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It's a reasonably well-known fact that insects grew to massive sizes due to the excessive concentration of oxygen in the prehistoric-Earth's atmosphere.

If one were to try to recreate this high-oxygen environment in a lab setting, would insects reared within grow to massive size?

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    $\begingroup$ Yep! In the group I work in we have a little pet project doing things like that. Different environmental conditions totally change insect’s body sizes. Getting up to truly massive sizes is a bit more tricky, but it works pretty well. $\endgroup$ – CalendarJ Aug 23 at 3:35
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    $\begingroup$ @CalendarJ it would be excellent if you draft this up as an answer with a bit more detail. Thanks! $\endgroup$ – theforestecologist Aug 23 at 16:48
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While @CalendarJ comments "Yep!" above, a quick search suggests that experimental hyperoxia typically only increases insect body sizes slightly. Everything below is from VandenBrooks et al. 2012 (of course there could be other sources that contradict this!)

A few beetles have been reported to reach larger sizes when reared in hyperoxia, but most of the species examined reach similar body sizes in 40% oxygen as in normoxia (Harrison et al. 2009).

and:

Here we investigated the impacts of Paleozoic oxygen levels (12–31%) on the development of Blatella germanica cockroaches. Body size decreased strongly in hypoxia, but was only mildly affected by hyperoxia.

and this figure:

enter image description here

These results show that male cockroaches grow much larger under present-day (normoxic; 21% oxygen, highlighted box) and elevated (hyperoxic; >= 24% oxygen) levels than under low (anoxic; 12%) conditions, but the hyperoxia-grown roaches are only a bit larger than normoxia-grown ones. (Similar results for females not shown.)


VandenBrooks, John M., Elyse E. Munoz, Michael D. Weed, Colleen F. Ford, Michael A. Harrison, and Jon F. Harrison. “Impacts of Paleo-Oxygen Levels on the Size, Development, Reproduction, and Tracheal Systems of Blatella Germanica.” Evolutionary Biology 39, no. 1 (March 1, 2012): 83–93. https://doi.org/10.1007/s11692-011-9138-3.

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The latest documentary I watched about the Meganeura (a giant Dragonfly from Pre-Cambrian) seems to confirm your intuition.

Based on experiments by John VandenBrooks at Midwestern University, with bugs in conditions A with less oxygen than current Earth atmosphere, and a 31% oxygen atmosphere in conditions B:

The bugs from group B were roughly twice as big. It was a bit frightening. (I don't know if there's a paper that shows the size difference statistics for several individuals.)

Here's the documentary for French TV "Science Grand Format":

  • there is a lightweight interview of the scientist at 06:15
  • experiments on cockroaches at 06:50
  • they show the bugs different sizes at 7:45.
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Some of the information contained in this post requires additional references. Please edit to add citations to reliable sources that support the assertions made here. Unsourced material may be disputed or deleted.

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    $\begingroup$ Do you have a citation/link to the documentary that you could provide? $\endgroup$ – CalendarJ Aug 23 at 3:35
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    $\begingroup$ @CalendarJ have added sources. Althought the scientist is interviewed and explain his results, I have not checked his research papers. $\endgroup$ – Stephane Rolland Aug 23 at 9:36
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I think you perhaps have a misconception. Body size is mainly controlled by genetics, thus we have species of "bugs" that are very small, others from closely-related lineages that are much larger. Take for instance flies, which come in sizes from fruit flies through house flies up to horseflies. They're all pretty similar (under magnification) to a non-specialist, yet it seems unlikely that raising a fruit fly in an oxygen-rich environment would make it grow to the size of a house fly, let alone a horse fly.

It doesn't take much thought to come up with many similar examples, such as beetles and butterflies, with species that come in a range of sizes. It seems obvious that they've evolved a size which fits their evolutionary niche, and if they grew too large, they would be unlikely to survive.

Oxygen concentration is one of the factors that puts limits on the size of insects, but to get real size increases, you need to evolve new species that take advantage of increased oxygen, not just grow your 1st generation offspring.

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  • $\begingroup$ Yes, this basically summarizes the answer I was seeking in this question; whether or not the difference in size of Paleozoic insects was caused by a genetic difference. $\endgroup$ – Andrendire Aug 26 at 20:16

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