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In my fantasy world, there are several species - both sapient and non-sapient. They all originally evolved from a group of insects that, after millions of years of isolation and no competition, have managed to evolve out of their anatomical constraints such as their respiratory system and exoskeleton.

Ignoring the sheer unlikelyhood of such a occurrence happening (which does have an in-universe explantation), what would these changes to the insect anatomy form and look like?

Let's assume oxygen levels stay (roughly) the same as modern day Earth without too much deviation, so no super bugs due to increased atmospheric oxygen.

Go, bring out the inner biologist within us all!

EDIT Due to fact that this question is apparently a duplicate, I would like to clarify. This question is asking about land based insects and how the respiratory, visual, circulatory, skeletal etc are changed due to an increase of size after enough time to evolve.

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  • $\begingroup$ So; If I've read this correctly you want to know what insects would look like if they had lungs and no exoskeleton? Short answer is that it's pretty hard to just 'swap' out an exoskeleton (shell) for an endoskeleton via evolution. The resulting transitional states would be a bit messy, to say the least. It's not even certain that they would retain their 6 limbs, which seems to me to be the only remaining insectile characteristic available to identify their origins. $\endgroup$ – Tim B II Oct 25 '19 at 4:16
  • $\begingroup$ This is close to asking - imagine something unimaginable happened, what would it look like? $\endgroup$ – Slarty Oct 25 '19 at 13:53
  • $\begingroup$ I strongly disagree with the classification of this as a duplicate; the earlier question only addresses the plausibility of a specific path to insect lung development, while this question is about what the plausible paths for lung and skeletal structure development are, which are not addressed in the existing answer to that earlier question. $\endgroup$ – Logan R. Kearsley Oct 25 '19 at 18:11
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The respiratory system and exoskeleton are not actually as strict of constraints as most people have been led to believe. For example, it's not actually the use of trachea in and of themselves that limits access to oxygen--it's reliance on passive diffusion. Improving the circulatory system and adding pumping to the function of the trachea (which some insects already have!) take care of that problem just fine.

Exoskeletons are more of a problem, but not in the way that is most obvious. Yes, they can be heavy, and that limits the growth of terrestrial arthropods somewhat--but in areas where there is little or no competition from tetrapods, arthropods can get really big already--and that's without even considering extinct clades like the famous carboniferous-era meganeurans. Consider, for example, the coconut crab, which can be up to 1 meter long!

The biggest problem with exoskeletons is the need to shed them as the animal grows. Molting gets harder as the organism gets larger, and it can become a very stressful and potentially deadly experience. Lobsters, for example, are sort-of functionally immortal, in that they show little to no signs of senescence, and just keep getting bigger as they age--but if they don't get eaten first, they will eventually die from the stress of necessary molting as they just keep growing. In fact, there are plenty of insects that just don't ever molt in adult form, because it's just not worth it--it wouldn't extend their lives enough to justify the risk. On the other hand, though, molting does come with advantages--it gives you a way to heal from massive injuries. Every molt is basically a full morphological reset.

Exoskeletons are thus not as good as endoskeletons for growing large, but they aren't inherently bad either. In competition with tetrapods, large arthropods tend to lose out--but your whole scenario is about eliminating that competition! Thus, I would expect to see quite a variety of quite large insects with little to no major development in their externally-obvious features.

To improve oxygen delivery, you'd want a more developed circulatory system, with more closed components; it need not evolve into a completely closed system, but it would be helpful to have better specialization of haemolymph vessels dedicated to picking up oxygen from the trachea and distributing it to the rest of the body. Additionally, you could see the evolution of "diaphragm" muscles designed to flex the body wall, thus changing internal volume to actively pump air in and out of the trachea; this sort of development has already happened once with the development of insect wings from the body wall.

In order to deal with the exoskeleton, you can imagine basic structural changes to make them more efficient at large size, like thinning out the skeletal wall on one side of a limb and thickening it on the other, with reinforcing spars extending into the interior of the limb, so it acts structurally more like an endoskeleton with muscles clumped on one side. None of that would be externally visible, though. The jackpot would be evolving a method of digesting and remodelling chitin after it has been laid down, thus eliminating the need for molting as the skeleton could then grow with the rest of the creature. That seems like the least likely option, but it would also not be externally visible.

Another way to sidestep the molting issue is to just not molt that much. You can have large eggs that produce large initial larvae, that grow even larger and metamorphose directly into a pretty-darn-big adult, which then does not have to molt as many times (or perhaps at all) to attain its final size. Adult molting may be retained in some clades as a response to serious injury, though--sure, the molt might kill you, but if you don't molt, you'll die anyway, so might as well roll the dice! If you win, you get a full reset and more years to reproduce.

As for what changes would actually be visible in adults--what they would look like--it's all basically square-cube stuff. You either get creatures that all stay very low-slung to the ground, distributing their weight over a large area, or you take the basic design of grasshopper back legs and apply that to all the weight-bearing limbs--i.e., move them closer to the body and more up-and-down, rather than splayed out to the side--to permit taller-standing creatures, and you increase the relative cross-sectional size of the limbs compared to the body as the whole creature increases in mass. Thus, a meter-scale mantid, for example, would probably have four enormous beefy grasshopper-like legs, with the front two oriented nearly straight up-and-down (and the back ones bent solely so that the positions of the feet form a large and more stable box than the close-together thorax attachment points would otherwise permit), rather than the wispy legs and wide-box stance typical of present-day mantids, while the non-weight-bearing forelimbs could remain essentially unchanged in relative scale.

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  • $\begingroup$ That last example sounds like a centaur. I love it! What about bipedalism? $\endgroup$ – Seraphim Oct 25 '19 at 17:27
  • $\begingroup$ @Seraphim I'm unsure bipedalism makes much sense for the insect body plan. It'd be difficult to stand upright because there's this big abdomen with most of the internal organs behind the limb girdle, so you're looking at bird/dinosaur-style bipedalism with the body axis remaining mostly-horizontal at best. And then you have to wonder, does it make sense to repurpose 4 legs as manipulatory, non-weight-bearing appendages? Or would you take advantage of the extra limbs to always keep 4 on the ground? I suppose I could see a Tyrannosaurid-type insect, with the 4 extra limbs all atrophied. $\endgroup$ – Logan R. Kearsley Oct 25 '19 at 18:06
  • $\begingroup$ Good point, I was just wondering due to the fact that I was thinking of having the elves of this world evolve from a mantis like organism that, much like us, evolved in trees so the middle pair of limbs moved up towards the “chest” area. Then, when they went back to the ground, they followed a similar evolution and bang! An elf that I sent a replicate of every single Tolkien elf ever. And the centaurs are just the ones who never went into trees and fairies are there smaller cousins. $\endgroup$ – Seraphim Oct 25 '19 at 18:23
  • $\begingroup$ I'd also want to point out that giant insects may not be limited to just an exoskeleton and may have a crude endoskeleton. The biggest weakness in molting is that you are basically boneless until the chitin hardens. Yet mammals are always soft on the outside. So why not take the best of both worlds? The exoskeleton may also be more segmented than in smaller insects, to make it easier to molt. The bigger the more segments. This could turn into a way to calculate how old the insect is. $\endgroup$ – user69494 Oct 25 '19 at 21:01
  • $\begingroup$ That would make sense. I also think that my be a transition body plan from exoskeleton to a endoskeleton, given enough time. Though that does leave me wondering, how would the eyes, antenna and (maybe) ears would fair during this whole process. I bet the fossil record would be fascinating to look at. $\endgroup$ – Seraphim Oct 26 '19 at 1:37
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The use of trachea limits their size very sharply. A higher partial oxygen pressure might help a little, but not much. You mentioned they are supposed to evolve beyond that, but what does that mean? You get a species that is ...

  • Egg-laying.
  • With compound eyes.
  • Six legs.
  • Endoskeleton.
  • Circulatory system for nutrient and oxygen transport.

Within these constraints, you could get something that looks almost like a dinosaur, or a mammal. The length of limbs is a minor change compared to the basics.

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Some insects like the Stalk-Eyed fly are able to grow their own anatomy to enormous size. Their eye stalks small when they are born, but within fifteen minutes or so after birth they rapidly expand them extremely long. It is reasonable to assume that fantasy insects would have functions like this.

https://thesmallermajority.com/2013/01/21/stalk-eyed-flies/

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  • $\begingroup$ These flies are usually about 1cm in length, hardly what most people would call "enormous". The OP seems to be after an absolute increase in size, not body parts that appear large relative to the rest of the body. $\endgroup$ – Nuclear Wang Oct 25 '19 at 13:28
  • $\begingroup$ These creatures often have eye stalks that are longer than the length of their body and are real world creatures. In a fantasy setting it is not much of a stretch to say that fictional insects might not develop something like this like for pincers, exoskeleton, wings etc. The OP asked if insects can evolve their basic anatomy to grow in size and stating that oxygen levels are the same, I have provided one example a plausible explanation where that can be possible in a fantasy world. $\endgroup$ – fnicke Oct 25 '19 at 13:38

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