result in a much larger size limit, Kaiser concluded.
The scientists presented their findings at a meeting of the American Physiological Society this week.
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