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https://www.youtube.com/watch?time_continue=95&v=tI6F57s78cI&feature=emb_logo-this video says that it was recently discovered there was a 2-meter long MILLIPEDE living in the Carboniferous period, but that's away BEFORE the O2 rise. It tells that it was all about the right conditions like food, humidity, etc. But why didn't any other similar arthropods become titans as well? And could the same conditions be met in an alternate world, but have more exciting results, like the bugs being the dinosaurs of the planet and they never go extinct, let's just say the odds were in their favor, but I'm wondering if humans could have evolved alongside them, of course, it didn't happen on earth but if the titan-bugs were established on a certain continent and the mammals on another could humans evolve to discover the titans?

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  • $\begingroup$ Can anything else be changed eg. atmospheric pressure (thus changing partial pressure of O2 and maybe buoyancy), or maybe making gravity less strong - maybe by having a bigger but less dense planet to achieve both at the same time? $\endgroup$ Jul 15, 2022 at 13:21
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    $\begingroup$ There are extant myriapods that reach lengths of over 30 cm (1 foot); for example Scolopendra gigantea or Archispirostreptus gigas. $\endgroup$
    – AlexP
    Jul 15, 2022 at 13:30
  • $\begingroup$ @AngryMuppet All the variables are the same as the giant millipede once roamed, the atmosphere was pretty much the same as present-day back then $\endgroup$
    – Mr.D
    Jul 15, 2022 at 13:32
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    $\begingroup$ "All the variables are the same as the giant millipede once roamed, the atmosphere was pretty much the same as present-day back then": The atmosphere may be similar, but the predators surely aren't. Myriapods are not known for their smarts and self-defense abilities. $\endgroup$
    – AlexP
    Jul 15, 2022 at 13:35
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    $\begingroup$ As written this doesn't look like a question about establishing some fact about a fictional world you are creating. It's more of an ask about what could be possible on some alternate world. That's a pretty open ended ask, and as worldbuilders you can always just say "On my world X is true" so, asking about what could be possible seems more a matter of idle speculation than a specific ask that would be a good fit for this site. $\endgroup$
    – sphennings
    Jul 15, 2022 at 15:02

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Short answer: no. The largest insects in modern times are in the range of a palm width body length, and it's my understanding that an insect's respiratory system limits them to that size.

Arthropods, even on land, can get bigger than this -- look up coconut crabs -- but larger ones have more efficient respiration than an insect's spiracles, that effectively carry the air to all parts of the body rather than directly oxygenating the blood.

So reducing gravity wouldn't help much; the combination of exoskeleton and square-cube law aren't the limiting factor here, it's oxygen uptake and transport.

The 2 meter millipede you mention isn't quite the same -- first, it's not an insect, but second, it has independent respiration in each segment, so body length isn't a limitation; rather, it's the size of the segments, which ought to be capable of approaching the mass/volume of a Hercules beetle or other near-maximum modern insect (meaning a millipede that size might well be possible today with our current atmosphere).

Concerning independent evolution on different continents: not happening. Not as long as you have birds or similar flying predators on one continent or the other. Birds have crossed oceans for as long as there have been multiple continents -- and many birds prefer to eat arthropods like insects, centipedes, and millipedes.

Alternatively, flying insects have crossed oceans (possibly carried by storms), meaning that if conditions were right for insects to become large enough to outcompete primitive mammals, they'd have done so globally. No rats means no early primates, means no apes, means no hominids...

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  • $\begingroup$ Is there any way to overcome the Square cube law? It has been a real pain. The info was very helpful I totally forgot that millipedes weren't arthropods, that makes a big difference $\endgroup$
    – Mr.D
    Jul 15, 2022 at 13:38
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    $\begingroup$ Millipedes are arthropods, but they aren't insects. Just like spiders, mites, sowbugs, etc. aren't insects, though they have many similarities. Square Cube law allows an arthropod to get a little bigger than a coconut crab, maybe a little bigger than that with some trickery (void spaces inside the body for lightness, for instance). $\endgroup$
    – Zeiss Ikon
    Jul 15, 2022 at 13:44
  • $\begingroup$ Oh, sorry, I did mean insects, so that they can get bigger, could ants get maybe up to the side of a dog and every other insect is scaled up accordingly? that would make a praying mantis like the one in the goosebumps movie, I hope that doesn't sound too big $\endgroup$
    – Mr.D
    Jul 15, 2022 at 13:54
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    $\begingroup$ They'd have to learn to breathe better. At the very least, book-lungs like what some land-going crabs have (essentially internal gills connected to a pumping system). $\endgroup$
    – Zeiss Ikon
    Jul 15, 2022 at 14:22
  • $\begingroup$ Maybe they could evolve an alternate respiratory system? $\endgroup$
    – Mr.D
    Jul 16, 2022 at 7:40
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I see three problems with your scenario, two concerning size and the deal breaker concerning distribution. Insects size is basically constrained by their gas exchange system and nutritional needs.

The gas exchange is a big problem so if you evolved them a more efficient for large bodies breathing system they can grow any size you want. But what do they eat? Trees? They also lay hundreds or thousands of eggs some lay millions over their lifetime.

The deal breaker is the larvae, insects larvae are often aquatic. And usually several times the adults size, so sea life is going to be very different and there's no stopping them moving between continents I would think. Hard to imagine what giant insect larvae would eat on land that's sustainable.

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